Living with Newton's Laws

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Arthur Hodges
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Task #1 - Newton s 1 st Law - This is a pain in the neck Let's suppose you are in your car, waiting at a stop light. Like any good driver, you have your seat belt buckled. (It's the law.

1 Task #1 - Newton s 1 st Law - This is a pain in the neck Let's suppose you are in your car, waiting at a stop light. Like any good driver, you have your seat belt buckled. (It's the law.) Suddenly, a car comes up from behind and hits your car in the rear. Once the surprise of the event wears off you notice that your head and neck hurt. 1. (a) Describe what you think happens to your head while your car is being hit from behind. (b) Suppose you are a passenger in the moving car and it hits the back of a stopped car. What happens to your head? We don't want to actually do this experiment in class, but let's look at some other situations that might help us understand this one better. Place a ball on a book that you hold out in front of you, flat like a tray. 2. Conduct the following experiments and record what happens to the ball: From standing still, suddenly walk quickly forward. From standing still, suddenly walk quickly backwards. Walk steadily forward, gently keeping the ball on the book with your other hand, then let go of the ball while walking steadily with the ball staying on the book. Then stop suddenly. These observations suggest a fundamental law of physics, which we call Newton's First Law of Motion. This law suggests an interesting idea about motion that was very radical when it was first articulated (actually by Galileo), and remains surprising to most people when they first encounter it. The natural state of motion of an object is to remain at rest, if it is already at rest, or to remain in motion at constant velocity, if it is already moving. The law is usually stated as follows: UNL page 1 of 9

An object at rest remains at rest, or an object in motion remains in motion at constant velocity, unless acted on by a net force. 3.

Now reconsider the situation where you are stopped at a red light and someone hits you from behind.

Now consider the situation where you are traveling in a car and hit a stopped vehicle in front of you.

The following figures show various whiplash injuries. Describe a scenario in which each one could occur.

2 An object at rest remains at rest, or an object in motion remains in motion at constant velocity, unless acted on by a net force. 3. Refer back to the observations of the ball on the book. Were they consistent with Newton's First Law? Explain. 4. Now reconsider the situation where you are stopped at a red light and someone hits you from behind. (a) Using Newton's First Law, predict what should happen to your head. (b) Where should the brain trauma occur in this type of accident? 5. Now consider the situation where you are traveling in a car and hit a stopped vehicle in front of you. (a) Using Newton's First Law, predict what should happen to your head. (b) Where should the brain trauma occur in this type of accident? 6. The following figures show various whiplash injuries. Describe a scenario in which each one could occur. (a) (b) Task #2A - Calibrating and exploring a new measuring tool: The "Force Plate" Today you will be using a new type of sensor called a force plate. Complete the following steps to calibrate this new sensor. Calibrating the force plate: A. Open the file MBL 1 Force Plate. B. Click on the "Setup" button to view the "Experiment Setup" window UNL page 2 of 9

b. With no mass on the force plate, enter 0 (zero) in the Low Point Value box and click the Low Point "Take Reading" button. c. Obtain 20 kg and distribute it evenly over the entire force plate.

3 Double click on Force Sensor icon C. Double click on the "Force Sensor" icon. You will now calibrate the sensor by setting known high and low point force values for the voltage signals coming from the sensor. a. Choose the "Calibration" tab near the top of the "Sensor Properties" window. b. With no mass on the force plate, enter 0 (zero) in the Low Point Value box and click the Low Point "Take Reading" button. c. Obtain 20 kg and distribute it evenly over the entire force plate. Enter this High Point Value in Newtons (20 kg * 9.8 m/s 2 ). Then click the High Point "Take Reading" button. d. Verify that the High Point Voltage and Value are greater than the Low Point Voltage and Value. Repeat steps if needed. e. You are now down calibrating so click the OK button. (c) Enter the weight on the force plate. Click here. (b) Enter zero in this box with no weight on the force plate. Click here. (e) Click OK when done calibrating D. Check your calibration: a. Place a known weight on the force plate (use a different weight than you used to calibrate the force plate). b. Click the Start button. Find the % difference between the known mass and the mean value for mass obtained from your data UNL page 3 of 9

