A fishing boat uses sonar to measure the depth of the sea.
The sonar sends a pulse of ultrasound down to the seabed and detects the echo.
The echo is detected 0.24 s after the pulse is sent.
The speed of ultrasound in sea water is 1500 m/s.
Calculate the depth of the sea below the boat.
The ultrasound has a frequency of 50 kHz50\ \text{kHz}50 kHz.
Calculate the wavelength of the ultrasound in sea water.
Give one reason why ultrasound is used rather than audible sound.
The boat moves to a new position. The echo now takes a longer time to return.
State what this shows about the sea at the new position.