14.2 Work and Machines
• When someone gets a flat tire, that person
cannot lift the car to remove the tire with only
his or her hands.
• The person needs a jack
Machines Do Work
• A machine is a device that changes a force.
• How do machines make work easier?
• 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.
Increasing Force
• Turning a screwdriver is using a small force
(it’s not as difficult as using your fingers) over
a large distance (the screwdriver is longer than
the screw) to become a large force (to remove
the screw) over a short distance (the screw
does not fly out).
• Picking up books one at a time takes less force
but a longer distance. Decreasing distance
decreases the amount of force needed.
Increasing Distance
• Some machines increase the distance over
which the force is exerted.
• When you row a boat, you should only move
the oars a short distance. The long oars have a
greater distance in the water and speeds up
travel time. However, this takes more force
than turning a screwdriver.
Changing Direction
• Some machines do work by changing the
direction of a force.
• For example, when you open a can of paint
with a metal wedge, you push the wedge
down while the other end of the wedge that is
between the lid and the can moves up
(opposite direction).
• Also, when a crew member rows an oar, the
short end is pulled to the person, and the long
end in the water moves away.
Work Input and Work Output
• You must remember that friction is still
present when doing work.
• For example, when you use a can opener to
open a can, the wheel you turn heats up a
little due to friction
• Because of friction, the work done by a
machine is always less than the work done on
a machine.
Sidebar: Perpetual Motion
• Scientists have been looking for self-sufficient
machines that will do the same work as is put
into the machine.
• One such machine that scientists are trying to
invent is a perpetual motion machine that
moves by itself over and over with no human
involvement past starting the machine.
• Sadly, friction makes this dream impossible
(perpetual motion is like expecting a bowling
ball to slide along an infinite alley forever).
Work Input to a Machine
• Input force is the force you exert on a
machine (how much you push or pull the
machine).
• Input distance is the length of the force
applied by yourself.
• Work input is the work done by the input
force over the input distance (work = force x
distance).
Work Output of a Machine
• Output force is the force exerted by the
machine.
• Output distance is the distance the output
force is exerted.
• Work output is the output force multiplied by
the output distance.
• You would need to decrease friction in order
to increase the output, and you can never
have a greater output than input.
Analysis of Output vs. Input
•
•
•
•
Example: screwdriver
Input occurs at the handle of the screwdriver.
Output occurs at the bit (pointy end).
The input distance is greater (the handle has a
larger circumference than the bit).
• The output force is greater (unless you have
arthritis, it is easy to use a screwdriver).
• The bit heats up (especially if electric), so
output work is lost due to friction.