A tall cylinder is filled with viscous oil. A round pebble is dropped from the top with zero initial velocity. From the plot shown in the figure, indicate the one that represents the velocity \((v)\) of the pebble as a function of time \((t).\)
 

1. 2.
3. 4.
(c) Hint: Use the concept of Stoke's law.
Step 1: Find the variation of velocity with time t.
When the pebble is falling through the viscous oil the viscous force is
                  F=6πηrv
where r is the radius of the pebble, v is instantaneous speed, η is coefficient of viscosity. As the force is variable, hence acceleration is also variable so the v-t graph will not be a straight line. First, the velocity increases and then becomes constant known as terminal velocity.