Outline

1. Outline Newton’s Third Law • Newton’s Third Law Examples • Quiz

2. NIII definition Newton’s 3rd Law “If object A exerts a force on object B, then object B exerts a force equal in magnitude and opposite in direction on A” Action  Reaction

3. Two Rules for Newton’s 3rd • Every Action force has a Reaction Force. Period. Always. • Action and Reaction forces ALWAYS act on different objects.

4. Action-Reaction Fmy hand ON block Example : block Action Force Fblock ON my hand Reaction Force

5. Example: A ball and the earth: Fearth on ball Fball on earth

6. The Math FA on B = - FB on A B Fapp A

7. Newton’s Third Law QQ58: N3 misconception A precocious child is given a wagon for her birthday. She refuses to use it. She says, “Newton’s Third Law says that no matter how hard I pull, the wagon will exert an equal but opposite force on me. So I will never be able to get it to move forward!” Is she right? Does Newton’s third law work for wagons?

8. Moving Objects QQ58: N3 misconception How can we walk ? – static friction Physicist Push on the earth Earth pushes back

9. Moving Objects QQ58: N3 misconception If Newton’s 3rd law always holds, how can objects move ??? Why does a car move forward ? v Wheels push backward on earth Earth pushes forward on wheels

10. Forces vs Accelerations QQ58: N3 misconception Newton’s 3rd law states that the forces are equal. However:Since F=ma a=F/mTherefore, given same F but different masses, the accelerations are NOT equal.

11. Forces vs Accelerations QQ58: N3 misconception Given an earth/ball system, the magnitude of the forces are equal, so Fearth on ball = Fball on earth mbg=meaeSo, ae=(mb/me)g = 2x10-24 m/s2 The ball, of course, falls down with 9.8 m/s2

12. Do for next class: • Read: Sections 8.1, 8.2, 8.3 • Try problems: 8.1, 8.5, 8.6, 8.8