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The velocity of an object is its speed in a particular direction. Velocity is a vector quantity because it has both a magnitude and an associated direction. To calculate velocity, displacement is used in calculations, rather than distance.

Unlike distance, which is a scalar quantity, displacement is a vector quantity. It includes:

• the distance travelled, measured in a straight line from start to finish
• the direction of the straight line

Learn more on acceleration in this podcast

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Acceleration

Acceleration is the rate of change of velocity. It is the amount that velocity changes per unit time.

The change in velocity can be calculated using the equation:

change in velocity = final velocity - initial velocity

The average acceleration of an object can be calculated using the equation:

This is when:

• acceleration (α) is measured in metres per second squared (m/s²)
• change in velocity (∆v) is measured in metres per second (m/s)
• time taken (t) is measured in seconds (s)

If an object is slowing down, it is decelerating (and its acceleration has a negative value).

Example

A car takes 8.0 s to accelerate from rest to 28 m/s. Calculate the average acceleration of the car.

final velocity, v = 28 m/s

initial velocity, u = 0 m/s (because it was at rest - not moving)

change in velocity, ∆v = (28 - 0) = 28 m/s