Potential Energy

1
Potential Energy Energy Conservation
2
Work Done by Gravity 1

• Example 1 Drop ball

S
S
mg
mg
y
y
x
x
10
3
Work Done by Gravity 2

• Example 2 Toss ball up

S
mg
y
x
13
4
Work Done by Gravity 3

• Example 3 Slide block down incline

?
h
S
mg
16
5
Example roller coaster
VO0
C
B
20m
15m
8m
D
A
Find the velocity of the coaster at points A,B,C,D
6
Example roller coaster
VO0
C
B
20m
15m
8m
D
A
Find the velocity of the coaster at points A,B,C,D
7
Example roller coaster
VO0
C
B
20m
15m
8m
D
A
10m
Car stops in 10m, what is the average force of
friction?
8
Example blocks and pulley
What is the velocity of the 3kg block right
before is hits the ground?
1.5m
9
Power (Rate of Work)

• P W / Dt
• 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
10
Summary

• Conservative Forces
• Work is independent of path
• Define Potential Energy U
• Ugravity m g y
• Uspring ½ k x2
• Work Energy Theorem

50