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A particle moves along a semi circular path of radius \(r\) in time \(t\) with constant speed, then its

  • A.average speed is \(\frac{\pi r}{2t}\)
  • B.average velocity is \(\frac{r}{t}\)
  • C.displacement is \(2\pi r\)
  • D.distance travelled is \(2r\)
  • E.average acceleration is \(\frac{2\pi r}{t^{2}}\)

Answer: E

Its average acceleration is \(\frac{2\pi r}{t^{2}}\).

The speed is \(v = \frac{\pi r}{t}\) (half the circumference in time t). At the end of the semicircle the velocity has reversed, so the change in velocity has magnitude 2v.

\[ \begin{aligned} a_{avg} &= \frac{|\Delta \vec{v}|}{t} = \frac{2v}{t} \\ &= \frac{2\pi r}{t^{2}} \end{aligned} \]

The others are wrong: the average speed is \(\frac{\pi r}{t}\), not \(\frac{\pi r}{2t}\); the displacement is 2r (the diameter), so the average velocity is \(\frac{2r}{t}\); and the distance is \(\pi r\), not 2r.

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