Newton’s Third Law of Newton’s Third Law of MotionMotion

Please read pages 64-69 for the Please read pages 64-69 for the first 10 min of class.first 10 min of class.

Newton’s Third LawNewton’s Third Law

• Newton’s Third Law of Motion states: for every action, there is an equal but opposite reaction.

• This means that when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted.

Action - ReactionAction - Reaction

• 1st & 2nd laws explain how the motion of a single object changes.

• 3rd law describes something else that happens when one object exerts a force on another object.

• According to Newton’s third law of motion, forces always act in equal but opposite pairs.

Action - ReactionAction - Reaction

• The forces exerted by two objects on each other are often called an action-reaction force pair.

• Either force can be considered the action force or the reaction force.

Action - ReactionAction - Reaction

• You constantly use action-reaction force pairs as you move about.

• When you jump, you push down on the ground.

• The ground then pushes up on you. It is this upward force that pushes you into the air.

Action: tire pushes on roadReaction: road pushes on tire

Action - ReactionAction - Reaction

• When the rocket fuel is ignited, a hot gas is produced.

• As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.

Action: rocket pushes on gases

Reaction: gases push on rocket

Consider hitting a baseball with a bat. If Consider hitting a baseball with a bat. If we call the force applied to the ball by the we call the force applied to the ball by the bat the bat the action forceaction force, identify the , identify the reaction reaction

forceforce..

(a) the force applied to the bat by the hands(a) the force applied to the bat by the hands

(b) the force applied to the bat by the ball(b) the force applied to the bat by the ball

(c) the force the ball carries with it in flight(c) the force the ball carries with it in flight

(d) the centrifugal force in the swing(d) the centrifugal force in the swing

(b) the force applied to the bat by the ball

Newton’s 3Newton’s 3rdrd Law Law

Question:Question:Suppose you are taking a space Suppose you are taking a space

walk near the space shuttle, and walk near the space shuttle, and your safety line breaks. How would your safety line breaks. How would you get back to the shuttle?you get back to the shuttle?

Newton’s 3Newton’s 3rdrd Law Law

The thing to do would be to take one of the tools The thing to do would be to take one of the tools from your tool belt and throw it is hard as you from your tool belt and throw it is hard as you can directly away from the shuttle. Then, with can directly away from the shuttle. Then, with the help of Newton's second and third laws, you the help of Newton's second and third laws, you will accelerate back towards the shuttle. As you will accelerate back towards the shuttle. As you throw the tool, you push against it, causing it to throw the tool, you push against it, causing it to accelerate. At the same time, by Newton's third accelerate. At the same time, by Newton's third law, the tool is pushing back against you in the law, the tool is pushing back against you in the opposite direction, which causes you to opposite direction, which causes you to accelerate back towards the shuttle, as desired. accelerate back towards the shuttle, as desired.

Section ReviewSection ReviewQuestion

If they are “equal but opposite,” why don’t action-reaction pairs cancel?

AnswerAnswer

Action-reaction pairs don’t cancel because Action-reaction pairs don’t cancel because they act on different objects, not on the same they act on different objects, not on the same object. Equal and opposite forces acting on the object. Equal and opposite forces acting on the same object would cancel.same object would cancel.

Conservation of MomentumConservation of Momentum

Law of conservation of momentum:Law of conservation of momentum:The total momentum of the objects that The total momentum of the objects that

interact does not change.interact does not change.

Pg. 68, figure 19Pg. 68, figure 19