Motion, forces and Newtonian mechanics
Motion, forces and Newtonian mechanics tests how to describe movement, interpret graphs and apply force laws.
Motion
- Speed = distance/time; velocity is speed in a stated direction; acceleration = change in velocity/time.
- Distance-time graph gradient = speed. Velocity-time graph gradient = acceleration; area under graph = distance travelled.
- Uniform acceleration equations: v = u + at; s = ut + ½at²; v² = u² + 2as.
Forces
- Resultant force changes motion. Newton's first law: no resultant force means constant velocity or rest. Second law: F = ma. Third law: equal and opposite forces on different bodies.
- Weight W = mg, with g about 10 N/kg near Earth. Mass is amount of matter (kg); weight is force (N).
- Friction opposes motion; terminal velocity occurs when drag equals weight.
- Hooke's law: extension proportional to load up to elastic limit; F = kx.
Worked example
A 1200 kg car accelerates from rest to 20 m/s in 5.0 s. Acceleration a=(20-0)/5.0=4.0 m/s². Resultant force F=ma=1200×4.0=4800 N. Distance travelled = area under velocity-time graph = ½×5.0×20=50 m. Using s=ut+½at² gives 0+½×4×25=50 m.
Traps
Confusing speed and velocity; reading gradient as distance; using g in kg; forgetting friction reduces resultant force; mixing up mass and weight.