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Chapter 12
Work, Energy and Power
Energy
• Is the ability to do work
• Expressed in Joules (dzuls)
• Forms of energy: Mechanical,
Heat, light, and Chemical
Energy
ENERGY
POTENTIAL
ENERGY
Gravitational
potential
energy
Elastic
potential
energy
KINETIC
ENERGY
POTENTIAL ENERGY
• Gravitational Energy
• Elastic Potential Energy
Gravitational Potential Energy
• Is the energy
possessed by a
body by virtue of
its position
PE = mgh
Sample Problem 1
• A 75.0kg child climbs a 6.00m long slide.
What is the potential energy of the child?
Given: m = 75.0kg, h = 6.00m, g = 9.8m/s2
Required: PE = ?
Sol’n: PE = mgh
= (75kg) (6m) (9.8m/s2)
= 4 410 kg m2/s2
= 4 410 J
Sample Problem 2
The potential energy of an apple is 6.00 joules.
The apple is 3.00-meters high. What is the
mass of the apple?
Given: h = 3.00m, PE =6.00 J, g = 9.8m/s2
Required: m = ?
Elastic Potential Energy
• Is the energy
possessed by a
spring or rubber
band when it is
stretched or
compressed
Kinetic Energy
• Is the energy
possessed by a
body because of
its motion
KE = ½ mv2
Sample Problem 3
What is the kinetic energy of a 2,000
kilogram boat moving at 5 m/s?
Given: m = 2,000kg, v = 5m/s
Required: KE = ?
Sol’n:
KE = ½ mv2
= ½ (2000kg) (5m/s)2
= 1000kg (25m2/s2)
= 25,000 J
Sample Problem 4
What is the velocity of an 500-kilogram
elevator that has 4,000 joules of energy?
Given: m = 5,000kg, KE = 4,000 J
Required: v = ?
Sample Problem 5
• What is the height of a raised object where
its mass is 25kg and the potential energy
is 34 J?
Sample Problem 6
What is the mass of an object where its
velocity is 25m/s and its kinetic energy is
38.03 J?