Potential Energy Notes

Chapter 5-2

Potential energy is stored energy--the energy is not being used now but could be used in the future--2 types:

gravitational potential energy (PEg)

elastic potential energy (PEe)

Gravitational potential energy

• the energy an object has based on position– depends on its mass, gravity, and its height above

a reference point– gravity is trying to pull the object back down to

the earth• Ex: apple on a tree, skier atop a hill, rock on cliff, etc.

Gravitational potential energy formula

Potential energy = mass x gravity x heightPEg = mgh (F x d where F = mg and d = h)

--units of Joules (kg x m/s2 x m)– Height is measured from a reference point• Usually the ground or the surface beneath the object• Can lead to a negative height– Ex. An object under water or below ground, any

object below the reference point

Sample problems

• How much potential energy does a 55 g apple have hanging 3.5 meters high in a tree?

• A 55 kg skateboarder is at the top of a ramp with a slope of 35 degrees. The ramp is 5.5 m long. How much potential energy does the person have?

Elastic Potential Energy

--the amount of energy in a stretched or compressed object--a stretched rubber band, a stretched spring, a compressed golf ball

http://www.youtube.com/watch?v=2Y57pw_iWlk

• the energy comes from the molecules wanting to return to their “normal” state– in stretching or compressing, the molecules get

pushed closer to or pulled further from their neighboring molecules.

– the energy comes from the attraction or repulsion of the molecules to return to their “relaxed” position

• amount of energy is based on the strength of the spring or stretched object and the distance it is stretched

• the spring constant (k) measures the strength of the spring or elastic object– the stronger the spring, the higher the k value– the farther the spring or elastic object is

stretched, the greater strain on the moleculesthe more potential energy

Elastic potential energy formula

• potential energy = ½ x spring constant (k) x distance from relaxed state2

PEe = ½ k x2

--the spring constant has units of (N/m) so the units become (N/m) x m2 which reduces to N x m or Joules (J)

Sample elastic potential problems

• A person jumping on a pogo stick compresses a spring with a spring constant of 9800 N/m a distance of 15 cm. How much energy is stored in the spring?

• A slingshot has a normal length of 26.0 cm. A person puts a marble in it and pulls it back to a length of 54.5 cm. The spring constant is 350 N/m. Find the energy of the rubber band.