Distance and displacement
- Distance is how far an object moves. It is a scalar: it has no direction.
- Displacement is the distance and direction in a straight line from the start to the finish. It is a vector.
Speed
- Speed is how fast something is moving. It is a scalar.
- Distance travelled = speed × time: s = v t.
- Typical speeds: walking about 1.5 m/s, running about 3 m/s, cycling about 6 m/s. The speed of sound in air is about 330 m/s.
- The speed of a moving object is rarely constant, so we often calculate an average speed: total distance ÷ total time.
Velocity
- Velocity is the speed of an object in a given direction. It is a vector.
- (Higher) An object moving in a circle at constant speed has a changing velocity, because its direction keeps changing. So it is accelerating.
Acceleration
- Acceleration is the rate of change of velocity: a = Δv ÷ t, measured in m/s².
- A negative acceleration is a deceleration: the object is slowing down.
- For uniform acceleration: v² − u² = 2 a s, where u is the initial velocity, v the final velocity and s the distance.
- Near the Earth's surface, an object falling freely under gravity accelerates at about 9.8 m/s².
Key terms
- Distance
- How far an object moves; a scalar.
- Displacement
- Distance in a stated direction from the starting point; a vector.
- Speed
- How fast an object moves; distance ÷ time.
- Velocity
- Speed in a given direction; a vector.
- Acceleration
- The rate of change of velocity, in m/s².
- Deceleration
- Slowing down; a negative acceleration.
- Uniform acceleration
- Acceleration that stays constant.