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# hw help

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Joined: Mar 2008
hw help

A wooden plank 2.5m long, 40 cm long and 60 mm thick floats in water with 15 mm of its thickness above the surface. Calculate the plank's mass.

i don't know how to go abot solving this problem. any help would be awsome.

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Joined: Nov 2007

GOODSTUDENT;77230 wrote:A wooden plank 2.5m long, 40 cm long and 60 mm thick floats in water with 15 mm of its thickness above the surface. Calculate the plank's mass.

i don't know how to go abot solving this problem. any help would be awsome.

Use Buoyancy Force, set it equal to mg. Buoyancy force points upward while mg points downwards. We know that 15 mm is above water, so 60-15 is under water. Which means that 45/60 percent of the block is under water

set rho V g = rho V g (density of the plank on one side, the density of water on the other)

then f = V(of water)/V (wood) = rho (wood)/rho(water) = 45/60 (simplified)

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