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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 mv2
KE = kinetic energy in joules (J)
m = mass in kg
v = velocity in m/s
KINETIC ENERGY
KE = ½ mv2
m = 2(KE/v2)
v = √2(KE/m)
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
g = 9.8 m/s2
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.