Forces, Motion, and Energy
Forces
A force is a push or pull measured in newtons. Contact forces include friction, air resistance, tension, and normal reaction; non-contact forces include gravity, magnetism, and electrostatic force. Forces are vectors: they have size and direction. Balanced forces give zero resultant force and no change in motion; unbalanced forces cause acceleration. Weight is the force of gravity on a mass: W = m × g, where g ≈ 10 N/kg on Earth. Mass is constant; weight varies with gravitational field strength. A classic trap is confusing mass (kg) with weight (N).
Motion
Speed = distance ÷ time (m/s); velocity includes direction; acceleration = change in velocity ÷ time (m/s²). On a distance–time graph, the slope gives speed; on a velocity–time graph, the slope gives acceleration and the area under the line gives distance travelled. Newton's laws: objects stay at rest or constant velocity unless a resultant force acts; F = ma; every action has an equal and opposite reaction.
Energy
Energy is conserved but transfers between stores: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, and sound. Work done = force × distance (J). Power = energy ÷ time (W). Efficiency = useful output ÷ total input × 100%. Energy is often wasted as heat through friction. Energy resources are renewable (solar, wind, hydroelectric, geothermal, biomass) or non-renewable (coal, oil, gas, nuclear).
Worked example
A 2 kg ball is lifted 3 m. GPE gained = mgh = 2 × 10 × 3 = 60 J. If dropped and all GPE converts to kinetic energy, KE = ½mv² = 60 J, so v² = 60, v ≈ 7.7 m/s.