1. Writing Prompt: 10/25/2006 How much work is done to lift a 40 N weight 2 meters in the air? How much potential energy does it have after the lift?

2. Force and Friction What’s under your shoe?

3. Review of Newton’s 3 rd Law A 30 N book rests on a table, how much force does the table push on the book? 30 N UP!!! Weight = 30 N Force Up = 30 N

4. Review of Newton’s 3 rd Law What is the name of this force from the table? We call this the “Normal Force” Weight = 30 N Force Up = 30 N

5. Normal Force This NORMAL does NOT mean “usual” or “common” Here, NORMAL means Perpendicular to The Normal force is ALWAYS 90° to the surface

6. Normal Force So what direction would the normal force from this surface be? A)B)C) ANSWER: C)

7. Normal Force So on a flat, horizontal table the normal force on an object is ALWAYS UP!!!

8. Friction Is friction a force? … well…. Does it –Affect motion? –Can it slow you down (decelerate you)? Then YES, friction is a force! Then YES, friction is a force!

9. Friction Question: If you pull on an object with 5 N of force and it doesn’t move because of friction, how much force is friction pulling back with? 5 N, No acceleration so the Net force is 0! PULL = 5 N Friction = 5 N

10. Friction To get the box to move you have to pull harder than the maximum friction! PULL > Friction Friction = 5 N

11. Friction What causes friction?

12. Friction So even smooth surfaces have got ridges and imperfections if you zoom in close enough.

13. Friction One final note… The COEFFICIENT OF FRICTION is something that tells us how much friction there is. We’ll see during the lab how to calculate this number. The symbol is this: μ (Greek letter “mu”)

14. Friction What do you think affects how much friction there is? Let’s find out!