Forces and motion 1: Identifying forces

Transcription

1 Forces and motion 1: Identifying forces University of York

2 Identifying forces All the questions in this set focus on the ability to identify the forces acting in everyday situations. Although there is no specific National Curriculum statement that says that pupils should be taught to do this, it is essential for many of the other statements. For instance, it is impossible to think about whether the forces acting on an object are balanced or unbalanced until you can identify what all the forces are. Many of the questions are suitable for use in Key Stage 2 classes, for example when teaching the ideas included in Unit 6E, Forces in Action in the QCA Scheme of work for Key Stages 1&2. They also relate directly to the ideas included in Unit 7K, Forces and their effects in the QCA Scheme of work for Key Stage 3. Some questions later in the pack return to situations considered in earlier ones, but with the addition of an extra, more challenging part-question. This enables you to choose the one best suited to the stage of understanding your pupils have reached. The questions deal with a small number of simple situations: two people pulling or pushing another object, which is not moving; a person pushing or pulling a heavy object, which is not moving; a person pushing or pulling a heavy object, which is moving at steady speed; an object hanging at rest from a support; an object sitting at rest on a surface. Some questions also include a rope or string connecting the interacting objects, thereby introducing a third object into the system. The first stage of understanding how the idea of forces can be used to explain examples of motion (or non-motion) is to appreciate which forces are involved in the situation without worrying too much about their relative sizes. Learning about their sizes can come later (and is probed much more fully in later sets of diagnostic questions on forces and motion). The key ideas underlying all the questions in this set are: a force is a push or a pull caused by an interaction between two objects; a force is always exerted by something, and on something (and it is important to be clear about which objects these are). A consequence of the first point above is that forces always come in pairs. If A exerts a force on B, then B exerts a force in the opposite direction on A. This always happens and doesn t depend on whether A or B moves or does not move, or on the relative sizes of A and B, or anything else. In these questions, the emphasis is on recognising that these partner forces arise every time a force is exerted. The set of questions Forces and Motion 5 probes understanding that the forces in an interaction pair are equal as well as opposite, in all situations. This basic point that forces come in pairs is not mentioned in the National Curriculum until Key Stage 4, but again it is difficult to see how a pupil can reach a clear understanding of simple examples of motion (or absence of motion) without it. For that reason, the questions in this set provide opportunities to develop pupils appreciation of this idea gradually, and in 6 University of York 2003

3 common everyday contexts where these ideas can more easily be related to experience. Questions 1-10 All of these ask about the forces acting on the main object in a given situation. Q1-3 are about two people pulling or pushing on an object; the questions ask about the forces on that object. Pupils are asked to mark forces using an arrow. Getting the direction of the force correct is more important than the relative sizes. It is important for pupils to be able to say which object each force identified is acting on. The exact point of application is less critical than the direction indeed for the extended objects involved in these situations, it is not always possible to be precise about the point of application of the forces. Q4-5 are about a person pushing or pulling on a box which is not moving. Pupils should identify a second force (due to friction) that is stopping the box moving. In order to emphasise that all forces are exerted by something on something, the friction force is referred to as the force exerted by the floor on the box. Q6-7 are about the same situations as Q4-5 but now with the box moving. Pupils may be less sure as to whether or not friction is now acting. Q8-9 consider an object hanging at rest. Q8 involves just two objects. Q9 is slightly more complicated by also involving a string linking the two. Here again, to emphasise the idea that forces are exerted by something on something, the force of gravity on an object is referred to as the force exerted by the Earth on the object. Finally Q10 considers the forces on a book lying on a table. Notes: 1 Some pupils may have difficulty with the idea that an inanimate object can exert a force. Perhaps the easiest way to get over this idea is to ask them to think about pushing on a spring with your hand. Although the spring is inanimate, it pushes back. In fact all objects distort like springs, though often by too small an amount to be seen. The origin of the pushing back force is exactly the same due to the restoring force of the material that has been distorted. 2 The choice of a box, sliding across the floor, rather than a trolley (or other object with wheels) rolling across it, is deliberate. The directions of the forces on wheeled vehicles, and the points at which they act, are complex so it is much easier to begin with examples in which the moving object slides. Questions These revisit the same situations as Q1-10, but now pupils are asked to label forces that are marked on the diagrams, rather than to mark specific forces on blank diagrams. Questions These questions revisit situations considered in earlier questions in the set. Parts (a) and (b) are repeats of earlier questions; in addition, however, these include a more difficult third (and in one case fourth) part which asks about the force on a second University of York

