Forces, Motion and Energy
Forces and motion
You must understand scalars and vectors, balanced and unbalanced forces, friction, weight and mass. Weight is a force measured in newtons and depends on gravity; mass is the amount of matter in kilograms. A trap is treating mass and weight as the same quantity.
Speed, acceleration and graphs
Use speed = distance ÷ time, and acceleration = change in velocity ÷ time. Interpret distance-time and velocity-time graphs: gradient of a distance-time graph is speed; area under a velocity-time graph is distance travelled. Read axes and units carefully.
Energy and work
Know energy stores and transfers, conservation of energy, and the equations for kinetic energy, gravitational potential energy and work. Power is the rate of energy transfer. Efficiency = useful output ÷ total input × 100%. A trap is forgetting that energy is dissipated as heat, so efficiency is always below 100%.
Worked example
A 2 kg ball falls 5 m. Gravitational potential energy lost = mgh = 2 × 10 × 5 = 100 J. If all is transferred to kinetic energy, ½mv² = 100, so v² = 100, giving v = 10 m/s. Show each step and state the assumption that air resistance is negligible.