This old chimney has a spiral staircase wrapped around it. The spiral staircase contains an inclined plane, or ramp. Do you see it? The stairs that wrap around the outside wall of the chimney make up the inclined plane. Together with the chimney, they represent a machine called a screw.
What Is a Screw
A screw is a simple machine that consists of an inclined plane wrapped around a central cylinder. No doubt you are familiar with screws like the wood screw in the left-hand side of the Figure below. The cap of the bottle pictured on the right is another example of a screw. Screws move objects to a greater depth (or higher elevation) by increasing the force applied to the screw. Many screws are used to hold things together, such as two pieces of wood or a screw cap and bottle. When you use a screw, you apply force to turn the inclined plane. The screw, in turn, applies greater force to the object, such as the wood or bottle top. You can watch an animation of a turning screw at the following URL.
To see some other examples of screws, go to this URL:
Q: Can you identify the inclined plane in each example of a screw pictured in the Figure above?
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 the Figure below. 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.
- 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.
Do the simple screw activity described at the following URL, and then answer the questions below.
- How does the activity model a screw?
- Which part of the screw is the long sloping side of the inclined plane?
- Which dimension of the screw is the same as the height of the inclined plane?
- What is a screw?
- How are screws used?
- Give an example of a screw that is not mentioned in the article.
- Why is the mechanical advantage of a screw always greater than 1?