Potential Energy & Energy Conservation

Work Done by Gravity 1 l Example 1: Drop ball Y i = h Y f = 0 mg S y x Y i = h Y f = 0 mg S y x 10

Work Done by Gravity 2 Example 2: Toss ball up Y i = h Y f = 0 mg S y x 13

Work Done by Gravity 3 Example 3: Slide block down incline h  mg S 16

Example: roller coaster 20m 15m 8m A C B D V O =0 Find the velocity of the coaster at points A,B,C,D

Example: roller coaster 20m 15m 8m A C B D V O =0 Find the velocity of the coaster at points A,B,C,D

Example: roller coaster 20m 15m 8m A C B D V O =0 Car stops in 10m, what is the average force of friction? 10m

Example: blocks and pulley 3kg 1kg 1.5m What is the velocity of the 3kg block right before is hits the ground?

Power (Rate of Work) P = W /  t  Units: Joules/Second = Watt l How much power does it take for a (70 kg) student to run up 5m high stairs in 7 seconds? 48

Summary  Conservative Forces »Work is independent of path »Define Potential Energy U n U gravity = m g y n U spring = ½ k x 2  Work – Energy Theorem 50