1. Newton’s Third Law Action and Reaction

2. A different way to look at force... u A force is really an interaction between two things--mutually u Forces always occur in pairs u Action Force u Reaction Force

3. For example... u You push on the wall & … u A hammer pushes on a nail &… u While walking, your foot pushes against the road &...

4. So you see... u It doesn’t matter which is the action force and which is the reaction force, they are partners in a single interaction...

5. So... u Action: object A exerts a force on object B u Reaction: object B exerts a force on object A

6. Newton’s 3 rd Law u For every action there is an equal but opposite reaction. u ie, same amount of force, but in the opposite direction!

7. If the forces are equal and opposite... u Let’s look at the interaction between the rifle and a bullet. u Action: rifle exerts a force on the bullet u Reaction: bullet exerts a force on the rifle.

8. Rifle F m a Bullet F m a It has to do with the 2 nd Law...

9. If the force is 20 N u And the rifle has a mass of 10 kg while the bullet has a mass of.02 kg… u Can we compare the acceleration of the bullet and the acceleration of the rifle?

10. Sure u 20 N = 10 kg ( a r ) u 20 N =.02 kg ( a b ) u a r = 2 m/s 2 u a b = 1000 m/s 2

11. u IT ALL BALANCES OUT!!!

12. Using Physics to Win Bets u An interesting implication of Newton’s third law u If all action-reaction forces must be equal and opposite, you can’t push any harder on an object than it can push back on you

13. Example: Boxing Champ Challenge u Hey Rocky…I’ll give you a million dollars if you can punch this paper with 45 pounds (200 N) of force! u Your money is safe because… u The paper can only exert a small force, that means Rocky exerts a small force too!