4 object involved in the situation the person pushing or pulling, or the support from which an object is hanging. This requires pupils to recognise the presence of the partner force in any interaction. Only six of the ten situations considered earlier are revisited; if you want to write similar questions for the other four situations, these provide a model to follow. Questions These questions take the earlier ideas a step further, by using a representation which isolates one object at a time from the situation being considered, and asks pupils to draw and label the forces acting on this object. A diagram of this sort is called a freebody diagram. Unlike earlier questions, these do ask about the relative size of the forces marked using the convention that a longer arrow means a larger force. Even so, direction and clear labels are still more important than sizes. Pupils may be more successful in identifying the forces on an object once they know about the link between force and motion, i.e. that: if an object is not moving, the forces on it must cancel each other out (add to zero); if the forces on an object do not cancel each other out (add to zero) it will move (indeed accelerate) in the direction of the total force. You may therefore think it more appropriate to use these questions after pupils have been taught about the connection between forces and motion, in Key Stage 3. Understanding of these ideas is not, however, probed explicitly by the questions in this set. They are the focus of the diagnostic questions in question set Forces and Motion 2. Q27-36 work well for small-group and whole-class discussion. One approach is to give each pupil a question sheet and allow them 5-10 minutes to write their own individual answers. Then put pupils into groups of three, and give each triad another blank copy of the question, perhaps enlarged to A3 size as a mini-poster. Encourage each group to discuss their answers and write an agreed best answer on the new sheet. This might take minutes. It is important that every member of the group is prepared to explain and justify this group answer. Then the groups answers can be pinned up on the wall, so everyone can see the whole set and the whole class can discuss what the answers should be. In this way, pupils may come to a better understanding of the ideas involved here, and of ways of representing them. Questions These questions are similar to Q11-20 in asking pupils to provide labels for marked forces. The forces marked are, however, more difficult ones to identify and label than those in Q11-20, which asked only about forces on the main object involved in the situation each time. In these questions, other forces in the various interaction pairs involved are also included. 8 University of York 2003

5 Questions The questions in this group are all in multiple choice format, so are quick to answer (and to mark). Q41-45 focus on the forces on the main object involved in the situation. Q46-49, on the other hand, look at the force exerted on the person pushing a box both when it does and doesn t move. This is a harder idea to grasp. The final two questions (Q48-49) also include the friction force on the pusher s feet. Similar questions (not included here) could be written about the situation of pulling an object. Q50-53 The final group of questions comprises four concept cartoons which explore ideas considered by earlier questions in the set. These could be displayed as posters while this topic is being taught, to encourage informal discussion. Or they could be used as individual or small-group activities. The response sheet provided with each cartoon is useful for making pupils think about their response to each of the comments of the cartoon characters. University of York

6 1 Mohammed and Pat both want to have the same book. Both are pulling on the book but it is not moving. Mohammed Pat (a) On this diagram, mark with an arrow the force exerted by Mohammed on the book. Mohammed Pat (b) On this diagram, mark with an arrow the force exerted by Pat on the book. Mohammed Pat 10 University of York 2003

7 2 Paolo and Pete are having a tug-of-war. Both boys are pulling on the rope but it is not moving. Paolo Pete (a) On this diagram, mark with an arrow the force exerted by Paolo on the rope. Paolo Pete (b) On this diagram, mark with an arrow the force exerted by Pete on the rope. Paolo Pete University of York

8 3 During a game of netball, Aisha and Beth are both pushing on the ball. But it is not moving. Aisha Beth (a) On this diagram, mark with an arrow the force exerted by Aisha on the ball. Aisha Beth (b) On this diagram, mark with an arrow the force exerted by Beth on the ball. Aisha Beth 12 University of York 2003

9 4 box not moving A construction worker is pushing a heavy box to move it across the floor. But the box is not moving. (a) On this diagram, mark with an arrow the force exerted by the construction worker on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). University of York

10 5 A furniture remover is trying to pull a heavy box across the floor. box not moving But the box is not moving. (a) On this diagram, mark with an arrow the force exerted by the furniture remover on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). 14 University of York 2003

