Q. A car is travelling at $72\ {kmh}^{- 1}$ and is 20 m from a barrier when the driver puts on the brakes. The car hits the barrier 2s later. What is the magnitude of the constant deceleration?
A) $7.2\ {ms}^{- 2}$
B) $10\ {ms}^{- 2}$
C) $36\ {ms}^{- 2}$
D) $15\ {ms}^{- 2}$
Explanation
(b) Initial speed of car,
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$u = 72\ {kmh}^{- 1} = 72 \times \frac{5}{18}{ms}^{- 1} = 20\ ms^{- 1}$
Distance from barrier $s = 20\ m$
Time taken by car to hit the barrier, $t = 2\ s$
Using first equation of motion $v = u + at$
$a = \frac{v - u}{t}$
$\therefore a = \frac{0 - 20}{2} = - 10\ {ms}^{- 2}$
$- ve$sign indicates that acceleration is retarding or it is deceleration which decreases the speed of car.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.