Name period date. Newton s Third Law POGIL. Every action has an equal but opposite reaction.

Transcription

1 Name period date Newton s Third Law POGIL Purpose To use Newton s Third Law of Motion to explain action and reaction force pairs. To use Newton s Third Law to explain the motion of a system. Part 1: Newton s Third Law Newton s Third Law of Motion states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force upon the first. More simply: Every action has an equal but opposite reaction. Action and reaction pairs follow a simple general rule: If the ACTION is A acting on B, then REACTION is B acting on A. The following illustrations show several action and reaction forces: Boy pushes wall to the right with a force of 100 N. Hammer hits the nail with a force of 200 N down. Wall pushes boy to the left with a force of 100 N. Nail pushes upward with a force of 200 N against the hammer. Boulder pulls Earth up with a force of 5000 N *Note that there are action/reaction pairs shown in the diagrams above. Earth pulls boulder down with a force of 5000 N

2 Critical Thinking Questions, part 1 1. Identify the missing action or reaction force for each of the situations shown below. (Draw a picture to help ) Action: Balloon pushes air down with a force of 2 N. Reaction: Action: Reaction: Exploding gunpowder pushes the cannonball to the right with a force of 3500 N. (c) Identify and describe the action and reaction forces that allow a car to travel down the road.

3 2. Neil Newton holds an apple weighing 1N at rest in the palm of her hand. (a) (b) (c) To say the weight of the apple is 1N is to say that a downward gravitational force of 1N is exerted on the apple by [Earth] [his hand]. Neil s hand supports the apple with normal force N, which acts in a direction opposite to W. We can say N [equals W] [has the same magnitude as W]. Since the apple is at rest, what is the net force? (d) Since N is equal and opposite to W, we [can] [cannot] say that N and W comprise an action-reaction pair. The reason is: (e) In accord with the rule stated earlier, if we say that the action is Earth pulling down on the apple, then the reaction is: (f) To repeat for emphasis, we see that N and W are equal and opposite to each other [and comprise an action-reaction pair] [but do not comprise an action-reaction pair]. Part 2: Action and Reaction and the influence of Inertia Mass plays a large role in the motion of objects that we see in the world around us. Recall that the more mass an object contains, the more inertia the object has. The greater the amount of inertia, the more resistant the object will be to changes in its motion. For example, when you jump into the air, the Earth pulls you towards the ground with a force equal to your weight (this is often referred to as the force of gravity, F g or W). What may not be obvious is that your body pulls the Earth upwards toward you with an equivalent force. Due to the enormous magnitude of the Earth s mass however, the upward movement of the Earth to our feet pales in comparison to your downward motion to meet the ground.

4 Critical Thinking Questions, part 2 1. Study the illustration below and answer the questions that follow: (a) How does the force exerted upon each cart compare when the compressed spring between them is released? (b) How does the resulting acceleration of each cart compare when the compressed spring is released?

5 Paradoxes of Newton's Third Law Suppose the horse in the picture below pulls the buggy to the left. (a) In the space below, draw a free-body diagram showing only those forces that act on the buggy. Don t forget to label the forces. (b) Identify the reaction forces to each of the action forces you labeled in (a). (c) In the space below, draw a free-body diagram showing only those forces that act on the horse. Don t forget to label the forces.

6 (d) Notice that when the horse pulls forward on the car, the car pulls back on the horse with an equal but opposite force. How, then, is the horse ever able to pull the car forward? In the drawing below, a tiny bug hits the windshield of a car traveling at 60 km/h. a. Which object experiences the greater force? Justify your answer. b. With Newton's Second Law of Motion in mind, which object experiences a greater acceleration? Justify your answer. c. Why is the damage to the bug so much greater than the damage to the car? Conclusions Write a in clear, complete sentences that addresses the following questions: How do the magnitude and direction of action-reaction pairs compare? Do action-reaction pairs only act upon one object? How will the resulting motion differ between action-reaction forces acting between objects of very different mass?