11 6 Whitney is pushing a large box across the floor. moving at a steady speed It is moving at a steady speed. (a) On this diagram, mark with an arrow the force exerted by Whitney on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). University of York

12 7 Ken is pulling a large box across the floor. moving at a steady speed It is moving at a steady speed. (a) On this diagram, mark with an arrow the force exerted by Ken on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). 16 University of York 2003

13 8 An apple is hanging on a branch. (a) On this diagram, mark with an arrow the force exerted by the Earth on the apple (the force of gravity). (b) On this diagram, mark with an arrow the force exerted by the branch on the apple. University of York

14 9 This football is hanging from a string. The other end of the string is tied to the ceiling. (a) On this diagram, mark with an arrow the force exerted by the Earth on the football (the force of gravity). (b) On this diagram, mark with an arrow the force exerted by the string on the football. 18 University of York 2003

15 10 A heavy book is sitting on a table. (a) On this diagram, mark with an arrow the force exerted by the Earth on the book (the force of gravity). (b) On this diagram, mark with an arrow the force exerted by the table on the book. University of York

16 11 Mohammed and Pat both want to have the same book. Both are pulling on the book but it is not moving. Mohammed Pat Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Mohammed Pat Label for this force: (b) Mohammed Pat Label for this force: 20 University of York 2003

17 12 Paolo and Pete are having a tug-of-war. Both boys are pulling on the rope but it is not moving. Paolo Pete Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Paolo Pete Label for this force: (b) Paolo Pete Label for this force: University of York

18 13 During a game of netball, Aisha and Beth are both pushing on the ball. But it is not moving. Aisha Beth Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Aisha Beth Label for this force: (b) Aisha Beth Label for this force: 22 University of York 2003

19 14 box not moving A construction worker is pushing a heavy box to move it across the floor. But the box is not moving. Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Label for this force: (b) Label for this force: University of York

20 15 A furniture remover is trying to pull a heavy box across the floor. box not moving But the box is not moving. Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Label for this force: (b) Label for this force: 24 University of York 2003

21 16 Whitney is pushing a large box across the floor. moving at a steady speed It is moving at a steady speed. Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Label for this force: (b) Label for this force: University of York

22 17 Ken is pulling a large box across the floor. moving at a steady speed It is moving at a steady speed. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Label for this force: (b) Label for this force: 26 University of York 2003

23 18 An apple is hanging on a branch. Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Label for this force: (b) Label for this force: University of York

24 19 This football is hanging from a string. The other end of the string is tied to the ceiling. Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Label for this force: (b) Label for this force: 28 University of York 2003

25 20 A heavy book is sitting on a table. Each of the diagrams below shows one of the forces involved in this situation. For each force, write a label that says clearly what the force is. Write it in the form: force exerted by [something] on [something] (a) Label for this force: (b) Label for this force: University of York

26 21 box not moving A construction worker is pushing a heavy box to move it across the floor. But the box is not moving. (a) On this diagram, mark with an arrow the force exerted by the construction worker on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). (c) Is there a force exerted by the box on the construction worker? If you think there is, mark it with an arrow on the diagram below. 30 University of York 2003

27 22 A furniture remover is trying to pull a heavy box across the floor. box not moving But the box is not moving. (a) On this diagram, mark with an arrow the force exerted by the furniture remover on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). (c) Is there a force exerted by the box on the furniture remover? If you think there is, mark it with an arrow on the diagram below. University of York

28 23 Whitney is pushing a large box across the floor. moving at a steady speed It is moving at a steady speed. (a) On this diagram, mark with an arrow the force exerted by Whitney on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). (c) Is there a force exerted by the box on Whitney? If you think there is, mark it with an arrow on the diagram below. 32 University of York 2003

29 24 Ken is pulling a large box across the floor. moving at a steady speed It is moving at a steady speed. (a) On this diagram, mark with an arrow the force exerted by Ken on the box. (b) On this diagram, mark with an arrow the force exerted by the floor on the box (the friction force on the box). (c) Is there a force exerted by the box on Ken? If you think there is, mark it with an arrow on the diagram below. University of York

