30. (a) The pressure (including the contribution from the atmosphere) at a depth of htop= L/2 (corresponding to the top of the block) is 5325topatmtop1.01 10 Pa(1030 kg/m )(9.8 m/s )(0.300 m)1.0410 Pappghρ=+=×+=×where the unit Pa (Pascal) is equivalent to N/m2. The force on the top surface (of area A= L2= 0.36 m2) isFtop= ptopA= 3.75 ×104N. (b) The pressure at a depth of hbot= 3L/2 (that of the bottom of the block) is 25botatmbot(1030 kg/m )(9.8 m/s )(0.900 m) 1.1010 Pagh=×+=×where we recall that the unit Pa (Pascal) is equivalent to N/m2. The force on the bottom surface isFbot= pbotA= 3.96 ×104N. (c) Taking the difference Fbot– Ftopcancels the contribution from the atmosphere
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