Motion, Force, Work, Energy, and Simple Machines
This domain tests motion quantities and their units, Newton's laws, the definitions of work, power, and energy, and the mechanical advantage of simple machines.
Core rules
- Speed is distance per unit time; velocity includes direction; acceleration is change in velocity per unit time. The SI unit of force is the newton, of work and energy the joule, and of power the watt.
- Newton's laws: an object stays at rest or in uniform motion unless a net force acts; force equals mass times acceleration; every action has an equal and opposite reaction.
- Work = force × distance moved in the direction of the force. Power = work done per unit time. Kinetic energy depends on mass and speed; potential energy depends on mass, height, and gravity.
- A simple machine such as a lever, pulley, or inclined plane multiplies force or changes its direction. Mechanical advantage = load ÷ effort; efficiency = useful output ÷ input × 100%.
Worked example
A person lifts a 10 kg box vertically through 2 m. Take g = 10 N/kg. The weight is mg = 10 × 10 = 100 N. Work = force × distance = 100 × 2 = 200 J. If the lift takes 4 s, power = 200 ÷ 4 = 50 W. The reasoning requires converting mass to weight before using the work formula.
Traps that cost marks
Do not use mass in newtons without multiplying by g. Work is zero if the force is perpendicular to motion, as when a person carries a load horizontally. Do not say a machine reduces the total work needed; it changes force and distance, and efficiency is always below 100% because of friction.
Key points
- Force is measured in newtons; work and energy in joules; power in watts.
- Work requires displacement in the direction of the force.
- Weight = mass × g, so convert mass before calculating work.
- Mechanical advantage compares load and effort; efficiency compares output and input.