30 25 An apple is hanging on a branch. (a) On this diagram, mark with an arrow the force exerted by the Earth on the apple (the force of gravity). (b) On this diagram, mark with an arrow the force exerted by the branch on the apple. (c) On this diagram, mark with an arrow the force exerted by the apple on the branch. 34 University of York 2003

31 26 This football is hanging from a string. The other end of the string is tied to the ceiling. (a) On this diagram, mark with an arrow the force exerted by the Earth on the football (the force of gravity). (b) On this diagram, mark with an arrow the force exerted by the string on the football. (c) On this diagram, mark with an arrow the force exerted by the football on the string. (d) On this diagram, mark with an arrow the force exerted by the string on the ceiling. University of York

32 27 Mohammed and Pat both want to have the same book. Each is pulling on the book but it is not moving. Mohammed Pat Represent forces: In the boxes below, you are asked to mark the forces acting on each person or object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the horizontal forces acting on the book. (Ignore any vertical forces.) (b) On this diagram, mark all the horizontal forces acting on Mohammed. (Ignore any vertical forces.) (c) On this diagram, mark all the horizontal forces acting on Pat. (Ignore any vertical forces.) 36 University of York 2003

33 28 Pete Paolo Paolo and Pete are having a tug-of-war. Both boys are pulling on the rope but it is not moving. Represent forces: In the boxes below, you are asked to mark the forces acting on each person or object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the horizontal forces acting on the rope. (Ignore any vertical forces.) (b) On this diagram, mark all the horizontal forces acting on Paolo. (Ignore any vertical forces.) (c) On this diagram, mark all the horizontal forces acting on Pete. (Ignore any vertical forces.) University of York

34 29 box not moving This construction worker is pushing a heavy box to try and move it along. But the box is not moving. Represent forces: In the boxes below, you are asked to mark the forces acting on each person or object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the horizontal forces acting on the box. (Ignore any vertical forces.) (b) On this diagram, mark all the horizontal forces acting on the construction worker. (Ignore any vertical forces.) 38 University of York 2003

35 30 This furniture remover is pulling a heavy box to try and move it along. box not moving But the box is not moving. Represent forces: In the boxes below, you are asked to mark the forces acting on each person or object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the horizontal forces acting on the box. (Ignore any vertical forces.) (b) On this diagram, mark all the horizontal forces acting on the construction worker. (Ignore any vertical forces.) University of York

36 31 Whitney is pushing a large box across the floor. moving at a steady speed It is moving at a steady speed. Represent forces: In the boxes below, you are asked to mark the forces acting on each person or object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the horizontal forces acting on the box. (Ignore any vertical forces.) (b) On this diagram, mark all the horizontal forces acting on Whitney. (Ignore any vertical forces.) 40 University of York 2003

37 32 Ken is pulling a large box across the floor. moving at a steady speed It is moving at a steady speed. Represent forces: In the boxes below, you are asked to mark the forces acting on each person or object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the horizontal forces acting on the box. (Ignore any vertical forces.) (b) On this diagram, mark all the horizontal forces acting on Ken. (Ignore any vertical forces.) University of York

38 33 An apple is hanging on the branch of a tree. Then it falls, and lands on the ground. On the diagrams below, draw all the forces acting on the apple: (a) when it is hanging on the branch, (b) while it is falling, (c) when it is sitting on the ground. (a) the apple is hanging on the branch. Represent forces: by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. Mark and label all the forces acting on the apple (b) the apple is falling. (c) the apple is sitting on the ground. 42 University of York 2003

39 34 This football is hanging from a string. The other end of the string is tied to the ceiling. Represent forces: In the boxes below, you are asked to mark the forces acting on each object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the forces acting on the ball. (b) On this diagram, mark all the forces acting on the string. University of York

40 35 A heavy book is sitting on a table. Represent forces: In the boxes below, you are asked to mark the forces acting on each object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the forces acting on the book. (b) On this diagram, mark all the forces acting on the table. 44 University of York 2003

41 36 This furniture remover has tied a rope to a heavy box. box not moving He is pulling the rope but the box is not moving. Represent forces: In the boxes below, you are asked to mark the forces acting on each person or object involved in this situation. by drawing arrows to show the direction of each force, with the length of the arrow representing the size of the force. Label each force to indicate what it is. (a) On this diagram, mark all the horizontal forces acting on the box. (Ignore any vertical forces.) (b) On this diagram, mark all the horizontal forces acting on the furniture remover. (Ignore any vertical forces.) University of York

