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Dropping ball-bearing of various diameters into a viscous liquid, glycerol, timing the fall between two markers that are a known distance apart.

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Asheeth Govindia


Physics : Data Analysis


In this experiment I dropped ball-bearing of various diameters into a viscous liquid, glycerol, timing the fall between two markers that are a known distance apart.

I will then be looking for a relationship between the velocity of descent and the diameter.



  1. I measured the diameter of the ball bearing.
  2. I dropped a steel ball bearing down the pipe filled with glycerol.
  3. As the ball bearing passed the first marker I started the stop clock.
  4. As the ball bearing passed the second marker I stopped the stop clock.
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Data Analysis

Stokes Law states that spheres falling through a fluid exhibit the following relationship.


Viscosity – any object moving through a viscous fluid is acted on by friction due to the fluid.  A higher viscosity will increase this friction that opposes its motion.

Calculating Viscosity

The force needed to separate molecules of the fluid according to Stokes is

Eq. (1) -> F = 6(pi)Rnvc,

Where R is the radius of the sphere, n is the viscosity of the fluid, and vc is the velocity of the sphere through the infinite fluid. This force can be set equal to the gravitational force modified to account for the buoyant effect as follows

Eq. (2) -> 6 (pi) R n vc = 4/3 (pi) R3 (pS-pL) g,

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Eq. (7) -> n = [2 g R2 (pS-pL) t] / [9L (1 + 2.4x) (1 + 1.65y)].

Viscosity of glycerol

2 x 9.8 x 0.0022 x (8.02 – 1.26) x 14.07 = 2.628 poise

9 x 0.515

Resultant Force = W – Fr – u

W = mg

W = 4/3πr3 x density of steel x g

u = 4/3πr3σg

At terminal velocity:

W – Fr – u = 0

Because there is therefore no acceleration


From my results I concluded that as the diameter of the ball increases the speed of the ball increases, but so does the drag. This means that the force pulling the ball the ball is greater than the frictional force pushing the ball upwards. So as the ball’s diameter increases the drag increased but still the speed at which it falls through the liquid increases.






Stokes Law



Calculating Viscosity


...read more.

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