Question

When a car driver travelling at a speed of 20

metre per second, applies brakes and brings the car

in rest at 20 second, then retardation will be

a.+2m/s²

b.-2m/s²

c. +1m/s²

d. -1m/s²​

Answer 1

Given :-

→ Initial velocity of the car = 20 m/s

→ Time taken to come to rest = 20 second

To find :-

→ Retardation of car.

Solution :-

Since, the car finally comes to rest, so it's final velocity will be zero.

______________________________

By the 1st equation of motion, we have :-

v = u + at

Where :-

• v is the final velocity.

• u is the initial velocity.

• a is acceleration.

• t is time taken.

⇒ 0 = 20 + a(20)

⇒ 0 - 20 = 20a

⇒ -20 = 20a

⇒ a = -20/20

⇒ a = - 1 m/s²

As we know that retardation is negative acceleration. Thus retardation is :-

= -(-1) m/s²

= + 1 m/s² [Option.c]

Answer 2

Answer:

Given :-

When a car driver travelling at a speed of 20  metre per second, applies brakes and brings the car  in rest at 20 second

To Find :-

Retardation

Solution :-

We know that

a = v - u/t

a = Acceleration

v = Final velocity

u = Initial velocity

t = time

a = 0 - 20/20

a = -20/20

a = -1

Retardation = -1 m/s²

Hence,

Option D is correct