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 (PE g ) elastic potential energy (PE e )

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 height PEg = mgh (F x d where F = mg and d = h) --units of Joules (kg x m/s 2 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 k

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 molecules  the more potential energy

Elastic potential energy formula potential energy = ½ x spring constant (k) x distance from relaxed state 2 PEe = ½ k x 2 --the spring constant has units of (N/m) so the units become (N/m) x m 2 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.