4 Use the mean value to find the % difference E. If you do not obtain results less than 8% difference, repeat the process for checking your calibration. F. If you are still having trouble, ask for help from your lab instructor. G. It might be a good time to save this file; give it a name that will identify it as yours. Exploring the Force Plate sensor Try out the calibrated force plate to answer the following question: How much force can you push with one hand compared to with both hands? Have each person try this out with the force plate sitting on the tabletop. 7. Write a summary of what you found. What conclusions could you make about the force with which people can push with their hands? Task #2B - Using the force plate to study Newton s 2nd Law Newton's second law of motion specifies the relationship between net force on an object and the resulting acceleration. 8. Based on your textbook, lecture notes, and/or lab partners, write Newton s 2 nd law in the form of an equation. Define any variables and/or constants. Activity #1 - Standing on a force plate Open the file you saved with the force plate calibration. If the file is already open, go to the Experiment drop-down menu and select Delete All Data Runs. Place the force plate flat on the floor and have one person in your group stand on it. Data Collection: Click the start button. After about 5 seconds hand a dumbbell weight to the person standing on the force plate UNL page 4 of 9

5 The person on the force plate should hold the dumbbell completely still for about 5 seconds. Then, accelerate the dumbbell upward as fast as you can (in a safe manner). Hold the dumbbell up high for about 5 seconds and then accelerate the dumbbell down. Hold the dumbbell still for about 2 seconds and then move the dumbbell up with a slow constant speed. Hold it up high for about 2 seconds and then lower the dumbbell with a slow constant speed. Click the Stop button to stop taking data. Data Analysis: Adjust the axes of the graph to best show the data and then print a copy for each person in your lab group. On the graph, identify which parts of the graph correspond to the different events described above. Using the DataStudio tools, record the following force values in a data table: The person standing still on the force plate (record the mean value) The person standing still holding the dumbbell (mean value) The person accelerating the dumbbell upward at a fast rate (max value) The person accelerating the dumbbell downward at a fast rate (max value) The person moving the dumbbell upward at a constant speed (mean value) The person moving the dumbbell downward at a constant speed (mean value) Invention Discussion Consider the dumbbell in the activity you just completed. As a class, draw a free body diagram representing the dumbbell. Keep the following in mind when drawing free body diagrams: (A) (B) (C) (D) Identify a body. Draw arrows representing the forces acting on that body only. For each force, identify an object that is exerting that force an object does not push/pull on itself! Write Newton's 2 nd Law from this free body diagram. 9. (a) What do you know about the forces acting on the dumbbell if a = 0? (b) What do you know about the forces acting on the dumbbell if a > 0? (c) What do you know about the forces acting on the dumbbell if a < 0? Data Analysis Continued: 10. Using Newton s 2 nd law and the data obtained from your graph, calculate: UNL page 5 of 9

6 (a) (b) (c) The mass (kg) of the person standing on the force plate. The mass of the dumbbell and % difference from its known value. The maximum acceleration of the dumbbell. 11. Considering Newton's 2 nd law, answer the following questions for the force plate/dumbbell activity: (a) How can you change the force exerted on the force plate? Cite examples from your graph that support your answer. (b) What changed when you added mass to the person on the force plate? What remained constant? (c) If the mass of the person did not change, how did the maximum force change in the last part of the activity? Activity #2 - Newton's 2 nd Law and riding in an elevator (Two groups can share the elevator) 12. Obtain a spring accelerometer (a spring scale with 500 g attached). Observe what happens to the spring accelerometer during the following events while you and your lab partners ride the elevator in Ferguson Hall from 3 rd floor to 1 st floor and back up to 3 rd floor. Record your visual observations in your lab notebook for (a d). (a) When the elevator is at rest (b) When the elevator is starting to move down (c) When the elevator is between floors (d) When the elevator is stopping at the bottom UNL page 6 of 9

