Question

A car moving at a rate of 72 kilometre per hour. How far does the car moves when it stops after 4 seconds?​

Answer 1

Answer:

Explanation:Assuming it starts at 72 kmph and hits a dead stop: Divide 72 by 60 for distance per minute. So, 1.2km per minute. 1.2km is 1200m and 4 seconds is one fifteenth of a minute.

Answer 2

Given:-

u = 72 km/hr = 72 $\rm \times \frac{5}{18} = 20\:m/s$

v = 0 m/s

t = 4 s

What to find:-

The distance travelled by car after 4 seconds.

How to find:-

First, we are going to find the acceleration of the car

Then, we are going to put the value of the acceleration in the second \ third equation of motion

Solution:-

u = 20 m/s

v = 0 m/s

t = 4 s

Using the first equation of motion for finding the acceleration,

v = u + at

0 = 20 + 4a

-20 = 4a

a = -5 m/$\rm s^2$

Using the second equation for finding the distance travelled by car,

$\rm s = ut + \frac{at^2}{2}$

$\rm s = 20 \times 4 + \frac{-5\times 4^2}{2} \implies 80 - 5 \times 8 = 80 - 40 = 40\:m$

Alternative,

Instead of using the second equation of motion, we can also use the third equation of motion,

$\rm v^2 - u^2 = 2as$

$\rm 0 - 400 = -10s \implies s = 40\:m$

Equation of motions:-

i) $\rm v = u+at$

ii) $\rm s=ut+\frac{at^2}{2}$

iii) $\rm v^2 - u^2 = 2as$