Transcript Chapter 4.2

Chapter 4.2
How Machines Do Work?
- How Machines Do Work
Input and Output Work

The amount of
input work done by
the gardener
equals the amount
of output work
done by the
shovel.
Input & Output work

Input force = the FORCE you exert on the
machine.

Output force = the FORCE the machine
exerts ON THE OBJECT
Calculating Input & Output work

Input work
 Input

force x Input distance
Output work
 Output

force x Output distance
When you use a machine, the amount of
output work can never be greater than the
amount of input work.
- How Machines Do Work
What Is a Machine?

A machine makes
work easier by
changing at least one
of three factors. A
machine may change
the amount of force
you exert, the distance
over which you exert
your force, or the
direction in which you
exert your force.
Mechanical Advantage

If you compare the input force to the
output force, you can find the advantage of
using a machine.

A machine’s mechanical advantage is the
number of times a machine increases a
force exerted on it.
Calculating Mechanical advantage

Mechanical advantage =
Output force
input force
Examples:
A machine that increases force: output force is
greater than input force.
ex. a can opener

For a machine that increases distance the
output force is less than the input force.
 Ex.

A wooden spoon vs. a regular spoon.
For a machine that changes direction the
input force will equal the output force
 Ex.
A weight pulley machine
 The mechanical advantage of these types of
machines will always be 1.
- How Machines Do Work
Mechanical Advantage

Interpreting Data:
 If
an 80-N input force
is exerted on Ramp 2,
what is the output
force?
400
N
Efficiency of Machines

So far you learned that input work is equal to
output work. This does not happen is real life.
 WHY???
FRICTION
In every machine, some work is wasted
overcoming friction. Force must be applied to
overcome the friction of the machine.
Efficiency of machines

The efficiency of a machine compares the
output work to the input work.
Efficiency is calculated as a percentage (%)
 Efficiency =
output work
X 100
input work

- How Machines Do Work
Calculating Efficiency

You do 250,000 J of work to cut a lawn with a hand
mower. If the work done by the mower is 200,000 J,
what is the efficiency of the lawn mower?

Read and Understand

What information have you been given?
Input Work (Winput) = 250,000 J
Output Work (Woutput) = 200,000 J


Calculating Efficiency
You
do 250,000 J of work to cut a lawn with a
hand mower. If the work done by the mower is
200,000 J, what is the efficiency of the lawn
mower?
Plan and Solve
What
quantity are you trying to calculate?
 The efficiency of the lawn mower = __
What formula contains the given quantities and the
unknown quantity?
 Efficiency = Output work/Input work X 100%
Perform the calculation.
 Efficiency = 200,000 J/250,000 J X 100%
 Efficiency = 0.8 X 100% = 80%
The efficiency of the lawn mower is 80 percent.
Real & Ideal Machines

Real machines
 Efficiency

is always less than 100%
Ideal machines
 Efficiency
is always 100%
( these machines do not exist)