Question

A toy car initially moving with uniform velocity of 18 km h^-1 comes to a stop in 2 s. Find the retardation of the car in S.I. units.

Answer 1

initial velocity (u) = 18 km/h^-1

final velocity (v) = 0 km/h

time (t) = 2 s

acceleration/retardation (a) = ?

To convert initial velocity u to m s^-1

18 km/h^-1 = `(18 " km")/(1 " h")`

= `(18 xx 1000 " m")/(60 xx 60 " s")`

⇒ 18 km/h^-1 = `(18 xx 10 " m")/(6 xx 6 " s")`

⇒ 18 km/h^-1 = 5 m s^-1

Acceleration (a) = `("Final Velocity "("v") - "Initial Velocity "("u"))/("time "("t"))`

a = `(0 - 5)/2` m s^-2

a = `-(5 "ms"^-1)/"2s"`

a = -2.5 m s^-2

Hence, acceleration of the body is -2.5 m s^-2. Negative sign shows that the velocity decreases with time, so retardation is 2.5 m s^-2.