Work and Power Notes

Work and Power

Fall 2023

Energy

Recap

Two types of energy:

• Potential Energy (PE)

• Kinetic Energy (KE)

Potential

Energy

Remember that potential
energy is stored energy in
order to do something

1.

Elastic

2.

Chemical

3.

Gravitational

Elastic
Energy

Energy stored in elastic
materials as the result of their
stretching or compressing

Chemical

Energy

This was seen in the bonds
between molecules.

Energy is released from bonds
during chemical reactions (i.e.
burning, consumption, etc.)

Gravitational

Energy

Potential energy something
has in relation to another
massive object due to
gravity

Remember, what goes up,
must come down!

Formula(s)

PEg = m*g*h
m = mass (kg OR N)
g = gravity (9.8 m/sec2)
h = height (m)

Gravitational

Energy

Practice

Problems

1. A baby carriage is sitting at the top

of a hill that is 21 m high. The
carriage with the baby weighs 12 N.
The carriage has ____________
energy. Calculate it.

1. A cinder block is sitting on a

platform 20 m high. It weighs 79
N. The block has
_____________ energy.
Calculate it.

Kinetic
Energy

Recall, this is energy in use or
motion

One example we saw of this
was in phase change (melting)

We can actually calculate
kinetic energy

Kinetic

Energy Practice

Problems

1.

You serve a volleyball with a mass
of 2.1 kg. The ball leaves your hand
with a speed of 30 m/s. The ball has
______________________
energy. Calculate it.

2.

What is the kinetic energy of a
3-kilogram ball that is rolling at 2
meters per second?

Law of

Conservation of

Energy

• Think about a weight bouncing on a

spring. It should return to the same
point each time, but it doesn’t.

• Think about a pendulum swinging.

It should swing to the same height
each time, but it doesn’t.

• Think about a roller coaster car.

When it starts off a hill, it should
return to the same height, but it
doesn’t.

WHY???

WORK, WORK, WORK,
WORK, WORK

Work
• It is the exchange of energy between objects

in the same direction of the force

• Amount of work depends on two factors

1.

Amount of force applied

Examples of forces include touch,
shoving, pushing, etc.

2.

Distance over which a force is applied

Calculated using the formula:

W = f * d

Disclaimer: No matter
how slow or fast you do
something, it is the
same amount of work!

Work Calculation

W = f * d

• W: work (measured in

joules (J))

• f : force (Newtons (N))
• d :distance over which

force is applied

Newtons

● 1 N = 1 kg*m/sec2
● If given pounds (lb),

multiply by 4.45 to get
Newtons

**1 N = 1 J ( joule)

Work

Calculation

1.

A 900N mountain climber scales
a 100m cliff. How much work is
done by the mountain climber?

2.

How much work does an elephant
do while moving a circus wagon 20
meters with a pulling force of
200N?

Power
This is the rate at which work
is done

P = w/t

w = work
t = seconds
P = power measured in watts
*1 watt = 1 J/sec
*1 hp = 746 watts

Horsepower = based
on the power an
average farm horse
could put out

Power

Practice

Problems

1) A horse performs 15,000 J of work pulling a

wagon for 20 seconds. What is the horse’s
power?

2) A box is pushed across the floor for a distance

of 5 meters with a force of 50 N in 5 seconds.

a) How much work is done?

b) What is the power?