An object weighs 315 N in air. When tied to a string connected to a balance and immersed in water,?

Question

it weighs 265 N. When it is immersed inoil,it weighs 269 N. Find:
A. the density of the object
B.the density of the oil

I need to know how you did this because it is a example for my test tomorrow! another problem just like this will be used but of course with different numbers! Thanks!

Answer 1

Water is density 1 Air is very near density 0.
an object of density 1 would have had the weight of 50Newtons
The ratio of the actual weight and the weight of an equivalent object of density one is equivalent to the definition of density

315/50=6.03
The answer to a is 6.03

the weight of the equivalent volume of oil is 4 newtons less than the water.
46/50=.92
the answer to b is .92

Occasionally grams or kilograms of mass are used instead of newtons. as long as one is consistent the math works the same. If there is a mixing of mass and force then you need to know how to convert to have just mass or just force. a Newton is a kilogram meter/second/second. The acceleration due to gravity is just about 10 meters/second/second. The number of newtons is 10 times the number of kilograms. English units of measure make the computations more complex in a mixed mode but if one is consistent then the math would work in pounds as well.

Answer 2

Let W(air) = weight in air, W(water) = weight in water, W(oil) = weight in oil.

W(air) = W(obj) - V(obj)*dens(air)
W(oil) = W(obj) - V(obj)*dens(water)

You can look up dens(air) (it may be small enough to ingore). The only unknowns are V(obj) and W(obj), so you can solve for those. You can get the dens(obj) from W(obj)/V(obj).

W(oil) = W(obj) - V(obj)*dens(oil) You know W(oil) and V(obj) and W(obj) so calculate dens(oil)