by sucking through a straw a student can reduce the pressure in his lungs to 750mm of mercury density=13.6gm/cm3 using the straw he can drink water from a glass upto a maximum depth of

a) 10cm b) 75cm 3) 13.6cm 4)1.36cm

Normal pressure is 760mmHg so the student can create a depression of 10 mmHg in his lungs. According to the law of floatation:weight of floating body = weight of displaced liquid.

here also wt. of water displaced from glass = wt. of air of lung reduced( interm of mmHg) 

wt. of water displaced from glass = density of water (d1) * volume of water displaced (v1)

wt. of air reduce from lung ( interm of mmHg) = density of Hg (d2) * volume of air displaced (v2)

Now d1 * v1 = d2 * v2

and v1 = d2 * v2 / d1 = 13.6gcm-3 * 10mm / 1gcm-3 = 136mm = 13.6cm 

Note : density of water = 1gcm-3

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