1. Lesson 3: Gravitational Potential Energy
Unit 6: Gravitation

2. Gravitational Potential Energy
The equation PE=mgh only works for h<<RE!
At higher heights, the gravitational force decreases as an object gets higher, and so does the potential energy.
5 min

3. Example 1
Find the escape velocity from planet Earth. (Ignore air resistance.)
If escaped, at infinity, potential energy is 0 J!
Start with the law of energy conservation: E∞ = Esurface. Then sub in PE and KE eq’ns.
Answer: m/s
10 min

4. Example 2
Find the maximum height reached by a cube of trash projected from the surface of Earth with an initial velocity of 10.0 km/s.
Start with the law of energy conservation...
Answer: 2.5x107 m
10 min

5. Brain Break!

6. Example 3 (Try it!)
Find a) the escape velocity from the Moon, and b) the velocity required to shoot a projectile up from the Moon to a height of 5000 km.
Hints: a) the escape velocity is that which is needed to obtain 0 J of energy at infinity,
b) remember to add RM to the height!
Answers: a) 2400 m/s b) 2000 m/s
20 min

7. Homework
Gravitation Problem Set #9-11
10 min