Experiment to determine the viscosity of motor oil using the method of falling sphere?

in the above experiment,why is that the sphere should be cleaned.also what would the presence of (a)temperature variation and(b) bubbles in the oil do to the accuracy of the experiment. Also for the purpose of the experiment,what complication is caused by a tube of finite dimensions being used to contain the oil instead of using a container of finite dimension.Experiment to determine the viscosity of motor oil using the method of falling sphere?
Particles on the ball add additional friction.





Temperature may change the viscosity e.g.; molasses.





Bubbles change (reduce) the viscosity of the fluid.


A lot of bubbles will also introduce the possibility that the ball will follow the path of least resistance and not fall straight down. This will screw up your measurements because the distance traveled by the ball will not be a constant factor.


.Experiment to determine the viscosity of motor oil using the method of falling sphere?
The falling sphere is acted upon by a constant force (gravity) and viscous friction. If the sphere is not clean you cannot use it as a standard because there is no standard dirt. In most liquids, viscous friction varies with temperature, so the speed of falling will vary. It should be obvious that falling through air is faster than falling through the liquid, so bubbles will cause the sphere to fall faster.


When the sphere falls in a tube, the liquid has to move from below the sphere to above it. This is work which will be subtracted from the energy of the falling sphere thus slowing it. In an infinite container the liquid is pushed aside which is less work.