Newton s Third Law A common (but confusing) statement of Newton s Third Law is "For every action there is an equal and opposite reaction." In this activity you will measure forces with force sensors and try to understand the real meaning of Newton s Third Law. Two objects, Cart A on the right and Cart B on the left are in contact. Assume friction is not negligible. When Cart A and Cart B are in contact, there is a horizontal contact force between them, meaning that Cart A pushes or pulls on Cart B, and Cart B pushes or pulls on Cart A. Compare the magnitudes of these two forces in different situations. All motion is along a straight line. Force of Cart A on Cart B = F BA Force of Cart B on Cart A = F AB Discuss your answers with the others in your group and reach consensus, recording your predictions in the chart below. Afterwards, test your predictions by using a LabPro or LabQuest, force sensors, and dynamics carts and record the actual results in the same chart. See experimental set-up on the next page for guidelines. Summary of predictions and actual data: mass B = mass A mass B < mass A scenario (a) Cart A and Cart B pull on each other in opposite directions but there is no motion. (b) Cart B pulls Cart A to the left at constant speed. F BA (>, =, <) F AB F BA (>, =, <) F AB predicted actual predicted actual (c) Cart B pulls Cart A to the left, both are accelerating to the left. (d) Cart B pulls Cart A up an incline at constant velocity (e) Cart B pulls Cart A up an incline, both are accelerating up the incline (f) Cart A and Cart B push on each other in opposite directions but there is no motion.** **For carts to push on each other, you need to put rubber stopper bumpers over the hooks on the force sensors OR replace the hooks with pegs with a rubber end. In either case, force sensors will need to be recalibrated.
Experimental Set-up To attach the force sensors to the dynamics carts, first screw a vertical metal pole to the cart. Put the hole in the force sensor over this pole and tighten the black knob on the side of the force sensor to hold the pole tightly. Connect the hooks of the force sensors with a paper clip or small loop of string. The friction pad is held on the dynamics cart by magnets in the end of the cart. (All ends of the carts may not be magnetic if I haven t yet put the magnets in, but one end of the non-plunger carts will have had magnets installed.) Increase the friction by lowering the felt pad the whole way with the screw. The extra mass sits on top of Cart A for experiments where mass A > mass B. The dynamics track can be inclined by raising one end. The Force Sensor responds to force directed parallel to the long axis of the sensor. Since you will be comparing the readings of two Force Sensors, it is important that they both read force the same way. In other words, you need to calibrate them. At the same time you will set the sensors so that one reads positive for a pull while the other reads negative. That way they will share the same coordinate system when the sensors are facing one another. A convenient way to apply a known force for calibration is to use a known mass as a hanging weight. 1 kg or 1000 g weighs 9.8 N on Earth.
Calibrating the force sensors if you are using a LabPro and a Computer: 1. Connect the Force Sensors to CH1 and CH2 of the LabPro. Make sure your force sensors are set on ±10 N (there is a button on the top). 2. Connect the LabPro to the computer and open Logger Pro 3 on the computer. 3. Open the file 11 Newton s Third Law in the Physics with Vernier folder. 4. Choose Calibrate from the Experiment menu. Select CH1: Dual Range Force. Click on the button. 5. Remove all force from the first sensor and hold it vertically with the hook pointed down. Enter a 0 (zero) in the Value 1 field, and after the reading shown for Reading 1 is stable, click. This defines the zero force condition. 6. Hang the 200 g mass from the sensor. This applies a force of 1.96 N. Enter 1.96 in the Value 2 field, and after the reading shown for Reading 2 is stable, then click. 7. Click to complete the calibration of the first Force Sensor. 8. Choose Calibrate from the Experiment menu. Select CH2: Dual Range Force. Click on the button. 9. Calibrate the second force probe in the same manner as the first. This time, however, in step #6, enter the force in value 2 as a negative number ( 1.96) since this sensor faces in the opposite direction. 10. You will be using the sensors in a different orientation than that in which they were calibrated. Zero the Force Sensors to account for this. Hold the sensors horizontally with no force applied, and click. Make sure both sensors are highlighted in the Zero Sensor Calibrations box and click to zero both sensors. This step makes both sensors read exactly zero when no force is applied. Calibrating the force sensors if you are using a LabQuest: 1. Connect the Force Sensors to CH1 and CH2 of the LabQuest. Make sure your force sensors are set on ±10 N (there is a button on the top). 2. Choose Calibrate CH:1 Force from the Sensors menu. 3. Hold the sensor attached to Channel 1 so that you can hang a weight from it but do not attach any weight now. 4. Select Calibrate Now. 5. Enter 0 as the known value for Reading 1. 6. When the voltage reading stabilizes, tap Keep. 7. Hang a 1.96 N weight (200 g) from the sensor. 8. Enter 1.96 as the known value for Reading 2 and tap Keep when the readings stabilize. 9. Select OK. 10. Choose Calibrate CH:2 Force from the Sensors menu. 11. Calibrate the second force sensor in the same way as the first, EXCEPT enter a negative value in step #8 ( 1.96) since the sensor faces in the opposite direction. 12. You will be using the sensors in a different orientation than that in which they were calibrated. Next you will zero the sensors. This step makes both sensors read exactly zero when no force is applied. To do this
Conclusions: a. Hold both sensors with the measurement axis horizontal and no force applied to the hooks. b. When the readings stabilize, choose Zero All Sensors from the Sensors menu. The readings for the sensors should be close to zero. 1. Below are freebody diagrams for scenario f (Cart A and B push but there is no motion and Cart A is more massive than Cart B). The freebody diagram for Cart B is on the left and for Cart A is on the right. a) Why is W BE drawn shorter than W AE? b) How do you know how long to draw N BT and N AT? c) Why is the friction on B is less than the friction on A? d) Which force represents B pushing on A? e) What is the reaction to the action of B pushing on A? Which force is this on the diagram? f) Do Newton s 3 rd Law action/reaction pairs act on the same object? Explain. g) How does N BP compare to N BA and f BT? Explain. h) How does N AP compare to N AB and f AT? Explain. i) How does N BP compare to N AP? Explain.
2. Draw freebody diagrams for Cart A and Cart B for scenario c (Cart B pulls Cart A to the left, both are accelerating to the left) where Cart A is more massive. Show the Newton s 3 rd Law action/reaction pair by marking the forces with the symbol / as shown in #1. Rank all the horizontal forces from largest to smallest. 3. This experiment is designed to help you learn something about Newton s Third Law. Write a clear summary about what you have discovered.