Question

A motorcycle is being driven at a speed of 20m/s when brakes are applied to bring it to rest in five seconds. The retardation will be * 1 point a) +4m/s^2 b) -4m/s^2 c) +0.25m/s^2 d) -0.25m/s^2

Answer 1

Given :

➳ Initial speed = 20m/s

➳ Final speed = zero (i.e., rest)

➳ Time interval = 5s

To Find :

➛ Retardation of motorcycle.

SoluTion :

➢ Applied brakes produces retardation in motorcycle.

➢ Here, Force due to brakes is constant, so it produces constant retardation in motorcycle therefore we can apply equation of kinematics to solve this question.

✴ First equation of kinematics :

• v = u + at

where,

v denotes final speed

u denotes initial speed

a denotes acceleration

t denotes time

⇒ v = u + at

⇒ 0 = 20 + a(5)

⇒ 5a = -20

⇒ a = -20/5

⇒ a = -4 m/s²

[Note : Negative sign shows retardation.]

Answer 2

Given ,

Initial velocity (u) = 20 m/s

Final velocity (v) = 0 m/s

Time (t) = 5 sec

We know that , the acceleration of a body is given by

$\boxed{ \sf{a = \frac{v - u}{t}} }$

Thus ,

a = (0 - 20)/5

a = -20/5

a = -4 m/s²

[ The negative sign of acceleration indicates it is retardation ]

Therefore ,

• The retardation of motorcycle is -4 m/s²