Question

A car running at a speed of 72 km/h is to be stopped in exactly 10 seconds. Find the uniform deceleration needed.

Answer 1

Explanations & Calculations

• The question mentions a uniform deceleration. For uniform acceleration/ deceleration 4 equations of motions can be used appropriately.

$\qquad\qquad \begin{aligned} \small v=u+at,\, v^2=u^2+2as, \,s=ut+\frac{1}{2}at^2,\, s=\frac{(v+u)t}{2} \end{aligned}$

• The appropriate equation to be used for this question is the simplest one which is $\small v=u+at$ as the initial speed ($\small 72kmh^{-1}$ ), final speed and the time are given.
• To begin with the calculations, you need to convert the given initial speed into $\small ms^{-1}$ as the time is given in seconds.

$\qquad\qquad \begin{aligned} \small 72\,kmh^{-1}&=\small \frac{72\times1000\,m}{3600\,s}\\ &=\small 20\,ms^{-1} \end{aligned}$

• Now you can try substituting the values to the equation and getting the answer.
• The answer should come with a negative sign as the finding is the deceleration, not the acceleration.