Question

A Car travelling at a speed of 20 m/s stops in 5 sec. after

application of brakes. What is the acceleration of the car?​

Answer 1

Answer:

–4 m/s²

Explanation:

As per the provided information in the given question, we have :

• Initial velocity (u) = 20 m/s
• Final velocity (v) = 0 m/s (As it stops)
• Time taken (t) = 5 s

We are asked to calculate the acceleration of the car.

Basically, in order to tackle this question, we can use two ways. Firstly, by using the acceleration formula and secondly, by using the first equation of motion.

By using the formula of acceleration :

$\longmapsto \bf {a = \dfrac{v-u}{t} }$

• v denotes final velocity
• u denotes initial velocity
• a denotes acceleration
• t denotes time

$\longmapsto \rm {a = \dfrac{0-20}{5} }$

$\longmapsto \rm {a =\cancel{ \dfrac{-20}{5}} }$

$\longmapsto \bf {a = -4 \; ms^{-2}}$

∴ Acceleration of the car is –4 m/s².

By using the first equation of motion :

$\longmapsto \bf {v = u + at }$

• v denotes final velocity
• u denotes initial velocity
• a denotes acceleration
• t denotes time

$\longmapsto \rm {0 = 20 + 5a}$

$\longmapsto \rm {0-20 = 5a}$

$\longmapsto \rm {-20 = 5a}$

$\longmapsto \rm {\cancel{\dfrac{-20}{5}} = a}$

$\longmapsto \bf {-4 \; ms^{-2} = a}$

∴ Acceleration of the car is –4 m/s².

$\pmb {\red{ Note :}}$ Negative sign implies that the speed is decreasing.

$\rule{200}2$

Points to remember :

• Acceleration is the rate of change in velocity.

• Its SI unit is m/s².

• It is vector quantity.

• Negative acceleration is called retardation.