1. Potential Energy and Conservation of Energy
Chapter 8
Potential Energy and Conservation of Energy
Dr. Jie Zou PHY 1151G Department of Physics

2. Dr. Jie Zou PHY 1151G Department of Physics
Outline
Gravitational Potential Energy
Conservation of Mechanical Energy
Examples
Dr. Jie Zou PHY 1151G Department of Physics

3. Gravitational Potential Energy
Gravitational potential energy U: The energy of a body due to elevated positions is called gravitational potential energy.
U = weight  height = mgh
The gravitational potential energy is relative to the reference level and depends only on mg and the height h.
Work done by a conservative force is equal to the negative of the change in potential energy.
Wc = - U = - (Uf – Ui) = Ui - Uf
Dr. Jie Zou PHY 1151G Department of Physics

4. Dr. Jie Zou PHY 1151G Department of Physics
Example 1
Find the gravitational potential energy of a 65-kg person on a 3.0-m-high diving board. Let U = 0 be at water level.
Dr. Jie Zou PHY 1151G Department of Physics

5. Dr. Jie Zou PHY 1151G Department of Physics
Example 2
An 82.0-kg mountain climber is in the final stage of the ascent of 4301-m-high Pikes Peak. What is the change in gravitational potential energy as the climber gains the last m of altitude? Let U = 0 be (a) at sea level and (b) at the top of the peak.
Dr. Jie Zou PHY 1151G Department of Physics

6. Conservation of Mechanical Energy
Mechanical energy E: The sum of the potential and kinetic energy of an object. E = U + K.
Conservation of mechanical energy: In systems with conservative forces (such as gravity) only, the mechanical energy E is conserved. Ui + Ki = Uf + Kf.
Assume there is no air resistance.
Dr. Jie Zou PHY 1151G Department of Physics

7. Conservation of Energy
Mechanical Ef = 90 J
Mechanical Ei = 100 J
When Friction exists
Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.
The universe, in short, has a certain amount of energy, and that energy simply ebbs and flows from one form to another, with the total amount remaining fixed.
Decrease in mechanical energy = Ei – Ef = 10 J = Thermal energy converted
Dr. Jie Zou PHY 1151G Department of Physics

8. Dr. Jie Zou PHY 1151G Department of Physics
Example 1
A player hits a 0.15-kg baseball over the outfield fence. The ball leaves the bat with a speed of 36 m/s, and a fan in the bleachers catches it 7.2 m above the point where it was hit. Assuming frictional forces can be ignored, find (a) the kinetic energy of the ball when it is caught and (b) its speed when caught.
Dr. Jie Zou PHY 1151G Department of Physics

9. Dr. Jie Zou PHY 1151G Department of Physics
Example 2
A 55-kg skateboarder enters a ramp moving horizontally with a speed of 6.5 m/s, and leaves the ramp moving vertically with a speed of 4.1 m/s. Find the height of the ramp, assuming no energy loss to frictional forces.
Dr. Jie Zou PHY 1151G Department of Physics

10. Dr. Jie Zou PHY 1151G Department of Physics
Example 3
If the height of the water slide is h=3.2 m, and the person’s initial speed at point A is 0.54 m/s, what is the horizontal distance between the base of the slide and the splashdown point of the person?
d=?
Dr. Jie Zou PHY 1151G Department of Physics

11. Dr. Jie Zou PHY 1151G Department of Physics
Example 4
Suppose the pendulum bob has a mass of 0.33 kg and is moving to the right at point B with a speed of 2.4 m/s. Air resistance is negligible.
(a) What is the change in the system’s gravitational potential energy when the bob reaches point A?
(b) What is the speed of the bob at point A?
Dr. Jie Zou PHY 1151G Department of Physics

12. Dr. Jie Zou PHY 1151G Department of Physics
Potential Energy of a Spring
U = (1/2)kx2
k = the force constant (N/m) of the spring
x = the displacement of the spring from its equilibrium position
Example: If k = 680 N/m and x = 2.25 cm, then U = (1/2)(680 N/m)( m)2 = J
Dr. Jie Zou PHY 1151G Department of Physics

13. Dr. Jie Zou PHY 1151G Department of Physics
Example
Experiments performed on the wing of a hawkmoth show that it deflects by a distance of x = 4.8 mm when a force of magnitude F = 3.0 mN is applied at the tip. Treating the wing as an ideal spring, find
(a) the force constant of the wing
(b) the energy stored in the wing when it is deflected
(c) what force must be applied to the tip of the wing to store twice the energy found in part (b)?
Dr. Jie Zou PHY 1151G Department of Physics

14. Dr. Jie Zou PHY 1151G Department of Physics
Homework
See online homework assignment at
Hand-written homework assignment:
Chapter 8, Page , Problems: #34, 86.
Dr. Jie Zou PHY 1151G Department of Physics