Unit 2 outcomes

By the end of this unit you will be able to:

• Identify and describe the mechanical advantage of screws and give examples.

What you should know

Before you start this unit, make sure you can:

• Describe what a simple machine is.

 Introduction

In this unit you will build on your knowledge of simple machines and forces to understand how a screw offers mechanical advantage. Once you understand the concept, you will be able to give examples of real-world applications in which screws help make work easier.

A screw is a cylinder with a head (solid top) at one end and a pointed tip (like a nail) at the other end. More importantly, it has sloped ridges winding around it. The correct term for the ridges (or grooves) around the shaft or cylinder is the  thread. The distance between threads is constant but different screws can have different spacing sizes between the threads. The distance between threads is called the  pitch.

Screws are useful for holding things together. They can pull or push an object together. They can be used to lift heavy items and tighten things.

How a screw functions as a simple machine

As a screw turns, the rotational movement is changed into a linear movement. It is easier to rotate the thread over a longer distance than to drive the screw vertically downwards.

A screw is essentially a continuous inclined plane. A screw has a long incline wrapped around a shaft, so its mechanical advantage can be approached in the same way as that of an inclined plane.

Inclined planes reduce the amount of effort force required by increasing the distance over which the force acts.

The mechanical advantage of a screw depends on the space between the threads and the length (and thickness) of the screw. The closer the threads are, the greater the mechanical advantage. It is easier to drive a screw into an object if the thread spacing is smaller. It takes less effort but more turns (essentially a longer distance). If the spaces between the threads are wider, it is harder to drill a screw into an object. It takes more effort but fewer turns (shorter distance).

How screws are commonly used in real-world situations

There are many applications for screws. Some applications are to hold items together and others are to lift or stretch objects.

Screws used to hold things together

The screw design can be used to secure, tighten, fasten or connect items. Look at some everyday examples in figure 2.

A bolt is a kind of screw but does not have a pointed tip. A bolt is not drilled into place, but rather, a hole is made for the bolt to go through. Then a nut is placed at the end to secure the bolt. Bolts are powerful tools used to hold things together.

Screw top lids on jars and bottles are another example of how two things can be held together tightly using the principle of a screw as a simple machine. Less force is required to close or open the lid by unscrewing than if you tried to lift the lid off vertically.

Light bulbs are fixed to electrical fittings by using the screw at the base of the bulb.

Screws used to lift or stretch objects

A corkscrew is used to lift a cork out of a bottle more easily. Guitars and violins are tuned by using a screw mechanism to tighten or loosen the strings. Some office chairs or stools use a screw mechanism to make them higher or lower.

Activity 2.1: Make an Archimedes screw to lift cereal

Suggested time: 30 minutes

What you need:

• internet access

What to do:

Watch this video on how to make an Archimedes screw.

If you have access to the items listed in the video, try can make your own Archimedes screw by following the steps.

How to make an Archimedes screw (Duration: 02:57)

What did you find?

Much less effort is required to lift the cereal using the Archimedes screw. The Archimedes screw concept can be adapted to raise water from a low level to a high level for the irrigation of crops.

Summary

• A screw is a simple machine based on a continuous inclined plane.
• The mechanical advantage of a screw depends on the pitch of the screw. The closer the threads of a screw, the greater the mechanical advantage.
• Screws can reduce the force required to hold things together, lift things or stretch things.
• There are many uses of screws in everyday life, such as screw top lids, clamps, bolts, corkscrews and swivel chairs.

Unit 2: Assessment

Suggested time to complete: 10 minutes

1. Describe how a screw functions as a simple machine.
2. What factors determine the mechanical advantage of a screw?
3. Explain how the space between the thread of a screw affects the mechanical advantage.
4. Give three examples of where screws are used to hold things together.
5. Give three examples of where screws are used in lifting or stretching.

The full solutions are at the end of the unit.

Unit 2: Solutions

Unit 2: Assessment

1. The thread of a screw acts like a continuous inclined plane. This increases the distance over which the force acts and therefore reduces the amount of effort (force) required.
2. .
   • length of the screw
   • thickness of the screw
   • pitch of the screw (space between the thread)
3. The closer the threads are, the greater the mechanical advantage. This is because the spiral distance is longer, therefore less effort is required. If the spaces between the threads are wider, the spiral distance is shorter, so more effort is required. It is harder to drill in a screw with a wide pitch into an object.
4. screw top lids, nuts, bolts, clamps, lightbulbs
5. tuning strings on musical instruments, swivel chairs and stools, Archimedes water screw, spiral staircase, spiral ramps in multi-storey carparks

Back to Unit 2: Assessment