Question

A car driver, travelling at a speed of 72 km/h applies brakes and the car comes to rest in 20 s. Calculate the acceleration.​

Answer 1

Explanation:

Here, 72 km/h = 72 × (5/18) m/s

= 20 m/s.

Using the formula, v = u - at.

v = 0, u = 20 m/s, t = 20 seconds.

0 = 20 - a×20

= 20a = 20

= a = 1 m/(s^2)

Hence, acceleration = -1 m/(s^2)

or retardation = 1 m/(s^2)

Answer 2

Explanation:

initial speed (u ) = 72 km / hr

= 72 × 5 / 18 m / s

= 4 × 5

= 20 m / s

final speed = 0

time ( t ) = 20 sec

acceleration =( final vel. - initial vel ) / time

= ( 0 - 20 ) / 20

= - 20 / 20

= - 1 m / s^2

negative acceleration shows car is coming to rest or decelerating.