Work and Machines

1. 14.2 Work and Machines
All cars come
equipped with
simple machines
designed to make
changing a tire a
fairly easy task.

2. 14.2 Work and Machines
Machines Do Work
How do machines make work easier?
A machine is a device that changes a force.
Machines make work easier to do. They
change the size of a force needed, the
direction of a force, or the distance over
which a force acts.

3. 14.2 Work and Machines
Increasing Force
Turning the jack handle allows the man to
raise the car.
Machines Do Work

4. 14.2 Work and Machines
Each complete rotation of a jack handle
applies a small force over a large distance.
• A small force exerted over a large distance
becomes a large force exerted over a short
distance.
• Each rotation lifts the car only a very short
distance.
Machines Do Work

5. 14.2 Work and Machines
Increasing Distance
A rower pulls an oar through a small
distance. The end of the oar in the water
moves through a large distance. The
increased travel of the oar through the water
requires the rower to exert a greater force.
A machine that decreases the distance
through which you exert a force increases
the amount of force required.
Machines Do Work

6. 14.2 Work and Machines
Changing Direction
Some machines change the direction of the
applied force. Pulling back on the handle of
the oar causes its other end to move in the
opposite direction.
Machines can change the amount of force
and the distance the force acts through as
well as the direction of the force.
Machines Do Work

7. 14.2 Work and Machines
Work Input and Work Output
How are work input and work output related
for a machine?
Because of friction, the work done by a
machine is always less than the work done
on the machine.

8. 14.2 Work and Machines
The oars of the boat act as machines that
increase the distance over which the force
acts.
Work Input and Work Output
Boat moves in
this direction.
Input force Input distance
Output force
Output distance

9. 14.2 Work and Machines
Work Input to a Machine
• The force exerted on a machine is the
input force.
• The distance the input force acts through
is the input distance.
• The work input equals the input force
multiplied by the input distance.
Work Input and Work Output

10. 14.2 Work and Machines
For an oar, the input force is the force
exerted on the handle, and the input distance
is the distance the handle moves.
The work input is the work done to move the
handle.
Work Input and Work Output

11. 14.2 Work and Machines
Work Output of a Machine
• The force exerted by a machine is called
the output force.
• The distance the output force is exerted
through is the output distance.
• The work output of a machine is the
output force multiplied by the output
distance.
Work Input and Work Output

12. 14.2 Work and Machines
The output work of the oars results from the
oars pushing against the water so that the
water pushes back against the oars.
Although nearly equal, the output work is less
than the input work because of friction. All
machines use some amount of input work to
overcome friction.
Work Input and Work Output

13. 14.2 Work and Machines
The only way to increase the work output is
to increase the amount of work you put into
the machine. You cannot get more work out
of a machine than you put into it!
Work Input and Work Output

14. 14.2 Work and Machines
Assessment Questions
1. What is the output distance of a machine that
requires 2 newtons of force exerted over 6 meters
and whose output force is 4 newtons?
a. 2 meters
b. 3 meters
c. 6 meters
d. 12 meters

15. 14.2 Work and Machines
Assessment Questions
1. What is the output distance of a machine that
requires 2 newtons of force exerted over 6 meters
and whose output force is 4 newtons?
a. 2 meters
b. 3 meters
c. 6 meters
d. 12 meters
ANS: B

16. 14.2 Work and Machines
Assessment Questions
1. The work output of a machine is always greater
than the work input to the machine.
True
False

17. 14.2 Work and Machines
Assessment Questions
1. The work output of a machine is always greater
than the work input to the machine.
True
False
ANS: F, never