Simple machines · Grade 3
Simple machines such as levers, pulleys, gears, ramps and wheels-and-axles make jobs easier by changing the size or direction of a force. A machine that lets you use a smaller force always makes you move further, so you never get more work out than you put in.
Key facts
- The six simple machines: lever, pulley, wheel and axle, inclined plane (ramp), wedge, screw.
- Mechanical advantage (MA) = load ÷ effort.
- Lever: effort × distance from pivot = load × distance from pivot.
- A pulley system with n supporting ropes: effort = load ÷ n (ideally), but you pull n times as much rope.
- Gears: the gear with fewer teeth turns faster; meshing gears turn in opposite directions.
- Ramp (no friction): effort × length = load × height. Efficiency = useful work out ÷ work in.
Worked examples
Example 1
A crowbar has the load 10 cm from the pivot and the effort 90 cm from it. What effort lifts a 450 N stone?
Answer: 50 N