Chapter 4: Work and Energy
This chapter will have a partner project, a vocabulary quiz
and a chapter test.
Vocabulary:
Work Kinetic Energy
Machine Potential Energy
Simple Machine Elastic Potential Energy
Compound Machine Chemical Potential Energy
Efficiency Gravitational Potential Energy
Mechanical Advantage Law of Conservation of Energy
Energy Mechanical Energy
System Power
Section 1: Work and Machines
Define work:
Work- the force applied through a distance.
Push against the desk. Did it move? If it does not move
you have not done any work on the desk.
When force is parallel to motion the equation for work is:
Work (joules) = applied force (newtons) x Distance
(meters)
W = Fd
Lets Practice!
1. A couch is pushed with a horizontal force of 80 N and
moves a distance of 5 m across the floor. How much work
is done?
2. How much work is done to lift a 100 N child 0.5 m?
3. The brakes on a car do 240,000 Joules of work in
stopping a car. If the car travels 40 m while the breaks
are being applied how large is the applied force that the
brakes exert on the car?
When a force is perpendicular to motion, the work from
that force is zero.
When you carry books while walking at a constant
velocity, how much work are you doing?
Why?
Machines are devices that change the force or increase
the motion from work.
How could you move a piano?
Machines can increase force, but can also increase speed
as well.
You go much faster on a bicycle than on foot. A bicycle is
a machine!
There are 6 Types of Machines
you will use one or all of these in your projects!
Simple Machines:
A Simple Machine is a machine that does work with only
one movement of the machine.
1.
2.
3.
4.
5.
6.
Lever
Wheel and Axle
Pulley
Inclined Plane
Wedge
Screw
Modified Lever
Modified Lever
Modified Inclined Plane
Modified Inclined Plane
A Compound Machine is a combination of two or more
simple machines:
A Pair of Scissors--> 2 wedges and 2 levers
Machines increase force or speed, but you cannot increase
both at the same time, so you are not increasing the
amount of work being done.
In Fact! You always put more work into a machine than
you get out of it.
It does not matter how good your machine is, this law
always holds true.
The efficiency is the ratio of output work to input work. It
is measured in percent using this equation:
Output Work (joules)
Efficiency (%) =
x 100
Input Work (joules)
WOUT
e=
x 100
WIN
What is the efficiency of a machine that does 800 J of
work if the input work is 2,000 J?
The input work on a pulley system is 75 J. if the pulley
system is 84% efficient, then what is the output work
from the pulley system?
Machines are useful in 3 major ways1-Increase speed
a person on a bicycle travels more quickly than a person
on foot, At the same time the force decreases. If you were
to ride up a hill however, you force would increase and
your time decrease.
2- Change the Direction of Force
an ax takes an applied downward force and turns it into a
force splitting a piece of wood apart-an outward force.
3- Increase Force
if you use a machine to increase force you will decrease
the speed. a car jack will help you exert a force to lift a
car, however, you move the jack handle faster than the
car lifts.
We can describe the effectiveness of the machine at
increasing force by its mechanical advantage.
Mechanical Advantage is the ratio of the output force to
the input force:
Output Force (Newtons)
Mechanical Advantage =
Input Force (Newtons)
FOUT
MA =
FIN
What is the Mechanical advantage of a hammer if the
input force is 125 N and the output force is 2,000N?