Simple Machines, Levers, Pulleys and Mechanical Advantage
This section covers Simple Machines, Levers, Pulleys and Mechanical Advantage. The exam tests lever classes, pulley systems, mechanical advantage, velocity ratio and efficiency.
Core concepts
- Mechanical advantage MA = load ÷ effort. Velocity ratio VR = distance moved by effort ÷ distance moved by load. Efficiency = (MA ÷ VR) × 100%.
- A first-class lever has the fulcrum between load and effort; a second-class lever has the load between fulcrum and effort; a third-class lever has the effort between fulcrum and load. For a lever, load × load arm = effort × effort arm in ideal balance.
- A fixed pulley changes direction but gives MA = 1. A movable pulley shares load between rope sections. In a simple pulley block, count the rope sections supporting the load to estimate VR.
- An inclined plane, wheel and axle, and screw jack are simple machines. Friction lowers efficiency, so real efficiency is below 100%.
Worked example
A first-class lever has a load of 600 N at 0.3 m from the fulcrum and effort at 1.5 m. For balance, effort = (600 × 0.3) ÷ 1.5 = 120 N. MA = 600 ÷ 120 = 5. If the effort moves 0.6 m while the load rises 0.12 m, VR = 0.6 ÷ 0.12 = 5, so ideal efficiency is 100%. The reasoning uses moments about the fulcrum.
Traps that cost marks
Do not mix MA and VR. Measure lever arms perpendicular to the force. Do not count the fixed pulley rope as an extra supporting section in a simple block. Efficiency above 100% is impossible.