# Homework Help: Calculating viscosity of a liquid with a falling object

1. Jun 1, 2017

### Beyar

1. The problem statement, all variables and given/known data

A small steel-bearing falls 25.0 cm in glycerol in 23.8 s and the same distance in castor oil in 15.1 s. The densities are for glycerol 1260 kg m−3 , for castor oil 961 kg m−3 , and for steel 7830 kg m−3 . The viscosity for glycerol is 1.490 Pa s. Calculate the viscosity for castor oil. All values are valid for 20 ◦C.

2. Relevant equations
I guess Viscosity=Density*Velocity where the velocity is equal to Distance/time.

3. The attempt at a solution
Thought I'd put the equation for the distance equal to each other and then rewrite it to get the viscosity of the castor oil, but I get the wrong value. It should be 0,988 Pas. I get the final equation to:
Viscosity of Glycerol= (Density of Castor Oil*Viscosity of Castor*Time the steel bearing fell in the Glycerol)/(Density of Glycerol * time steel bearing fell in Castor oil)

The equation has not regarded the denisty of the steel-bearing though, so that might be the problem but I don't see how I would get around to fit it in.
It is very frustrating.

Last edited by a moderator: Jun 1, 2017
2. Jun 1, 2017

### BvU

Well then, could it be that the net driving force for falling is a density difference ?

3. Jun 1, 2017

4. Jun 2, 2017

5. Jun 2, 2017

### BvU

Call it $R$, cross your fingers and hope it divides out in the answer