Calculating Specific Gravity in Water

[davidatwayne]

Homework Statement

A sample of an unkown material weighs 200 N in air and 150 N when immersed in water. What is the specific gravity of this material?

Homework Equations

specific gravity = (density/density of water)
density of water is 1000 kg/m^3

The Attempt at a Solution

I don't even know where to start. All my textbook says is the specific gravity of a substance is the ratio of its density to the density of the water at 4 degrees C. I don't know how to relate this to an equation or begin to solve it.

 

[AlephZero]

Hint: use Archimedes' Principle (or the Law of Buoyancy - a different name for the same thing)

 

[ShawnD]

The key is to solve one thing at a time and not try to make one big impossible equation.

The volume of this thing is related to 50N of water (200 - 150)
50 = (v)(d)(g)
50 = (v)(1)(9.8)
v = 5.102 litres i think

200 = (5.102)(d)(9.8)
d = ~4

You can also do it as an expression in one shot. The in-water weight is 150, which is related to (d-1), and your normal weight affected by d alone is 200. Put the stand alone conditions as the numerator on each side and the in-water as the denom.:
200/150 = d/(d-1)
d = 4

 

[davidatwayne]

Thanks a lot guys, it makes a lot more sense now.