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.
1. Two objects, a truck on the left, and a car on the right,
are on a horizontal rough surface (friction present) and are in contact. The
vehicles can have their motors on or their brakes applied, or be in neutral.
When the truck and car are in contact, there is a horizontal contact force between
them, meaning that the truck pushes or pulls on the car, and the car pushes
or pulls on the truck. Compare the magnitudes of these two forces in
different situations. All motion is along a straight line.
In Parts (a) to (d) vehicles are of equal mass, in Parts (e) and (f)
the truck is much more massive than the car.
Discuss your answers with the others in your group and reach consensus.
|
Force of car on truck is
(>, =, <) the Force of truck on car. |
Measurements
(later) |
|
| (a) Equal masses, Truck and car push but there is no motion. | ||
| (b) Equal masses, Truck pushes car to the right at constant speed. | ||
| (c) Equal masses, Truck pushes car to the right, both are accelerating to the right. | ||
| (d) Equal masses Truck pushes car and both move to the right, car sets brakes and they are slowing down, accelerating to the left. | ||
| (e) Very Massive truck moves to the right and rear ends a light car that is also moving right. They separate after the collision. | ||
| (f) Light car moves to the right and rear ends a massive truck that is also moving right. They separate after the collision. |
2. Get a computer, LabPro, two force sensors with two adaptors, and two massive bars. Connect the LabPro to the computer. Use the adaptors at the ends of the force probe cables, and connect the force probes to CH1 and CH2 of the LabPro. The truck is the sensor connected to CH 1, and the car is the sensor connected to CH2.
• Be sure the slide switch on the force sensors is at 50N.3. Now set up experiments to check the results that you predicted in parts (a) to (d). Enter your observations into the appropriate column in the table on the previous page.
• Open the file “Newton’s Third Law” from MyDocuments (Drag it there from the server if it is not there already).
• Zero both sensors (CNTRL-0), and check the zero by collecting data with the sensors horizontal and not touching. Beware the Tare: There is a tare button on the side of the force sensor. Be very careful not to accidentally press it. If you do, you will need to re-zero the sensors.
• Check the calibration of the sensors by hanging 550 grams from a hook attached to the sensor. You should get about 5.4 N for each sensor. If you do not ask an instructor how to calibrate the sensor.
• Replace the hooks on the force sensor with rubber bumpers.
• Push on each bumper with your hand while collecting data to see how data are collected.
Click on the clock icon (or on menu, Experiment: Data Collection) and a window
will open. In the window, make the length of the experiment 0.5 s, and collect
1000 samples per second. Then click the Triggering tab and check Enable Triggering.
Check parts (e) and (f) of your predictions.
5. This experiment is designed to help you learn something about Newton’s
Third Law. Write a clear summary about what you have discovered.
This page maintained by Anne G. Young. Last modified 04-Jan-2004 .