Recall and use the equation: acceleration (metre per second squared, m/s²)…

The idea

Acceleration measures how quickly velocity changes. The equation is acceleration = change in velocity ÷ time taken, with acceleration in metres per second squared (m/s²), change in velocity in metres per second (m/s) and time in seconds (s). Change in velocity means final velocity minus initial velocity. For example, a car speeds up from 4 m/s to 16 m/s in 6 s: change in velocity = 16 m/s − 4 m/s = 12 m/s, so acceleration = 12 m/s ÷ 6 s = 2 m/s². A negative result means deceleration. Rearranging gives change in velocity = acceleration × time, and time = change in velocity ÷ acceleration.

What earns the marks

• States the equation as acceleration equals change in velocity divided by time taken.
• Calculates change in velocity by subtracting the initial velocity from the final velocity.
• Substitutes values with correct units into the equation to find acceleration in m/s².
• Rearranges the equation to find change in velocity or time when required.
• Interprets a negative acceleration as deceleration.

Common mistakes and how to put them right

• Uses the final velocity alone instead of the change in velocity.

  Put it right: Subtract the initial velocity from the final velocity first.

• Divides time by change in velocity.

  Put it right: Divide the change in velocity by the time taken.

• Omits the squared unit and writes acceleration in m/s.

  Put it right: Give acceleration in m/s².

• Ignores the sign of a negative result.

  Put it right: Recognise that a negative acceleration means deceleration.

Exam tips

• Write down the values you know with their units before substituting into the equation.
• Show the subtraction that gives the change in velocity as a separate step.
• Check that your final answer carries the unit m/s².