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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.
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Solution
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.
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