42 37 Winston is pushing a box along the floor. It is moving at a steady speed. moving at a steady speed d a b e c Some of the forces acting in this situation are marked on the diagram. Say what each of these forces is, by giving it a label of the form: force exerted by [something] on [something] The first one has been done for you. Fill in the blanks below, by writing similar labels for the other forces. Force a force exerted by Winston on the box Force b Force c Force d Force e 46 University of York 2003

43 38 Ken is pulling a box along the floor. It is moving at a steady speed. moving at a steady speed d a b c e Some of the forces acting in this situation are marked on the diagram. Say what each of these forces is, by giving it a label of the form: force exerted by [something] on [something else] The first one has been done for you. Fill in the blanks below, by writing similar labels for the other forces. Force a force exerted by Ken on the box Force b Force c Force d Force e University of York

44 39 Winston is pushing a box along the floor. It is moving at a steady speed. moving at a steady speed d a b e c Some of the forces acting in this situation are marked on the diagram. From the table below, pick out the correct label for each of the forces a-e. Write one letter (a, b, c, d or e) in the first column in the table, beside the correct label for that force. Write one letter (a, b, c, d or e) here Label for the force force exerted by the floor on the box (friction) force exerted by Winston on the box force exerted by the box on Winston force exerted by the floor on Winston (friction) force exerted by the floor on the box (reaction) 48 University of York 2003

45 40 Ken is pulling a box along the floor. It is moving at a steady speed. moving at a steady speed d a b c e Some of the forces acting in this situation are marked on the diagram. From the table below, pick out the correct label for each of the forces a-e. Write one letter (a, b, c, d or e) in the first column in the table, beside the correct label for that force. Write one letter (a, b, c, d or e) here Label for the force force exerted by the floor on the box (friction) force exerted by Ken on the box force exerted by the box on Ken force exerted by the floor on Ken (friction) force exerted by the floor on the box (reaction) University of York

46 41 Kwai is pushing a large box, but it is not moving. box not moving Which of the following best describes the forces acting horizontally on the box? (Ignore any forces acting up and down.) Tick ONE box ( ) The only force on the box is Kwai s push. The only force on the box is friction. There are two forces on the box: Kwai s push and friction. There are no forces acting on the box. 50 University of York 2003

47 42 Kwai is pushing a large box. It is moving at a steady speed across the floor. moving at a steady speed Which of the following best describes the forces acting horizontally on the box? (Ignore any forces acting up and down.) Tick ONE box ( ) The only force on the box is Kwai s push. The only force on the box is friction. There are two forces on the box: Kwai s push and friction. There are no forces acting on the box. University of York

48 43 An apple is hanging from a branch of a tree. Which of the following best describes the forces acting on the apple? Tick ONE box ( ) There are no forces acting on the apple. The only force on the apple is the downward pull of gravity. The only force on the apple is the upward pull of the branch. There are two forces on the apple: the downward pull of gravity, and the upward pull of the branch. 52 University of York 2003

49 44 A large crate is sitting on the ground. Which of the following best describes the forces acting on the crate? Tick ONE box ( ) There are no forces acting on the crate. The only force on the crate is gravity, pulling it downwards The only force on the crate is the upward push of the ground. There are two forces on the crate: the downward pull of gravity, and the upward push of the ground. University of York

50 45 A large crate is sitting on a weighing machine. Which of the following best describes the forces acting on the crate? Tick ONE box ( ) There are no forces acting on the crate. The only force on the crate is gravity, pulling it downwards. The only force on the crate is the upward push of the weighing machine. There are two forces on the crate: the downward pull of gravity, and the upward push of the weighing machine. 54 University of York 2003

51 46 Winston is pushing a large box, but it is not moving. box not moving force exerted by Winston on the box The diagram above shows the force exerted by Winston on the box. Which of the following best describes the force exerted by the box on Winston? Tick ONE box ( ) The box exerts a force forwards on Winston s hands. The box exerts a force backwards on Winston s hands. The box does not exert a force on Winston. University of York

