Work in / Work out

3.2.2 Work/Energy Principle

• potential energy: STORED ENERGY– can be in a FIELD or DEVICE

• gravitational PE is stored in gravitational fields

• elastic PE is stored in elastic objects (springs)

Definitions – Types of Energy

hmgPE

2

2

1kxPES

2.0 m

30 kg

• A 30 kilogram object is positioned 2.0 meters above the surface of the Earth. – What is the gravitational potential energy of

the object?

Example #1 – Gravitational PE

ΔPE = mgΔh

ΔPE = (30 kg)(9.81 m/s2)(2.0 m)

ΔPE = 588.6 J

• The length of a spring with a spring constant of 100 newtons per meter is changed by 4.0 centimeters. – What is the elastic potential energy stored in

the spring?

Example #2 – Elastic PE

PES = ½ kx2

PES = ½ (100 N/m)(0.04 m)2

PES = 0.08 J

10 cm 14 cm

• kinetic energy: ENERGY OF MOVING OBJECTS– depends on mass and velocity

Definitions – Types of Energy

2

2

1mvKE

• A 5.0 kilogram toy car is moving at 2.0 meters per second. – What is the kinetic energy of the toy car?

Example #3 – Kinetic Energy

KE = ½ mv2

KE = ½ (5.0 kg)(2.0 m/s)2

KE = 10 J

• internal energy: TEMPERATURE/HEAT

Definitions – Types of Energy

Q

When work is done ‘on a system’ there is a net gain of one or more types of energy by that system. Energy types can change within a

system.

Work/Energy Principle

WORKTEFdW

Gravitational PE

Elastic PE

Kinetic Energy

Internal Energy

if “h” increases

if “x” increases

if “v” increases

if temp. increases

Work/Energy Principle – Scenario #1

A car is moving along a flat surface with an increasing speed. What type(s) of energy

are changing?

PEg PES KE Q X X ↑ ?

A car is moving up an incline with constant speed. What type(s) of energy are

changing?

PEg PES KE Q ↑ X X ?

Work/Energy Principle – Scenario #2

A car is applying its brakes and slowing down as it moves down a slope. What

type(s) of energy are changing?

PEg PES KE Q ↓ X ↓ ↑

Work/Energy Principle – Scenario #3

A cart is moving on a flat surface toward a wall and is stopped by a spring on its front.

What type(s) of energy are changing?

PEg PES KE Q X ↑ ↓ ?

Work/Energy Principle – Scenario #4

A rocket flies upward with an increasing velocity [ignore air resistance]. What type(s)

of energy are changing?

PEg PES KE Q ↑ X ↑ X

Work/Energy Principle – Scenario #5

End of 3.2.2 - PRACTICE