Motion, Forces, and Energy
Core concepts
Speed is distance over time; velocity adds direction; acceleration is change in velocity over time. On a distance-time graph, slope is speed; on a velocity-time graph, slope is acceleration and area is displacement. Newton's laws describe inertia, F = ma, and action-reaction pairs. Friction and gravity are common forces; weight is mass times gravitational field strength. Work is force times distance in the direction of force; power is work over time. Kinetic energy is ½mv²; potential energy depends on height and mass. Energy converts between forms but total energy is conserved.
Traps to watch
- Treating speed and velocity as identical.
- Confusing mass with weight.
- Forgetting that acceleration includes direction changes, not just speeding up.
- Using total distance when displacement is asked.
- Assuming no work is done when a force is applied but no motion occurs in its direction.
Worked example
A 2 kg cart accelerates at 3 m/s². Net force = ma = 6 N. If it travels 4 m in 2 s from rest, average speed = 2 m/s. Its kinetic energy at 2 m/s is ½(2)(2²) = 4 J.
Rule of thumb
Identify the graph type first, then read slope or area. Check units and whether the question asks for a vector or scalar.