Forces, motion and Newton’s laws
Physics · GCSE · The School
Three laws
FIRST: an object stays still or moves at constant velocity unless a RESULTANT force acts. SECOND: resultant force = mass × acceleration, F = ma. THIRD: for every action there is an equal and opposite reaction, acting on a DIFFERENT object — which is the part that gets missed, and the reason the two forces never cancel out.
Using F = ma
Find the RESULTANT force first by combining everything acting along the line of motion, then divide by mass to get acceleration. A 1,200 kg car with 3,600 N of resultant force accelerates at 3 m/s². If the forces balance, the resultant is zero and the acceleration is zero — which means constant velocity, not necessarily stationary.
Stopping distance
STOPPING DISTANCE = THINKING DISTANCE + BRAKING DISTANCE. Thinking distance grows with speed and with reaction time, which is lengthened by tiredness, alcohol, drugs and distraction. Braking distance grows with the SQUARE of speed and is worsened by wet or icy roads, worn tyres and worn brakes. Doubling speed roughly quadruples the braking distance, which is the whole argument for speed limits.
A 1,200 kg car has a driving force of 4,000 N and experiences 1,000 N of drag and friction. Find the RESULTANT first: 4,000 − 1,000 = 3,000 N. Then a = F ÷ m = 3,000 ÷ 1,200 = 2.5 m/s². Now suppose it is travelling at steady speed: the resultant must be zero, so drag has risen to 4,000 N and the acceleration is 0 — even though the engine is working just as hard. Always find the resultant before dividing.