University of Michigan - Department of Astronomy

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I can calculate the motion of heavenly bodies, but not the madness of people.

-- Isaac Newton

Introduction: Why Do Things Move?

Sir Isaac Newton defined three laws of motion. They are provided here as a quick refrence.Your textbook has a more in-depth discussion, or check one of the resources below.

  1. First Law (Law of Interia): A body at rest will remain at rest unless acted upon by a force. A body moving in a straight line at a constant speed will continue moving in a straight line at a constant speed unless acted upon by a force.
  2. Second Law: Force (F) is equal to the mass (m) times the acceleration (a), or F = ma. Force is in Netwons (N); mass is in kg; acceleration is in meters/second/second (m/s/s).
  3. Third Law: Every action has an equal and opposite reaction. A body applying a force will experience a force equal to and opposite form the force it applies; that is, if object 1 applies a force onto object 2, then object 1 feels an equal force applied in the opposite direction.


Part 1: Newton's Laws

Each group should have three blocks. Set them up as shown below and push them across the desk at a constant speed. Keep in mind that there is friction between the bottom of the blocks and the desk.

  1. Describe the motions of system A and system B:

  2. While under uniform (constant) motion, compare the net force on system A to that on system B. Explain how you arrived at your answer.

  3. A Free-body Diagram considers systems A and B to be a single point in space, and arrows are drawn into or away from that point which describe the direction of the forces acting on the systems. The length of the arrows are scaled to represent the size of the force and the arrows are labeled to describe them. Draw the FBDs for system A and system B:
  4. Is the magnitude of the horizontal force exerted by system A on system B larger, smaller, or equal to the force exerted on system A by system B. Explain your answer.

  5. Newton's third law means that whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction. Identify the forces in systems A and B which are paired from Newton's 3rd law. How did you decide which pairs to match up?

  6. Suppose the friction was suddenly cut in half, but the force you apply to push the blocks remains the same. What happens to the total magnitude of the horizontal forces on:



Last updated: 9/20/12 by SAM Original by CMM, edits by EB

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