9.11 There are different types of machines

In this topic, you will learn that:

• inclined planes such as a ramp can provide a mechanical advantage • a wedge or screw can reduce the effort needed to split or enter an object • a wheel and axle is a special lever that turns about a fulcrum.

Figure 1 An escalator is an example of a ramp.

ramp a sloping surface joining two different levels

wedge a piece of wood, metal or other substance that tapers to a thin edge and is driven between two objects or parts of an object to secure or separate them

screw a sharp-pointed metal object with a spiral thread running along its length and a slotted head

thread the spiral ridge of a screw

Figure 2 An axe is an example of a wedge. Many different machines have been developed through the centuries that make less work for us. Ancient Greek mathematician Archimedes (c. 287– 212 bce) developed a screw that carried water to the top of a house. The screw was just a hollow pipe with an inclined plane (a simple machine) wound around the inside.

As well as pulleys and levers, other simple machines are ramps, wedges, screws and wheels and axles.

Ramps

Ramps are the simplest types of inclined planes. A ramp is used to lift heavy objects (the load) up to a higher level. For example, a piano mover might use a ramp to get a piano from the ground onto a truck. Ramps are used to bridge gaps between uneven surfaces. Escalators are moving ramps with steps (Figure 1). A ramp is called a simple machine because it makes moving a load easier. Going up the ramp might take longer than a single step up, but it requires a lot less force from your legs.

Wedges

A wedge is an inclined plane that moves through another object and changes the direction of a downward force to a sideways force. An axe is a wedge. When an axe hits a log, the downward force is changed to a sideways force, which splits the log (Figure 2).

Humans discovered the benefits of wedges when they used the jagged edges of rocks to cut animal flesh and skin. It is more than likely that you have used a wedge today: a knife is a wedge and so are your teeth. Each tooth in a zipper is a tiny wedge that fits tightly with the adjacent teeth.

Figure 3 Corkscrews are used to pull the cork out of a bottle.

Screws

You might be surprised to know that a screw, like the wedge and the ramp, is also an inclined plane. The indent that spirals around a screw, called the thread, looks almost like a road (a ramp) spiralling up the side of a mountain. Screws penetrate materials such as wood or cork by using the turning effect of a force. The effort needed to turn a screw into an object is much less than that required to hammer the screw into the same object.

Wheels and axles

If you have used a circular door handle or travelled in a car, bus or train today, you have used a wheel-and-axle simple machine. A wheel is a type of lever that turns in circles about its centre – the fulcrum or pivot point. An axle usually links the lever and the wheel. For example, when you turn a doorknob, you apply an effort force to the door handle and the

axle exerts a force on the load (the latch), which opens the door.

A wheel and axle is sometimes a force magnifier. For example, you apply a small effort to a doorknob to move a larger load, the latch. This is because the outside edge of the wheel, or doorknob, moves a larger distance than the axle, or latch. A Ferris wheel is an example of a wheel and axle (Figure 4).

Wheel-and-axle machines can also act as distance magnifiers. When you pedal a bike, you apply a force to the pedals (Figure 5). This force causes the larger wheels to turn. The distance the wheel travels is much further than the distance the pedal travels. The distance has been magnified.

wheel and axle a type of lever that can rotate about its centre, magnifying force or distance

Figure 4 A Ferris wheel is an example of a wheel and axle. Figure 5 A bike wheel is an example of a distance magnifier.

9.11 Check your learning

Remember and understand

1 List the six types of simple machines. 2 Identify which of the following is not an inclined plane. a knife used to cut bread

b screwdriver used to turn a screw

c nail driven into a piece of wood d spear thrust into a tree 3 Identify the part of a circular doorknob that is a wheel and the part that is an axle. Draw a labelled diagram to support your answer.

Apply and analyse

4 Compare a wedge with a ramp. 5 Identify a circular doorknob as either a force magnifier or a distance magnifier. 6 Explain how the can opener shown in Figure 6 is acting as a wheel and axle by identifying the effort, load, fulcrum, wheel and axle.

Figure 6 A can opener is a lever.