screw

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 1.2. 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.

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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 1.1. 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. Q: Can you identify the inclined plane in each example of a screw pictured in the Figure 1.1? 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.

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instructional diagrams

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questions

examples of screws include

a) wood screws.

b) spiral staircases.

c) bottle caps.

-->  d) all of the above

uses of screws include

a) raising objects.

b) lowering objects.

c) holding objects together.

-->  d) all of the above

screws help us do work by increasing the

-->  a) amount of force that is applied.

b) distance over which force is applied.

c) speed with which force is applied.

d) two of the above

the input force is applied to the inclined plane of a screw.

-->  a. true

b. false

the mechanical advantage of a screw is always less than 1.

a. true

-->  b. false

in the picture above, it takes it takes more turns of the screw on the right to go the same distance into the wood as the screw on the left.

-->  a. true

b. false

diagram questions

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