1. Physics 101: Lecture 10 Potential Energy & Energy Conservation

2. Review Preview Work: Transfer of Energy by Force WF = |F| |S| cos
Kinetic Energy (Energy of Motion)
K = 1/2 mv2
Work-Kinetic Energy Theorem:
SW = DK 
Preview
Potential (Stored) Energy U 07

3. Work Done by Gravity 1 Example 1: Drop ball Yi = h Yi = h S S mg mg y
Yf = 0
Yf = 0 x x 10

4. Work Done by Gravity 2 Example 2: Toss ball up Yi = h S mg y Yf = 0 x13

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

6. Example: Pendulum
As the pendulum falls, the work done by the string is
1) Positive 2) Zero 3) Negative
How fast is the ball moving at the bottom of the path?
W = F d cos q. But q = 90 degrees so Work is zero.
Conservation of Energy (Wnc=0) h 30

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

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

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

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

11. 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 the stairs in 141 (5 meters) in 7 seconds? 48

12. Summary Conservative Forces Work is independent of path
Define Potential Energy U
Ugravity = m g y
Uspring = ½ k x2
Work – Energy Theorem
Chapter 6, problems 27, 31, 35, 37
Good Luck on EXAM! 50