52 47 Winston pushes a large box and it moves across the floor at a steady speed. moves at a steady speed force exerted by Winston on the box The diagram above shows the force exerted by Winston on the box. Which of the following best describes the force exerted by the box on Winston? Tick ONE box ( ) The box exerts a force forwards on Winston s hands. The box exerts a force backwards on Winston s hands. The box does not exert a force on Winston. 56 University of York 2003

53 48 box not moving Kwai is pushing a large box, but it is not moving. Which of the following best describes the forces acting horizontally on Kwai? (Ignore any forces acting up and down.) Tick ONE box ( ) The only force on Kwai is a forward push of the box on his hands. The only force on Kwai is a backward push of the box on his hands. There is a backward push of the box on his hands and a forward push of the ground on his feet. There is a forward push of the box on his hands and a backward push of the ground on his feet. Kwai is not moving, so there are no forces acting on him. University of York

54 49 moves at a steady speed Kwai is pushing a large box. The box moves at a steady speed. Which of the following best describes the forces acting horizontally on Kwai? (Ignore any forces acting up and down.) Tick ONE box ( ) The only force on Kwai is a forward push of the box on his hands. The only force on Kwai is a backward push of the box on his hands. There is a backward push of the box on his hands and a forward push of the ground on his feet. There is a forward push of the box on his hands and a backward push of the ground on his feet. The box is moving along steadily, so there are no forces acting on Kwai. 58 University of York 2003

55 50 Bottle on a shelf A bottle is sitting on a shelf. What forces are acting on the bottle? The bottle is not moving. There are no forces on it. The only force on the bottle is the force of gravity pulling it downwards. There are two forces on the bottle the force of gravity and the push of the shelf upwards, which balances it. A shelf cannot push. It is just in the way of the bottle and stops it falling. University of York

56 Bottle on a shelf What do you think? Tick a box in each row to show if you agree or disagree with each of the pupils: Asha The bottle is not moving. There are no forces on it. Agree Disagree Not sure Ben Colin Diana The only force on the bottle is the force of gravity pulling it downwards. There are two forces on the bottle the force of gravity and the push of the shelf upwards, which balances it. A shelf cannot push. It is just in the way of the bottle and stops it falling. Please explain your answer in the space below: 60 University of York 2003

57 51 Box on a table A box is sitting on a table. What forces are acting on the box? The box is not moving, so I don t think there are any forces here. The only force is gravity, pulling the box down. The table is pushing the box upwards and this balances the force of gravity. A table cannot push. It is just in the way of the box and stops it falling. University of York

58 Box on a table What do you think? Tick a box in each row to show if you agree or disagree with each of the pupils: Andrea Bob The box is not moving, so I don t think there are any forces here. The only force is gravity, pulling the box down. Agree Disagree Not sure Charlie Denise The table is pushing the box upwards and this balances the force of gravity. A table cannot push. It is just in the way of the box and stops it falling. Please explain your answer in the space below: 62 University of York 2003

59 52 Hanging ball A ball is hanging from a string which is tied to the ceiling. What forces are acting on the ball? The ball is not moving. There are no forces acting on it. The only force on the ball is the force of gravity pulling it down. The string is pulling upwards on the ball and this balances the force of gravity. A string cannot pull. It just stops the ball from falling. University of York

60 Hanging ball What do you think? Tick a box in each row to show if you agree or disagree with each of the pupils: Alan The ball is not moving. There are no forces acting on it. Agree Disagree Not sure Barbara Carole David The only force on the ball is the force of gravity pulling it down. The string is pulling upwards on the ball and this balances the force of gravity. A string cannot pull. It just stops the ball from falling. Please explain your answer in the space below: 64 University of York 2003

61 53 Pushing a heavy box A construction worker is pushing a heavy box, but it is not moving. What forces are acting on the box? The box is not moving. There are no forces acting on it. The only force on the box is the construction worker s push. box not moving There are two forces on the box: the construction worker s push, and a friction force in the opposite direction, which cancels it out. Friction is stopping the box moving. But friction is a kind of resistance, not a force. University of York

62 Pushing a heavy box What do you think? Tick a box in each row to show if you agree or disagree with each of the pupils: Ann The box is not moving. There are no forces acting on it. Agree Disagree Not sure Brian The only force on the box is the construction worker s push. Cathy Dan There are two forces on the box: the construction worker s push; and a friction force in the opposite direction. Friction is stopping the box moving. But friction is a kind of resistance, not a force. Please explain your answer in the space below: 66 University of York 2003

