A girl shouts in front of a cliff and hears the echo 2.5 s later…

Contributed by StemPy Lab

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

A girl shouts in front of a cliff and hears the echo $2.5\ \text{s}$ later. How far is the cliff? (Speed of sound $340\ \text{m/s}$ )

Need a reminder? The key ideaShow

An echo travels to the reflector and back: $d = \dfrac{vt}{2}$ . A separate echo needs at least $0.1\ \text{s}$ (a wall at least $17\ \text{m}$ away in air). SONAR uses ultrasound in sea water, about $1500\ \text{m/s}$ .

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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