EnergyPotential Energy

Kinetic EnergyMechanical Energy

Potential EnergyStored energy of an objectExamples

Gravitational potential energyElastic potential energy

Elastic Potential Energythe energy stored in elastic materials as the

result of their stretching or compressing.Examples

Rubber bandsBungee chordsTrampolinesArrow drawn into a bow

Gravitational Potential Energythe stored energy of position possessed by an

object.PE = mghequal to the work done in lifting an object to

a certain height (W = PE)If you do 50 J of work lifting a box to a shelf,

the box will have 50 J of gravitational potential energy.

Problem #1How much work is done lifting a 1 kg book

from the floor to a shelf that is 6 m high?W = Fd = (1.0 kg x 9.8 m/s2) (6.0 m) = 59 J

How much potential energy will this book have on top of the shelf?

PE = W = 59 J

Problem #2A vase is placed on a shelf that is 10 m high

giving the vase a PE equal to 60 J. If you move the vase to a lower shelf that is half as high as the first one, how with this change the PE of the vase? Can you determine the mass of the vase?The vase’s PE will be half its original value. PE = mgh60 J = m (9.8 m/s2) (10 m)0.61 kg = m

Kinetic Energythe energy of motion Examples

vibrational (the energy due to vibrational motion)

rotational (the energy due to rotational motion)translational (the energy due to motion from

one location to another).

Kinetic EnergyKE = ½ mv2

The kinetic energy of an object is equal to the work done to set it in motion.

Problem #3Missy Diwater, the former platform diver for

the Ringling Brother's Circus, had a kinetic energy of 12 000 J just prior to hitting the bucket of water. If Missy's mass is 40 kg, then what is her speed? Show your calculation.

KE = ½ mv2

12 000 J = ½ (40 kg) v2

24 m/s = v

Mechanical EnergyPotential Energy of an object.Kinetic energy of an object.Sum of potential and kinetic energy of an

object.

Law of Conservation of EnergyEnergy cannot be created nor destroyed.Energy can be transformed from one form to

another.Total amount of energy never changes.

Problem #4You do 75 J of work lifting an object 10 m

from the ground.a. How much potential energy would this

object have 10 m from the ground?W = PE = 75 J

b. If the object falls from this height, how much kinetic energy will it have as it hits the ground?

PE lost = KE gained KE = 75 J

Problem #5A forklift lifts a 20 kg crate 4 m from the

ground.a. How much potential energy did the crate

gain?PE = mgh = (20. kg)(9.8 m/s2)(4.0 m) = 780 J

b. How much work was done lifting the crate at this height? W = PE = 780 J

c. The forklift breaks causing the crate to fall. Halfway to the ground, what is the potential energy of the crate? What is its kinetic energy?PE = 390 J KE = 390 J

Problem #5A forklift lifts a 20 kg crate 4 m from the

ground.d. What is the potential and kinetic energy of

the crate as it hits the ground?PE = 0 J KE = 780 J

e. How fast was the crate moving as it hits the ground? KE = ½ mv2 780 J = ½ (20 kg) v2

8.8 m/s = vf. What is the mechanical energy of the crate?

Mechanical energy is 780 J