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.