Simple Machines
Primary 6 · Science and Elementary Technology
Simple Machines
Lesson 4 of 5
14 min
Let's begin
Look at the road that climbs from the valley to the top of a hill in Rwanda. It does not go straight up. It winds and zigzags from side to side.
The road is longer that way, so why build it like that? Because a long, gentle slope is much easier to climb than a short, steep one. That road is a simple machine.
An inclined plane is a sloping surface, such as a ramp. Instead of lifting a load straight up, you push or pull it up the slope.
Examples: a ramp for loading a lorry, a wheelchair ramp at a school or hospital, the zigzag roads up Rwanda's hills, and a slide.
Worked example
A 50 kg drum must go onto a lorry 1 m high. Lifting it straight up needs an effort equal to lifting all 50 kg. Pushing it up a ramp 4 m long needs only about a quarter of that effort, roughly the effort of lifting 12 or 13 kg, plus a little extra because of friction. The drum travels 4 m instead of 1 m.
A wedge is two inclined planes joined back to back, thick at one end and thin at the other. When you push or hit the thick end, the thin edge is forced in and the sloping sides push the material apart, sideways.
Examples: an axe head splitting firewood, a knife or panga, the point of a nail, a chisel, the blade of a hoe, your front teeth, and a doorstop (which uses the wedge to hold a door).
A screw is an inclined plane wrapped round a cylinder. The raised spiral ridge running round it is called the thread. Each turn moves the screw forward only a short distance, but with great force, and the thread holds things together tightly.
Examples: wood screws, nuts and bolts, bottle caps and jar lids, the screw of a bench vice and a screw jack for lifting.
See it for yourself. Cut a right-angled triangle from paper. Its long sloping edge is an inclined plane. Now roll the triangle round a pencil, starting from one of the short sides. The sloping edge becomes a spiral, just like the thread of a screw.
Tip
Three machines, one family:
Inclined plane = a ramp. Wedge = two ramps back to back. Screw = a ramp that goes round and round.
Try each one first, then tap Show answer to check.
Mutesi can load a sack onto a lorry using a 2 m ramp or a 5 m ramp. Which needs less effort? Which needs a longer push?
Answer: The 5 m ramp needs less effort, but it is also the longer push.
Which simple machine is the blade of an axe, and what does it do to the wood?
Answer: A wedge. It turns the downward blow into a sideways push that splits the wood apart.
Name the simple machine in a jar lid.
Answer: A screw. The lid and the jar have threads.
Why do roads up steep hills zigzag?
Answer: A zigzag road is a longer, gentler inclined plane, so vehicles and people need less effort to climb it.
What you learned today
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Quick check
Match each simple machine to what it is.
Tap one on the left, then its partner on the right.
