Recall and apply: acceleration (m/s²) = change in velocity (m/s) / time (s)

The idea

Acceleration measures how quickly velocity changes, so it is the change in velocity divided by the time taken for that change. Find the change in velocity by subtracting the initial velocity from the final velocity, keeping direction in mind: a car slowing from 20 m/s to 8 m/s in 6 s has a change of 8 − 20 = −12 m/s, giving a = −12 ÷ 6 = −2 m/s², a deceleration. Units follow from the division: (m/s) ÷ s = m/s². Rearranged forms are a × t = Δv and t = Δv ÷ a. In a multiple-choice question, compute the value first, then check the unit and sign before matching an option, since distractors often divide by the wrong quantity or drop the negative sign.

What earns the marks

• Selects the option equal to the change in velocity divided by the time taken, with the unit m/s².
• Calculates the change in velocity by subtracting the initial velocity from the final velocity before dividing.
• Keeps a negative sign when the object slows down and identifies the result as deceleration.
• Rearranges the relationship correctly to find time as change in velocity divided by acceleration.
• Rejects options whose unit is m/s or m, since acceleration is measured in m/s².

Common mistakes and how to put them right

• Divides the final velocity by the time without subtracting the initial velocity.

  Put it right: Subtract the initial value first to obtain the change.

• Drops the negative sign for a slowing object.

  Put it right: Retain the sign so the answer shows deceleration.

• Uses the unit m/s for acceleration.

  Put it right: Write m/s² because velocity is divided by time.

Exam tips

• Write down the values of initial velocity, final velocity and time before touching the options.
• Check the sign of the change in velocity, especially when the object is slowing.
• Confirm the unit of your answer matches m/s² before selecting an option.