Newton’s Third Law

1. Newton’s Third Law Action/Reaction Force Pairs

2. Review of Newton’s Laws • Newton’s First Law - tells you what happens to an object if the net force on it is zero. • Newton’s Second Law - tells you what happens to an object if the net force on it is not zero. • What could Newton's Third Law possibly tell you?

3. Newton’s Third Law • In a sense, Newton's First and Second Laws tell you what forces do. Newton's Third Law tells you what forces are. Newton's Third Law is often stated: For every action there is an equal and opposite reaction.

4. "action...reaction" means that forces always occur in pairs. • Forces are interactions between objects. • Single, isolated forces never happen. • The two forces involved are called the "action force" and the "reaction force.“ • Either force in an interaction can be the "action" force or the "reaction" force. See Identifying Action and Reaction Forces.

5. Difficulties with Newton's Third Law: • "If A pushes B, then B pushes A with an equal and opposite force. If these forces are equal and opposite, they cancel, producing a net force of zero. • This means that neither object can accelerate, which means that Newton's Laws predict that nothing can ever move."

6. Only the forces that act on the same object can cancel! • Forces that act on different objects don't cancel. • The Force on the wall is equal and opposite to the force on the ball, BUT:

7. The Solution: • These forces DO NOT CANCEL because they influence the motion of different objects. • The force that the wall exerts on the ball influences the ball’s motion, and the force that the ball exerts on the wall influences the wall’s motion.

8. Why does the skater move?

9. What are the action/reaction forces? F wall and F skater • What objects are each of the forces acting on? F wall – on the skater F skater – on the wall • Are the force pairs equal and opposite? Yes, but they are acting on different objects so they do not cancel. • Why does the skater move but the wall does not? the wall is too massive to move with the force exerted by the skater, but the wall exerts a force large enough to move the skater.

10. Ropes and Tension • • Tension is always a pulling force. This means it is directed away from an object. • Fnet = FT - Fg

11. Practice • You are helping to repair a roof by loading equipment in a bucket that workers hoist to the rooftop. If the rope is guaranteed not to break as long as the tension does not exceed 450 N and you fill the bucket until it has a mass of 42 Kg. What is the greatest acceleration that the workers can give the bucket as they pull it to the roof?

12. Solution • Fnet = FT –Fg • ma = FT – mg • (42 kg)a = FT – (42 kg)(9.8 m/s2) • (42 kg) a = 450 N - 412 N • a = 450 N – 412 N = 0.91 m/s2 42 kg