Force, work, position, and move are words we hear every day: force work position Move force, work position

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1 Force, work, position, and move are words we hear every day: The door is stuck force it open! Carrying all those groceries up the stairs is going to be much work. The outfielder was out of position and missed the ball. Move closer if you need to see the board. Scientists use words like force, work, and position to describe moving objects. Take a moment to try to define each word. How do you use it in everyday life? Do you think scientists use it differently? What is position? What is motion? Let us start with position. What does it mean? You may have seen a sign in a mall with an arrow pointing to a spot on a map. The arrow tells you: You are here. You might wonder, How does the sign know where I am? Actually, how do you know where you are? You know your position in the mall because you can compare that point to other things on the map. The important thing to realize is that you cannot tell the position of something without knowing the This map shows your position in a building. position of other things around it. The same is true with motion. You cannot tell if something is moving unless you can compare it to the position of something else. If the distance between two things is changing, you know at least one of them is moving. Sometimes, you can tell which is moving because one object is staying still. Imagine you are flying in an airplane high above the ocean. First, all you see is water in all directions. Then you see a boat floating in the water. How can you tell if the boat is moving? What if the boat is near an island? 1

2 What is force? The scientific meaning of force is easy a force is a push or a pull. You probably know what it feels like to push a cart or to pull a rope. If you have ever played tug-ofwar, you also know what happens to you if you are pushed or pulled. What does force have to do with motion? Nothing changes its motion unless a force acts on it. The act of pushing or pulling changes motion in several ways: speeding up slowing down changing direction Speeding up is when an object starts to move from a standstill. It takes force to change the direction of motion. This is true even if the speed stays the same. Suppose you are running and someone bumps into you from the side. Your speed may stay the same, but your direction will change. This is because a force acted on you. The kicker s foot applies force to the ball. The force changes the ball s motion. Each force acts on one particular object with both strength and a direction. Look at the image of the soccer player above. What direction do you think the object (the soccer ball) is going to move? An object at rest typically has multiple forces acting on it, but they add together to give zero net force on the object. When forces do not add to zero, it causes a change in the object s speed or direction of motion. Objects in contact also exert forces on each other. For example, after the soccer player kicks the ball, will it continue moving forever? No! But, why not? That is because another force acts on the ball as it rolls, slowing it down. This force is called friction. Friction is the force between an object and the surface it is moving on. What is work? A famous writer is writing a book. She climbs two flights of stairs to her study where she does her writing. For eight hours, she types away at the This pole vaulter performs work as she moves over a distance. 2

3 keyboard of her computer and finishes the second chapter. You could say she worked hard all day, but how much work did she really do? According to scientists, the writer did much more work climbing the stairs than she did writing. Now we will see why this is true. Work is force multiplied by distance. What does that really mean? Suppose you climbed up two flights of stairs. You did more work than someone who only climbed one flight of stairs! Both of you did more work than someone who stayed put at the bottom of the stairs. If you re not moving, no work is being done. If the writer stops typing and just sits in her chair, she is not moving. If she is not moving, she is not working at least not in a scientific sense. What tools can we use to show that work is being done? Tools help us do work. Power tools help us by doing some of the work for us. These tools have an electric motor or a gasoline engine that applies the force needed to do work. For other tools, we supply all the force. These are usually called hand tools. Hand tools make work easier in two ways. They can increase the amount of force. They can also increase the speed at which the force is applied. A pry bar applies a large force to the head of the nail. A pry bar (also called a crowbar) increases force. You probably cannot pull a nail out of a board with your fingers (it would take a great amount of force that you probably are not strong enough to apply), but you can with a pry bar. The effort or force you apply to the pry bar gets increased by the tool on the head of the nail. A baseball bat and an axe help us do work by increasing the speed at which the force is applied. We apply a large force to the tool s handle over a short distance. This makes the other end of the tool go very fast over a longer distance. Now Find a long board, a brick, and a weight (for example, a five-gallon water container). The weight should be heavy, but you should still be able to lift it. Put the brick under the middle of the board, and put the weight on one end. Push down on the other end of the board. How hard is it to make the weight go up? Try moving the brick closer to the weight. Try moving it farther from the weight. How does the force needed to lift the weight change when you change the distance? 3

4 Getting Technical: Using a Come-Along A come-along is also called a ratchet lever hoist. This tool can exert a great amount of force. You can use it to lift something as heavy as a car. A come-along is made up of a lever, a pulley wheel, and a rope or chain. The chain is connected to the load, which is whatever you want to lift. When you pull the lever down a long distance, the pulley wheel turns just a little. The turning wheel pulls the chain a short distance. The chain moves the load up a little. This is the result: You pull a long distance with a little force, and you lift a heavy weight a short distance. Why is this tool called a come-along? When you pull the lever, whatever is attached to the chain will come along. This hoist can lift a very large weight. You can use all of the tools below to make work easier. Each tool has a place where a person applies force. Each tool also has a place at which force acts to do work. For each tool, write an A where you apply a force. Write a W where the work is done by each tool. Pliers Spade Hand Drill Bucket and Pulley 4

5 Working With Pulleys A set of pulleys is a tool that can be used to increase pulling force. Pulleys can also change the direction of the applied force (your effort). For this investigation, you will need these materials: Two pulleys with hooks attached (available at a hardware store) About 10 meters of rope A spring scale A bucket Water or gravel 1. Increase the weight of the bucket by adding water or gravel. Make sure you can still lift the bucket without straining yourself. 2. Use the spring scale to weigh the bucket. 3. Attach one pulley overhead but not out of reach. The wheel should hang down. 4. Pass the rope through this pulley. Then tie one end of the rope to the bucket and the other end to the spring scale. 5. Pull on the spring scale until the bucket leaves the ground (see Figure A below); record the reading on the scale. 6. Untie the rope from the bucket. Tie that end of the rope overhead, several feet from the pulley. 7. Attach a second pulley to the handle of the bucket; the wheel should face up. Then pass the rope through the bucket pulley and then through the overhead pulley. 8. Pull on the spring scale until the bucket leaves the ground (see Figure B below); record the reading on the scale. Look for examples of past and present uses of pulleys and compare them to your pulley system. 5

6 Ask your child if they see any patterns in the results of the experiment. Use these questions to help clarify these patterns. How do pulleys make work easier? How is the number of pulleys related to the applied force? Do pulleys reduce the amount of work that has to be done? Explain your answer. 6