When a foreign object lodged in the trachea (windpipe) forces a person to cough, the diaphragm thrusts upward causing an increase in pressure in the lungs. This is accompanied by a contraction of the trachea, making a narrower channel for the expelled air to flow through. For a given amount of air to escape in a fixed time, it must move faster through the narrower channel than the wider one. The greater the velocity of the air- stream, the greater the force on the foreign object. X rays show that the radius of the circular tracheal tube contracts to about two-thirds of its normal radius during a cough. According to a mathematical model of coughing, the velocity v of the air- stream is related to the radius r of the trachea by the equation

v ( r ) = k ( r 0 − r ) r 2     1 2 r 0 ≤ r ≤ r 0

where k is a constant and r₀ is the normal radius of the trachea. The restriction on r is due to the fact that the tracheal wall stiffens under pressure and a contraction greater than 1 2 r 0 is prevented (otherwise the person would suffocate).

(a) Determine the value of r in the interval [ 1 2 r 0 , r 0 ] at which v has an absolute maximum. How does this compare with experimental evidence?

(b) What is the absolute maximum value of v on the interval?

(c) Sketch the graph of v on the interval [ 0 ,   r 0 ] .