# A wave of period 5.0 ms travels through a medium. The graph shows the variation with distance d of the displacement x of points in the medium. What is the average speed of a point in the medium during one full oscillation?

## a) 0m/s b) 4m/s c)16m/s d) 400m/s The answer is C, but why is it not D?

The point moves from $- 2$ $c m$ to $+ 2$ $c m$ and then back to $- 2$ $c m$ (for example) in one oscillation, a total distance of $8$ $c m$ or $0.08$ $m$.
The period is $5.0$ $m s$ = $5.0 \times {10}^{-} 3$ $s$
The average speed is just ${v}_{a v} = \frac{d}{t} = \frac{0.08}{5 \times {10}^{-} 3} = 16$ $m {s}^{-} 1$