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Speed and acceleration

Speed tells how far something moves per unit of time, and acceleration tells how quickly the speed changes. Positive acceleration means the speed increases, negative that it decreases. The unit of speed is meters per second, and of acceleration meters per second squared. Time must be in seconds and distance in meters. A speed given in kilometers per hour is divided by 3.6 to get meters per second.

Try it yourself

Velocity–time graph

v = \frac{\Delta s}{\Delta t}

average speed

a = \frac{\Delta v}{\Delta t}

average acceleration

Symbols

$v$	speed	m/s
$a$	acceleration	m/s²
$\Delta s$	distance	m
$\Delta t$	time	s

Example

A bicycle covers 150 m in 6 s: $v = 150/6 = 25$ m/s.

The speed rises from 10 m/s to 25 m/s in 5 s: $a = (25 - 10)/5 = 3$ m/s².

Always convert to meters and seconds before substituting into the formulas.

Practise motion for free →

Equations of motion →

Part of Physics 1: Motion.