A car's velocity is 18 m s⁻¹. After the brake is pressed it has a deceleration of 2 m s⁻². What will its velocity be after 3 sec?

Correct answer: b 12 m s⁻¹

For a deceleration a = −2 m s⁻², so v = u + at = 18 − 2 × 3 = 12 m s⁻¹.

Why the other options are wrong

• a 27 m s⁻¹: 27 m s⁻¹ comes from adding the deceleration (18 + 2 × 3). After braking the velocity falls, so it has to be subtracted.
• c 9 m s⁻¹: 9 m s⁻¹ is 18 divided by 2. The velocity comes from v = u − (deceleration × time), not from a division.
• d 3 m s⁻¹: 3 is just the time given in the question, not a velocity. v = 18 − 2 × 3 = 12 m s⁻¹.