63 Answers and discussion Identifying forces 1 (a) (b) The book is not moving. So the two forces are equal in size. The arrows should be equal in length. 2 (a) (b) The rope is not moving. So the two forces are equal in size. The arrows should be equal in length. 3 (a) (b) The ball is not moving. So the two forces are equal in size. The arrows should be equal in length. 4 (a) (b) The box is not moving. So the two forces are equal in size. The arrows should be equal in length. University of York

64 5 (a) (b) The box is not moving. So the two forces are equal in size. The arrows should be equal in length. 6 (a) (b) The resultant force (or total force) on an object moving at a steady speed is zero. So the two forces are equal in size. The arrows should be equal in length. 7 (a) (b) The resultant force (or total force) on an object moving at a steady speed is zero. So the two forces are equal in size. The arrows should be equal in length. 8 (a) (b) The apple is not moving. So the two forces are equal in size. The arrows should be equal in length. 68 University of York 2003

65 9 (a) (b) The football is not moving. So the two forces are equal in size. The arrows should be equal in length. 10 (a) (b) The book is not moving. So the two forces are equal in size. The arrows should be equal in length. 11 (a) force exerted by Mohammed on the book (b) force exerted by Pat on the book 12 (a) force exerted by Pete on the rope (b) force exerted by Paolo on the rope 13 (a) force exerted by Aisha on the ball (b) force exerted by Beth on the ball 14 (a) force exerted by the construction worker on the box (b) force exerted by the ground on the box (friction) 15 (a) force exerted by the furniture remover on the box (b) force exerted by the ground on the box (friction) 16 (a) force exerted by Whitney on the box (b) force exerted by the ground on the box (friction) 17 (a) force exerted by Ken on the box (b) force exerted by the ground on the box (friction) 18 (a) force exerted by the Earth on the apple (gravity) (b) force exerted by the branch on the apple University of York

66 19 (a) force exerted by the Earth on the football (gravity) (b) force exerted by the string on the football 20 (a) force exerted by the Earth on the book (gravity) (b) force exerted by the table on the book (reaction) 21 (a) and (b): same as Q4 (c) Yes The forces in (a) and (c) are an interaction pair. So they are the same size. The arrows should be the same length. 22 (a) and (b): same as Q5 (c) Yes The forces in (a) and (c) are an interaction pair. So they are the same size. The arrows should be the same length. 23 (a) and (b): same as Q6 (c) Yes The forces in (a) and (c) are an interaction pair. So they are the same size. The arrows should be the same length. 70 University of York 2003

67 24 (a) and (b): same as Q7 (c) Yes The forces in (a) and (c) are an interaction pair. So they are the same size. The arrows should be the same length. 25 (a) and (b): same as Q8 (c) The forces in (a) and (c) are an interaction pair. So they are the same size. The arrows should be the same length. 26 (a) and (b): same as Q9 (c) (d) The forces in (b) and (c) are an interaction pair. So they are the same size. The arrows should be the same length. The tension in a string must the same throughout. So the forces in (b) and (d) are the same size, and the arrows should be the same length. University of York

68 27 (a) force exerted by Mohammed force exerted by Pat (b) force exerted by the book force exerted by the ground (friction) (c) force exerted by the book force exerted by the ground (friction) All six forces are equal in size. 28 (a) force exerted by Paolo force exerted by Pete (b) force exerted by the rope force exerted by the ground (friction) (c) force exerted by the rope force exerted by the ground (friction) All six forces are equal in size. 72 University of York 2003

69 29 (a) (b) force exerted by the construction worker force exerted by the box force exerted by the ground (friction) force exerted by the ground (friction) All four forces are equal in size. 30 (a) (b) force exerted by the furniture remover force exerted by the box force exerted by the ground (friction) force exerted by the ground (friction) All four forces are equal in size. 31 (a) (b) force exerted by Whitney force exerted by the box force exerted by the ground (friction) All four forces are equal in size. force exerted by the ground (friction) 32 (a) (b) force exerted by Ken force exerted by the box force exerted by the ground (friction) force exerted by the ground (friction) All four forces are equal in size. University of York

