physics
posted 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

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= mgvolume*densitywater*g
The weight of the rock in air is mg
comparing them..
wuw/Wair= (mgvolume*densitywater*g)/mg
= 1volume*densitywater/m)
but m is volume*densityrock
Wuw/Wair= 1densitywater/densityrock
check that math