# physics

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a 100 kg boulder is placed under the surface of water. its volume is 0.5 x 10^5 cm^3. what is the weight of the rock while under water; compare that to the weight of the rock in air (sitting on the ground)

i just found the weight of the rock in air by multiplying 100 kg by 9.8 and got 980 kg. but i don't know what to do next

• physics -

No. The question is what is the weight of the rock as compared to in air.

The weight of the rock under water is

Wuw= mg-volume*densitywater*g

The weight of the rock in air is mg

comparing them..

wuw/Wair= (mg-volume*densitywater*g)/mg
= 1-volume*densitywater/m)

but m is volume*densityrock
Wuw/Wair= 1-densitywater/densityrock

check that math

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