Potential Energy
Gravitational Potential Energy — stored energy due to position: PE = mgh
What is happening to the kinetic energy of the disk as time passes?
II. Energy
(Joules) — 2 kinds
Potential & Kinetic Energy
A) Potential Energy - stored energy that an object has due to its position or condition.
“At what position(s) or condition(s) do these objects have the most PE?”
1. Gravitational P.E.g
(PE = Potential Energy)
Two factors determine PE
Height, weight
| P.E.g = | (mg)h = | (weight)h |
|---|---|---|
| Joules | Kg (m/s2)m | N m |
| (Not in reference) |
g on earth = 9.8 m/s2
mg = weight
(force it could exert)
h = height
(distance force could be exerted)
| ΔP.E. | = mgΔh | = wΔh |
|---|---|---|
| (Not in reference) |
Ex 1
What P.E. is gained when a 100. kg object is raised 4.00 m straight up?
m = 100. kg
h = 4.00 m
Ex 2
What PE would be gained if an object were moved 4 m to the right?
Ex 3
An object weighing 15. N is lifted from the ground to a height of .22 m.
Find object's change in PE.
w = 15. N
h = .22 m
| ΔP.E. | = mgΔh | = wΔh |
|---|---|---|
| (Not in reference) |
“Crow bars reduce the force needed to move a body”
“...remember what you learned on a seesaw?”
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. Gravitational potential energy is best described as
2. How much potential energy is gained when a 2.0 kg object is raised 5.0 m straight up? (g = 9.8 m/s²)
3. An object weighing 15 N is lifted from the ground to a height of 0.22 m. What is its change in PE?
4. A crate is slid 4 m to the right across a level floor. What is its change in gravitational potential energy?
5. Which two factors determine an object's gravitational potential energy near Earth?
Pumped-Storage Hydropower
Power companies store extra energy by pumping water uphill into a reservoir when demand is low. When demand is high, the water flows back down through turbines to generate electricity. In one cycle, 1.0 × 106 kg of water is raised 50. m.
How much gravitational potential energy is stored in the Earth–water system?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. Which sequence of energy transformations occurs as the stored water generates electricity?
C2. The system returns 80% of the stored gravitational potential energy as electrical energy. Substitute numerical values into a mathematical representation that can be used to determine the electrical energy produced in one cycle. Then calculate the electrical energy.
C3. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
C4. An engineer measures heights from the lower reservoir’s surface, and another measures from sea level. How do their calculations compare?
C5. In a real pipe, 25% of the system’s energy is transformed into thermal energy by friction as the water flows down. Construct an energy bar graph for the Earth–water system at the instant the water reaches the turbine inlet, just before entering the turbine. The zero level for gravitational potential energy is the turbine inlet, and the water started from rest in the upper reservoir.
Click a box to set each bar’s height; click it again to clear the bar to 0. The ET bar is drawn for you.
C6. The power company is choosing between two sites: Site A raises the water 50. m, and Site B raises the same mass of water 100. m but is in steep mountains far from the city. Identify one benefit and one drawback of choosing Site B.