Physics · Space, time and motion. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Displacement is the straight-line distance from start to finish, with a direction. Distance is the length of the path travelled. Velocity is the rate of change of displacement, so it is a vector; speed is a scalar. An average value covers a whole journey. An instantaneous value belongs to one moment.
When the acceleration is constant, the four equations of motion link s, u, v, a and t. On a graph, a gradient gives a rate and an area gives a total.
A projectile has two independent motions: a constant horizontal velocity and a vertical acceleration g downwards. Time links the two. Air resistance reduces the range and the height. A falling object reaches terminal speed when the drag equals its weight.
Students say that a projectile has zero velocity or zero acceleration at the top of its path. Only the vertical component of velocity is zero there; the acceleration is still 9.8 m s−2 downwards.
Answer: About 16 m s−1.
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