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)