Simple Machines
Senior 2 · Physics
Simple Machines
Lesson 1 of 4
22 min
Let's begin
A builder in Kigali lifts a 50 kg sack of cement onto a lorry. A farmer draws water from a deep well. A mechanic loosens a tight wheel nut. None of them is strong enough to do it with bare hands alone, so each uses a simple machine. Knowing how to measure what a machine gives you, and what it costs you, is the key to choosing and improving tools.
A simple machine is a device that makes work easier by changing the size or the direction of a force. The force you apply is the effort (E). The force the machine overcomes is the load (L). The main types are the lever, the inclined plane (with the wedge and the screw), the wheel and axle, and the pulley.
A machine cannot give out more energy than is put in. If it lets a small effort move a large load, the effort must move through a longer distance.
MA and VR are ratios, so they have no units.
A builder uses a machine to raise a 600 N sack of cement. He applies an effort of 150 N, and his hands move 2.0 m while the sack rises 0.40 m.
Some of the input work is wasted: it is done against friction in the moving parts (which becomes heat) and in lifting the moving parts of the machine itself (such as a pulley block or a lever's arm). So the MA is always less than the VR, and η < 100%. For an ideal, frictionless and weightless machine, MA = VR.
Tip
MA = L ÷ E (forces). VR = distance of E ÷ distance of L (distances). η = MA ÷ VR
If your efficiency comes out above 100%, you have divided the wrong way round somewhere. Oiling a machine raises its MA and its efficiency, but it never changes its VR.
A machine lifts an 800 N load with an effort of 250 N. Its velocity ratio is 4. Find the MA and the efficiency.
Solution: MA = 800 ÷ 250 = 3.2. η = 3.2 ÷ 4 × 100% = 80%.
A machine with VR = 6 is 75% efficient. What effort lifts a load of 900 N?
Solution: MA = 0.75 × 6 = 4.5, so E = 900 ÷ 4.5 = 200 N.
Why can the efficiency of a real machine never reach 100%?
Solution: Some input work is always wasted against friction (as heat) and in lifting the machine's own moving parts.
A student says her machine has MA = 5 and VR = 4. Comment.
Solution: This is impossible. It would mean η = 125%, so the machine would give out more work than was put in. She has probably made a measurement error.
Summary
Sources and credits
The Rwandan examples, worked examples and practice items are written by Ganzaa.