7 Earlier someone took a force plate along for an elevator ride in Oldfather Hall. Open up the file called Elevator data. You can use this data to answer the following questions. Data Analysis: Find and record the minimum, maximum, mean values, and draw a free body diagram for the person riding in the elevator for each of the following: When the elevator is at rest When the elevator is accelerating at the beginning When the elevator is moving at a constant rate When the elevator is accelerating at the end Print off a graph for each person in your group and tape it into your lab notebook. Using the data collected from the graph to make the following calculations. 13. (a) The mass of the person riding this elevator. Explain how you know. (b) The acceleration of the elevator when it is starting to move. (c) The net force acting on the person when the elevator is moving (in between the starting and stopping). (d) The elevator s acceleration when it is stopping. 14. (a) What is the motion of the elevator when the net force is zero? (b) What part of the ride do you appear to have less weight? Why is this? (c) What part of the ride do you appear to have more weight? Why is this? 15. Calculate what force would be acting on you as you accelerate at the beginning of the elevator ride if you were the one riding in the elevator in Oldfather Hall. Task #3 - Using the force plate to study Newton s 3 rd Law Keep in mind that, Net force is defined as: the sum of all force vectors: F net = Writing Newton s 2 nd law as a vector equation we have is always in the same direction as the net force. F F net = m a, where the acceleration Activity #1 - How net force and motion relate to each other Physically push on the wall of the classroom. Describe the motion of the wall when you push on it. Record your observations and draw a free body diagram for the situation UNL page 7 of 9

8 16. (a) From the observations you made in the activities you did previously for Newton s 2 nd law, what do you know about the net force when an object is at rest? Has a constant motion? What has to happen to the net force on an object for it to accelerate? (b) Describe the forces and the net force for the pushing situation you tried in activity #1 for Newton s 3 rd law. The preceding experiments suggest another law of motion, now known as Newton s Third Law. This law can be stated as follows. Whenever one object exerts a force on another object, the second object exerts a force back on the first one that is equal in magnitude to the first but opposite in direction to it. This suggests that forces always occur in pairs, sometimes referred to as action-reaction pairs. Sometimes this law is stated as follows. For every action there is a reaction equal in magnitude but opposite in direction. It is important to keep in mind that the action-reaction pair of forces act on different objects. They are not acting on the same object. Activity #2 - Using 2 Force plates to investigate Newton s 3 rd law In this activity, you will need to use two force plates that have each been calibrated. A special station has been set up in the room with this equipment. You should go to this station to complete this activity. You may have to get together with another lab group since there is not extra equipment. This station has two calibrated force plates with handles attached. They were calibrated in the same manner as the calibration you performed earlier in this lab. Have two members of your group each pick up a force plate and push them against each other (so the Vernier Force Plate label of each plate is against each other). Be sure to hold the plates completely vertical and have the two surfaces touching each other completely. Try each of the following configurations and record force data for each plate: (a) Held vertically, but NOT touching each other (about 5 seconds) UNL page 8 of 9

9 (b) (c) (d) Gentle push (about 5 seconds) Moderate push (about 5 seconds) Hard push (about 5 seconds) Print the relevant graph(s) and determine the mean force values for each plate for each configuration (a-d). Record these values in a data table for force plate A and B. 17. Do both force plates have the same reading when pushed on gently, moderately hard, and hard? Explain why. 18. In your own words state Newton s 3 rd law and explain what evidence in this activity helps to support Newton s 3 rd law UNL page 9 of 9

What Do You Think? For You To Do GOALS. The men s high jump record is over 8 feet.

What Do You Think? For You To Do GOALS. The men s high jump record is over 8 feet. Activity 5 Run and Jump GOALS In this activity you will: Understand the definition of acceleration. Understand meters per second per second as the unit of acceleration. Use an accelerometer to detect acceleration.