Potential and Kinetic Energy

• Energy: is the ability to do work

Kinetic Energy= the energy an object possesses

because of its motion

• The amount of Kinetic energy is dependent on the mass of the object in motion and it’s velocity.

VELOCITY

Velocity = distance ÷ time

Distance = metersTime = seconds

Calculating kinetic energy

If we know the mass of an object and its velocity we can determine the amount of kinetic energy possessed by using the following formula:

kinetic energy = 1/2 (mass of object)(velocity of object)2

or KE = 1/2 mv2

or KE = 0.5mv2

The SI unit for kinetic energy is the Joule (J).

A Joule is kg m2/s2

A bicycle with a mass of 14 kg traveling at a velocity of 3.0 m/s

east has how much kinetic energy?

KE = 0.5mv2 = 0.5(14 kg)(3.0 m/s)2

= 0.5(14 kg)(9.0 m2/s2)

= 63 kg x m2/s2 = 63 J

A 1200 kg automobile is traveling at a velocity of 100 m/s northwest.

What is the kinetic energy of the automobile?

KE = 0.5 mv2 = 0.5(1200kg)(100 m/s)2

= 0.5(1200 kg)(104 m2/s2)

= 6 x 106 kg m2/s2 = 6 x 106 J

Potential Energy

• The energy of position

• The amount of energy contained in an object at rest

Determining Potential Energy

• By its position and its weight (mass X gravity)

PE = (mass)(gravity)(height) = mgh

• where m is mass in kg

• g is the force of gravity = 9.8 m/s2

• h is the height

• The SI unit that represents potential energy is the Joule (J) (kg m2/s2).

Examine an example of potential energy

A flower pot with a mass of 15 kg is sitting on a window sill 15 meters above the ground. How

much potential energy does the flower pot contain?

• PE = (mass)(gravity)(height)• = (15 kg)(9.8 m/s2)(15 m)• = 2205 kg m2/s2

• = 2205 J• = 2.2 x 103J

Examine an example of potential energy

• If the flower pot is lowered to a window sill that is 10. m from the ground. Does this change the potential energy of the flower pot?

PE = (mass)(gravity)(height)= (15 kg)(9.8 m/s2)(10. m)= 1470 kg m2/= 1470 J= 1.5 x 103J

Potential or Kinetic Energy?

• Moving car

• Tree branch

• Bent car fender

• Balloon filled with air

• Balloon squirting around room

• Person inside a moving car

• What type of energy does the space shuttle have at lift off?

Conversion of Potential to Kinetic Energy

• In this picture both kinds of energy are evident. Can you point them out?

• The water at the top has potential energy

• When water falls to a lower level, the potential energy is converted to kinetic energy.

Example