# Chapter 13 - Worked Solutions

### to questions from the OUP text Senior Physics - Concepts in Context by Walding, Rapkins and Rossiter

My thanks to Peter Finch from St Joseph's College, Gregory Terrace, for these (pdf) solutions.

Solutions-Ch-13-Finch.pdf

The rest are by Greg Rapkins:

Q37.

The sound that is reflected from the stand has to travel 300 m further then the other sound.
Therefore
v = d/t = 300m /0.9 s = 333 m s-1

Q38.

In general: L = nl/4, where n = 1, 2, 3, 4, 5, .......

Q39.
Sound is reflected from the fish and from the bottom.
Since the sound has traveled to the fish (and bottom) and back. It took 0.1 s to reach the fish and 0.125 s to reach the bottom. That is the sound took 0.025 s to travel from the fish to the bottom.
v = d/t
d = v.t
= 1450 x .025 = 36.25 m
the fish are 36 m from the bottom.

Q41.

Solutions-Ch13-Q41-Finch.pdf
(a)
Points A, B, C, are points on the spring and therefore are moving outwards or inwards.
When the wave has gone 1 m further on point A is the same distance form the equilibrium.
Point B will have moved up to about the position o the 18 cm mark - a change of 38 cm.
Point c has moved soen to the -40 cm mark - a change of about -20 cm .
Therefore B has moved the greatest distance in the same time and therefore has the greatest velocity.
(b)
The velocity of the wave is 5 m s-1 and therefore goes 1 m in 0.2 of a second. When the wave has gone 1 m, C has moved down 20 cm.
Speed = distance / time = 20 cm/0.2 s = 100 cm s-1 = 1 m s-1.
(c)

(d)
Points have to instantaneously change direction.

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