$(a)$ Briefly mention three uses of ultrasound in the field of medicine.
$(b)$ $A$ ship which is stationary is at a distance of $2800 \ m$ from the sea-bed. The ship sends an ultrasound signal to the sea-bed and its echo is heard after $4 \ s$. Find the speed of sound in water.

(N/A) Three uses of ultrasound in medicine are:
$(i)$ Echocardiography: Ultrasonic waves are reflected from various parts of the heart to form an image of the heart.
$(ii)$ Ultrasound scanning: It is used to obtain images of internal organs like the liver,gall bladder,uterus,and kidneys to detect abnormalities like stones or tumours.
$(iii)$ Prenatal examination: Ultrasonography is used during pregnancy to monitor the growth of the foetus and detect congenital defects.
$(b)$ Given: Distance $(S) = 2800 \ m$,Time $(t) = 4 \ s$.
The sound travels to the sea-bed and back,so the total distance covered is $2S$.
Speed $(V) = \frac{\text{Total Distance}}{\text{Total Time}} = \frac{2S}{t} = \frac{2 \times 2800 \ m}{4 \ s} = 1400 \ m/s$.
Thus,the speed of sound in water is $1400 \ m/s$.