Simple Machines – JSS2 Intermediate Science Lesson Note (Third Term)

A simple machine is a tool that makes work easier by reducing the effort needed or by changing the direction of the force.

Behavioural objectives

1. Define a simple machine, load, effort and fulcrum.
2. State the types of simple machines and give one example of each.
3. Explain how simple machines make work easier.
4. State the formula for mechanical advantage.
5. Calculate the mechanical advantage of a machine from its load and effort.

What is a simple machine?

A simple machine is a tool that makes our work easier and faster.

• Purpose: it lets us use a small force (effort) to move a heavy object (load).
• Everyday example: a tin of Milo is hard to open with fingernails, but a spoon used to pry the lid up opens it easily.
• Machine in the example: the spoon acts as a lever.

What are the key terms used with simple machines?

There are four key terms: load, effort, fulcrum and mechanical advantage.

• Load: the object you are trying to move.
• Effort: the force you apply to the machine.
• Fulcrum (pivot): the fixed point on which a lever turns or rests.
• Mechanical advantage (M.A.): the number of times a machine multiplies the effort.

How do simple machines make work easier?

Simple machines do not do the work for us, they change the way we apply force.

1. Increase the force: a small effort moves a large load, as with a crowbar.
2. Change the direction of the force: we pull down to lift up, as with a pulley.
3. Increase the speed of the work: the load moves faster or further, as with a bicycle gear.

What are the types of simple machines?

There are six basic simple machines: the lever, pulley, inclined plane, wheel and axle, screw and wedge. The gear is a related machine taught with them.

1. What is a lever?

A lever is a stiff bar that turns around a fixed point called the fulcrum.

• Parts: the load, the effort and the fulcrum.
• Examples: scissors, see-saws, wheelbarrows, crowbars and bottle openers.

2. What is a pulley?

A pulley is a wheel with a groove for a rope or cable.

• Use: it lifts a load upward by pulling downward.
• Examples: the rope used to raise a flag on a flagpole, and the rope used to draw a bucket of water from a well.

3. What is an inclined plane?

An inclined plane is a slanting surface, also called a ramp.

• Use: it is easier to push a heavy drum up a ramp than to lift it straight up.
• Examples: a wheelchair ramp, and a plank used to load bags of rice into a truck.

4. What is a wheel and axle?

A wheel and axle is a large wheel joined to a smaller rod called the axle.

• Use: when the wheel turns, the axle turns too, but with more force.
• Examples: a doorknob, a screwdriver and a steering wheel.

5. What is a screw?

A screw is an inclined plane wrapped around a cylinder.

• Use: it holds things together or lifts things.
• Examples: jar lids, screw-type light bulbs and a car jack used to lift a vehicle to change a tyre.

6. What is a wedge?

A wedge is two inclined planes joined back to back to form a sharp edge.

• Use: it splits or cuts things when pushed in.
• Examples: an axe, a knife, a nail and a chisel.

What is a gear?

A gear is a wheel with teeth that fit into the teeth of another wheel.

• Use: when one gear turns, it makes the other turn, so gears change speed or direction.
• Examples: the chain and gears on a bicycle, and the inside of a clock.

Summary table of simple machines

What is mechanical advantage?

Mechanical advantage is the ratio of the load to the effort, and it tells us how many times a machine multiplies our force.

• Formula: Mechanical Advantage (M.A.) = Load ÷ Effort.
• Units: load and effort are both measured in newtons (N), so M.A. has no unit.
• M.A. greater than 1: the machine multiplies the force, so the work is easier.
• M.A. less than 1: the effort is bigger than the load, but we usually gain speed or distance.

How do we calculate mechanical advantage?

1. Write down the load and the effort in newtons.
2. Write the formula: M.A. = Load ÷ Effort.
3. Put in the values and divide.
4. State the answer with a short meaning.

Worked example 1

A lever lifts a stone weighing 500 N (load) with an effort of 100 N. Find the M.A.

• Step: M.A. = Load ÷ Effort = 500 ÷ 100 = 5.
• Meaning: the lever multiplies the effort 5 times.

Worked example 2

A pulley lifts a load of 600 N with an effort of 200 N. Find the M.A.

• Step: M.A. = 600 ÷ 200 = 3.
• Meaning: the pulley multiplies the effort 3 times.

Evaluation

1. Why do we say a pair of scissors is a simple machine?
2. Name the type of simple machine found on a flagpole.
3. Which simple machine is basically a ramp?
4. If a machine has a load of 200 N and an effort of 50 N, calculate its mechanical advantage.
5. State three ways in which simple machines make work easier.

Assignment

List five simple machines you use at home or in school. For each one, name its type and state whether it increases force, changes direction or increases speed.