Asked by john

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
Answered by bobpursley
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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