Module 3 Lesson 2

Module 3—Effects of Force on Velocity

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Newton’s Third Law

Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.

The modern translation of this law is that all forces occur in pairs, equal in magnitude and opposite in direction. Expressed as an equation, it is

Lesson 2 Lab: Newton’s Third Law

An illustration uses female and male figure skaters to demonstrate Newton’s third law of motion.

In simple terms, Newton’s third law means that if one object exerts a force on another, the other object simultaneously exerts an equal force back on the first object. For example, if a skater pushes on a smaller skater with , then the smaller skater pushes back on the larger skater with, which is equal in magnitude but opposite in direction.

The applet used for this lab lets you simulate two skaters pushing away from one another. You can learn more about the simulation and how to use it by reading Show Me found at the top of the simulation screen.

Problem

If one object exerts a force on another, will the other object simultaneously exert an equal force back on the first object?

Open the Momentum Conservation simulation that will demonstrate Newton’s third law; then continue with the procedure. You may be required to login with a username and a password. Contact your teacher for this information, if needed.

Procedure

Reset the applet by pressing “Reset” ().
Press the “Data” button () to display information about each skater.
Press “Play” (), and observe the skaters moving apart. Keep the mass of skater 1 at 60.0 kg and the mass of skater 2 at 45.0 kg.

Observations and Analysis

Module 3: Lesson 2 Assignment

Remember to submit the answers to LAB 1, LAB 2, LAB 3, and LAB 4 to your teacher as part of your Module 3: Lesson 2 Assignment.

LAB 1. Record the information from the following chart, and complete the calculations for skater 2. (The calculations for skater 1 have been done for you as examples.) Let motion to the right be positive and motion to the left be negative.

Skater 1	Skater 2
initial velocity = 0.00 m/s	initial velocity = 0.00 m/s
final velocity = –0.23 m/s	final velocity = ___________
change in velocity = –0.23 m/s	change in velocity = ___________
time interval over which the force is applied = 0.10 s *Note: This is not displayed on the simulation.	time interval over which the force is applied = 0.10 s *Note: This is not displayed on the simulation.
calculate the acceleration of skater 1	calculate the acceleration of skater 2
mass of skater 1 = 60.0 kg	mass of skater 2 = __________
calculate the force applied to skater 1	calculate the force applied to skater 2 * Round your answer to 2 significant digits.

LAB 2. What do you notice about the magnitude of the force applied to each skater?

LAB 3. What do you notice about the direction of the force applied to each skater?

LAB 4. Restate Newton's third law using the force values you calculated for each skater.

Conclusion

You saw that each skater was acted on by a force. You also saw that the magnitudes of these two forces were equal. The difference in the forces was their directions, which were opposite, and the fact that each force acted on a different person.