1. Gravitational Potential Energy
Stored energy of height
§ 6.3

2. Potential Energy The energy of relative position of two objects
gravity
springs
electric charges
chemical bonds

3. Potential Energy Energy is stored doing work against a potential
Potential energy increases when “the potential” does negative (< 0 ) work
lifting a weight
stretching a spring

4. Gravitational Potential Energy
the work to raise an object to a height
Ug = mgh

5. Gravity Doing Negative Work
Source: Young and Freedman, Figure 7.2b.

6. Work from Potential Energy
When a potential does > 0 work on a body:
The body’s potential energy decreases
The body’s kinetic energy increases

7. Poll Question When a cute furry animal moves upward in free-fall:
Its gravitational potential energy increases.
Its kinetic energy increases.
Both A and B.
Neither A nor B.

8. Gravity Doing Positive Work
Source: Young and Freedman, Figure 7.2a.

9. Poll Question When a cute furry animal moves downward in free-fall:
Its gravitational potential energy increases.
Its kinetic energy increases.
Both A and B.
Neither A nor B.

10. Board Work Problem
A 50-g egg released from rest from the roof of a 30-m tall building falls to the ground. Its fall is observed by a student on the roof of the building, who uses coordinates with origin at the roof, and by a student on the ground, who uses coordinates with origin at the ground. What values do the two students find for:
Initial gravitational potential energy Ug0?
Final gravitational potential energy Ugf?
Change in gravitational potential energy DUg?
Kinetic energy just before impact Kf?

11. Mechanical Energy The energy available to do work
Kinetic + potential = K + U

12. Conservation of Mechanical Energy
If the only force doing work is gravity, mechanical energy does not change.
E1 = E2
K1 + Ug1 = K2 + Ug2
1/2 mv12 + mgy1 = 1/2 mv22 + mgy2

13. Board Work Problem
A 50-g egg released from rest from the roof of a 30-m tall building falls to the ground. Its fall is observed by a student on the roof of the building, who uses coordinates with origin at the roof, and by a student on the ground, who uses coordinates with origin at the ground. What values do the two students find for:
Initial total energy E0?
Final total energy Ef?
Change in total energy DE?