The diagram shows an experiment to measure the speed of a small ball falling at constant speed through a clear liquid in a glass tube.
There are two marks on the tube. The top mark is positioned at 
on the adjacent rule and the lower mark at 
. The ball passes the top mark at 
and passes the lower mark at 
.
The constant speed of the ball is calculated by 
.
Which expression calculates the fractional uncertainty in the value of this speed?
