Question

Suppose the glass of the previous problem is covered by a jar and the air inside the jar is completely pumped out.

(a) What will be the answers to the problem ?

(b) Show that the answers do not change if a glass of different shape is used provided the height, the bottom area and the volume are unchanged.

Answer 1

If the glass will be covered by a jar and the air is pumped out, the atmospheric pressure has no effect. So,

(a) Force exerted on the bottom

= $\left(h{p}_{w}g\right)\times A$

= $(20\times {10}^{-2}\times {10}^{-3}\times 10)20\times {10}^{-4}$

= 4 N

(b) mg = $h\times {\rho }_w\times g\times A+F_a$

$\Rightarrow F_a=5-4=1N$

(c) If glasses of different shapes used have the same volume,, height and area no change in answer will occur.