A sound wave with a frequency of 510 Hz and a wavelength of 3.5 m is directed toward the bottom of a lake to measure its depth. If the echo of the sound from the bottom is heard 0.39 seconds later, how deep is the lake?

Approximately .

Explanation:

In that , this sound wave travelled from the surface of the lake to the bottom, got reflected, and travelled back from the bottom to the surface. The sound wave travelled from the surface to the bottom (without bouncing back) in only that much time. In other words, it took only for the sound wave to travel from the surface to the bottom of the lake.  

The speed of sound in cold water (, ) is approximately .

In , that sound wave would have travelled a distance of:

.

Therefore, the depth of the lake is approximately .

a.metals can become magnetic if they have many spinning electrons that create magnetic fields.d.only some metals are attracted by magnets; these metals are called ferromagnetic.

d.wrapping an insulated wire around a metal with ferromagnetic properties and applying an electric current.