1. Science Starter! A 1200 kg car is traveling along an icy road. Suddenly, the brakes fail! Because of friction, the car slows down at a rate of 0.98 m/s 2. (a) Draw a free-body diagram of the car. (b)Write an “F Net Equation” for the vertical and horizontal directions. (c)Determine the force of friction between the car and the road. (d) If the car was originally traveling at 30 m/s, how far will the car travel before coming to a stop?

2. Friction – Section 5.2

3. Frictional Force Static Friction: friction that exists between two surfaces that are touching but NOT MOVING. (applied force > static friction to cause motion) Kinetic Friction: friction that exists between two surfaces that are moving against each other

4. Equation F f = μF N μ = coefficient of friction (depends on the material and has no units) F N = Normal Force Two different situations (still or moving?) μ s = coefficient of static friction μ k = coefficient of kinetic friction

5. Example 1 A 20 kg cart is pushed with 25 N of force along a rough surface where μ = 0.10. (a)Draw a free-body diagram of the cart. (b)Write a “F NET Equation” in the horizontal and vertical directions. (c)Determine the acceleration of the cart.

6. b) F NET y = F N – F g F NET x = F – F f c) ma y = F N – F g ma x = F – μF N (20)(0) = F N – (20)(9.8) (20)(a) = 25 – (0.1)(196) 0 = F N – 196 (20)(a) = 25 – 19.6 F N = 196 N a = 0.27 m/s 2

7. Self - Quiz You help your mom move a 41 kg bookcase to a different place in the living room. You push with a force of 65 N, and the coefficient of friction between the carpet and the bookcase is 0.15. (a)Draw a free-body diagram of the bookcase. (b)Write a “F NET Equation” in the horizontal and vertical directions. (c)Determine the acceleration of the bookcase.