Potential Potential Energy & Energy Energy & Energy Conservation Conservation
Work Done by Gravity 1Work Done by Gravity 1
Example 1: Drop ball
Yi = h
Yf = 0
mgS
y
x
Yi = h
Yf = 0
mgS
y
x
10
Work Done by Gravity 2Work Done by Gravity 2
Example 2: Toss ball up
Yi = h
Yf = 0
mgS
y
x
13
Work Done by Gravity 3Work Done by Gravity 3
Example 3: Slide block down incline
h
mgS
16
Example: roller coasterExample: roller coaster
20m 15m
8mA
C
B
D
VO=0
Find the velocity of the coaster at points A,B,C,D
Example: roller coasterExample: roller coaster
20m 15m
8mA
C
B
D
VO=0
Find the velocity of the coaster at points A,B,C,D
Example: roller coasterExample: roller coaster
20m 15m
8mA
C
B
D
VO=0
Car stops in 10m, what is the average force of friction?10m
Example: blocks and pulleyExample: blocks and pulley
3kg
1kg1.5m
What is the velocity of the 3kg block right before is hits the ground?
Power (Rate of Work)Power (Rate of Work)
P = W / t Units: Joules/Second = Watt
How much power does it take for a (70 kg) student to run up 5m high stairs in 7 seconds?
48
SummarySummaryConservative Forces
» Work is independent of path
» Define Potential Energy UUgravity = m g yUspring = ½ k x2
Work – Energy TheoremUKWnc
50