http://www.ck12.org Chapter 4. Motion and Forces
To see some other examples of screws, go to this URL:
http://www.beaconlearningcenter.com/weblessons/simplemachines/machines015.htm
FIGURE 4.89
Q: Can you identify the inclined plane in each example of a screw pictured in theFigure4.89?
A: The inclined plane of the screw on the left consists of the ridges, or threads, that wrap around the central cylinder
of the screw. The inclined plane of the cap on the right consists of the ridges that wrap around the inner sides of the
cap.
Mechanical Advantage of a Screw
The mechanical advantage of a simple machine is the factor by which it multiplies the force applied to the machine.
It is the ratio of the output force to the input force. The force applied by the screw (output force) is always greater
than the force applied to the screw (input force). Therefore, the mechanical advantage of a screw is always greater
than 1.
Look at the two screws in theFigure4.90. In the screw on the right, the threads of the inclined plane are closer
together. This screw has a greater mechanical advantage and is easier to turn than the screw on the left, so it takes
less force to penetrate the wood with the right screw. The trade-off is that more turns of the screw are needed to do
the job because the distance over which the input force must be applied is greater.
Q: Why is it harder to turn a screw with more widely spaced threads?
A: The screw moves farther with each turn when the threads are more widely space, so more force must be applied
to turn the screw and cover the greater distance.
Summary
- A screw is a simple machine that consists of an inclined plane wrapped around a central cylinder. Screws
move objects to a greater depth (or higher elevation) by increasing the force applied to the screw - The mechanical advantage of a screw is always greater than 1 because the output force applied by the screw
is greater than the input force applied to the screw.