Potential Energy Kinetic Energy Mechanical Energy

Potential Energy Stored energy of an object Examples Gravitational potential energy Elastic potential energy

Elastic Potential Energy the energy stored in elastic materials as the result of their stretching or compressing. Examples Rubber bands Bungee chords Trampolines Arrow drawn into a bow

Gravitational Potential Energy the stored energy of position possessed by an object. PE = mgh equal 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 #1 How 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 #2 A 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 = mgh 60 J = m (9.8 m/s2) (10 m) 0.61 kg = m

Kinetic Energy the 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 Energy KE = ½ mv2 The kinetic energy of an object is equal to the work done to set it in motion.

Problem #3 Missy 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 Energy Potential Energy of an object. Kinetic energy of an object. Sum of potential and kinetic energy of an object.

Law of Conservation of Energy Energy cannot be created nor destroyed. Energy can be transformed from one form to another. Total amount of energy never changes.

Problem #4 You do 75 J of work lifting an object 10 m from the ground. 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 #5 A forklift lifts a 20 kg crate 4 m from the ground. How much potential energy did the crate gain? PE = mgh = (20. kg)(9.8 m/s2)(4.0 m) = 780 J How much work was done lifting the crate at this height? W = PE = 780 J 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 #5 A 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 = v f. What is the mechanical energy of the crate? Mechanical energy is 780 J