70 33 (a) (b) (c) force exerted by the branch air resistance force exerted by the ground (reaction) force exerted by the Earth (gravity) force exerted by the Earth (gravity) force exerted by the Earth (gravity) In (a) and (c), the two forces are equal in size. The forces in (c) should be shown in line, but have been drawn a little apart so they can be more clearly seen. 34 (a) (b) force exerted by the string force exerted by the ceiling force exerted by the Earth (gravity) force exerted by the football All four forces are equal in size. 35 (a) (b) force exerted by the book force exerted by the table (reaction) force exerted by the Earth (gravity) force exerted by the floor (reaction) force exerted by the Earth (gravity) force exerted by the floor (reaction) In (a), the two forces are equal in size. In (b), the sum of the two downward forces is equal to the sum of the two upwards forces. The downward force exerted by the book in (b) is equal to the upwards reaction force exerted by the table in (a), as these are an interaction pair. The two forces in (a) should really be in line. They are drawn slightly apart so that their lengths are clearer. 74 University of York 2003

71 36 (a) (b) force exerted by the rope force exerted by the rope force exerted by the ground (friction) force exerted by the ground (friction) All four forces are equal in size. 37 b force exerted by the floor on the box (reaction) c force exerted by the floor on the box (friction) d force exerted by the box on Winston e force exerted by the floor on Winston (friction) 38 b force exerted by the floor on the box (reaction) c force exerted by the floor on the box (friction) d force exerted by the box on Ken e force exerted by the floor on Ken (friction) 39 c, a, d, e, b 40 c, a, d, e, b Note: For questions 41-49, answer options are referred to by number (1, 2, 3 ), in the order in which they appear in the question University of York

72 50 Colin s comment here is correct. The absence of motion does not mean there are no forces acting there may be balanced forces (as in this case) which add to zero. Colin and Diana may be thinking along similar lines. In fact there is an interaction between the bottle and the shelf. The shelf is slightly compressed (even apparently hard objects can be compressed microscopically) and tends to spring back to its normal position. This causes an upwards force on the bottle. 51 The ideas put forward here are similar to those in Q50. Charlie s comment is correct. 52 Again the ideas involved here are similar to those in Q50 and Q51. The only difference is that here it is the tension of a string that is involved, rather than the reaction of a surface. The underlying reasoning is the same. The string stretches slightly and tends to spring back to its natural length. This causes an upward force on the ball. So Carole s comment is correct. 53 As in Q50, the absence of motion does not mean there are no forces acting, so Ann s comment is incorrect. Cathy s comment is correct two forces which balance each other and add to zero. Dan expresses an idea which some students may share that friction is not a force but a kind of resistance. Friction is really the name for the interaction between two surfaces that try to slide over each other. This interaction gives rise to a pair of forces, one acting on each of the objects involved. We can refer to these, as Cathy does, as friction forces. 76 University of York 2003

73 Acknowledgements Some of the questions in this pack are based on probes used by science education researchers to explore learners understanding of key ideas about forces and motion. These have been modified, often quite substantially, to improve their clarity or to make them easier to use in class. Others are new questions, written for EPSE Project 1: Using Diagnostic Assessment to Enhance Teaching and Learning in Science. Questions which are knowingly based on previous probes are listed below, with an indication of the original source. If precursors of any questions in this pack are not acknowledged below, we would be grateful to have this pointed out, so that it can be rectified in any subsequent publication. The emphasis in this pack on clear identification of the forces acting and of what is exerting each force, and which object it is acting on was influenced by: Hart, C. (2002). If the Sun burns you, is that a force? Some definitional prerequisites for understanding Newton s Laws. Physics Education, 37 (3), Hart, C. (2002). Forceful Experiences A Unit on Forces for Year 7 to 10 students. Unpublished teaching notes. Department of Science and Mathematics Education, University of Melbourne. Q26-36 The format of these questions is based on an idea developed by Pam Mulhall and Brian McKittrick at Monash University. They refer to items of this format as Conceptual Understanding Procedures (CUPs). A selection can be downloaded from URL: Q50-53 These are new items, but are based on the Concept Cartoon format developed by Stuart Naylor and Brenda Keogh: Naylor, S. & Keogh, B. (2000). Concept Cartoons in Science Education. Sandbach: Millgate House Publishers. University of York

74 78 University of York 2003