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A motor-car is uniformity retarded and brought to rest from a speed of 90km/h in ...


Question

A motor-car is uniformity retarded and brought to rest from a speed of 90km/h in 10s. Find its acceleration.

Options

A) 2.5 ms\(^{-1}\)

B) 2.5 ms\(^{-2}\)

C) -2.5 ms\(^{-2}\)

D) -2.5 ms\(^{-1}\)

The correct answer is B.

Explanation:

\(u = \cfrac{90\text{km}}{h} = \cfrac{90000}{3600}\text{ms}^{-1} = 25\text{ms}^{-1}\)
\(t=10\text{s}\)
\(v=u+at\)
\(0=25+a(10)\)
\(-25=10a\)
\(a=\cfrac{-25}{10}=-2.5\)m/s\(^2\)
Acceleration = -(-2.5m/s\(^2\)) = 2.5m/s\(^2\)

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Dicussion (1)

  • \(u = \cfrac{90\text{km}}{h} = \cfrac{90000}{3600}\text{ms}^{-1} = 25\text{ms}^{-1}\)
    \(t=10\text{s}\)
    \(v=u+at\)
    \(0=25+a(10)\)
    \(-25=10a\)
    \(a=\cfrac{-25}{10}=-2.5\)m/s\(^2\)
    Acceleration = -(-2.5m/s\(^2\)) = 2.5m/s\(^2\)