The lower end of a long plastic straw is immersed below the surface of the water in a plastic cup. An average person sucking on the upper end of the straw can pull water into the straw to a vertical height of 1.1 m above the surface of the water in the cup.

(a) What is the lowest gauge pressure that the average person can achieve inside his lungs?

(b) Explain why your answer in part (a) is negative.

a) - 10780 Pa

Explanation:

b) Initially the pressure inside lungs and atmospheric pressure are equal. When the gauge pressure of the lungs becomes negative, its pressure becomes less than the atmospheric pressure. It creates a pressure difference between the pressure acting on the open surface of water and surface of water inside the pipe which is connected to the lungs. This difference pulls the water inside the pipe.

Pressure difference = gauze pressure of lungs = h d g = 1.1 x 1000 x 9.8

= 10780 Pa.