Speed
The speed of an object is the distance it travels per unit time
Speed is a scalar quantity
This is because it only contains a magnitude (without a direction)
For objects that are moving with a constant speed, use the equation below to calculate the speed:
Where:
Speed is measured in metres per second (m/s)
Distance travelled is measured in metres (m)
Time taken is measured in seconds (s)
Average Speed
In some cases, the speed of a moving object is not constant
For example, the object might be moving faster or slower at certain moments in time (accelerating and decelerating)
The equation for calculating the average speed of an object is:
The formula for average speed (and the formula for speed) can be rearranged with the help of the formula triangle below:
Velocity
The velocity of a moving object is similar to its speed, except it also describes the object’s direction
The speed of an object only contains a magnitude - it’s a scalar quantity
Velocity is therefore a vector quantity because it describes both magnitude and direction
e.g. ‘15 m/s south’ or ‘250 mph on a bearing of 030°’
This means velocity can also have a negative value
E.g. a ball thrown upwards at a velocity of 3 m/s comes down at a velocity –5 m/s, if upwards is considered positive
However, their speeds are still 3 m/s and 5 m/s respectively
The equation for velocity is very similar to the equation for speed:
Where:
v = velocity in metres per second (m/s)
s = displacement, measured in metres (m)
t = time, measured in seconds (s)
Velocity is a vector quantity, so it uses displacement, s, rather than distance which is scalar.