CHAPTER 4: ENERGY
SECTION 1: The Nature of Energy
WARM-UP:
 What is energy?
 List some different types of energy.
LEARNING GOALS
 Distinguish between kinetic and potential
energy.
 Calculate kinetic and gravitational
potential energy.
 Describe different forms of potential
energy.
WHAT IS ENERGY?
 Changes are constantly occurring in the
world.
 Examples: light bulbs heating the air,
sunlight on window, you breathing
WHAT IS ENERGY?
 Every change requires
energy.
 Energy: ability to cause
change
FORMS OF ENERGY
 Electromagnetic energy: energy that
travels in waves
 Example: UV, Infrared, Radio
 Mechanical energy: energy in physical
movement or position
 Example: moving car, flying baseball
FORMS OF ENERGY
 Electrical energy: energy carried by
moving electrons
 Example: any kind of electricity
 Thermal energy: energy related to
temperature
 Example: cup of hot tea
UNITS OF ENERGY
 Joule (J) – SI unit of energy
 Other units:
 calorie (cal)
 Calorie (Cal)
 Kilowatt Hour (kWh)
KINETIC ENERGY
 Kinetic Energy: the energy that a
moving object has because of its
motion.
 Depends on the mass and speed of the
object.
KINETIC ENERGY
KE = ½
2
mv
 KE = kinetic energy in joules (J)
 m = mass in kg
 v = velocity in m/s
KINETIC ENERGY
KE = ½
2
m = 2(KE/v )
v = √2(KE/m)
2
mv
EXAMPLE
 A jogger with the mass of 60 kg is moving at a speed
of 3 m/s. What is the jogger’s kinetic energy?
EXAMPLE
 A sprinter has a mass of 80.0 kg and a kinetic energy
of 4,000 J. What is the sprinter’s speed?
EXAMPLE
 A baseball is moving at a speed of 40 m/s and has 120
J of kinetic energy. What is the mass of the baseball?
POTENTIAL ENERGY
 Potential Energy: stored energy due to an
object’s position.
 Three types:
 elastic
 chemical
 gravitational
POTENTIAL ENERGY
 Elastic Potential Energy: energy stored
by something that can stretch or
compress
 Ex: rubber bands, springs
POTENTIAL ENERGY
 Chemical Potential Energy: energy
stored in chemical bonds
 Ex: gasoline, food, batteries
POTENTIAL ENERGY
 Gravitational Potential
Energy: energy stored in
objects due to their position
above the Earth’s surface
 Ex: a boulder on a cliff
POTENTIAL ENERGY
GPE = mgh
 GPE = gravitational potential energy (J)
 m = mass in kg
2
 g = 9.8 m/s
 h = height in m
POTENTIAL ENERGY
 GPE = mgh
 m = GPE/(gh)
 h = GPE/(mg)
 g = GPE/(mh)
EXAMPLE
 What is the gravitational potential energy of a ceiling
fan that has a mass of 7 kg and is 4 m above the
ground?
EXAMPLE
 How high above the ground is a baseball with a mass
of 0.15 kg and a GPE of 73.5 J?
EXAMPLE
 A rock climber is 200 m above the ground and has a
GPE of 117,600 J. What is the rock climber’s mass?
CHANGING ENERGY
 If all of the object on the shelves
have the same mass:
 Which has the most potential
energy?
 Which will be moving faster if
they were to all fall?
CHANGING ENERGY
 As an object begins to fall, it has both
GPE and KE.
 As the object gets closer to the ground,
it has less GPE and more KE.
EXAMPLE
 An 80 kg diver jumps from a 10 m platform.
 What is the GPE of the diver at the top of the
platform?
 What is the GPE of the diver after falling 5 m?
CHECK-IN:
 Explain how the kinetic energy of a
truck could be increased without
increasing the truck’s speed.