A small metal sphere is falling through castor oil. Which of these forces change? why?

A small metal sphere is falling through castor oil. Name the forces acting on the metal sphere.

A small metal sphere is falling through castor oil. Name the velocity of the sphere when the unbalanced force on it is zero

A small metal sphere is falling through castor oil. Write down the expression for terminal velocity in terms of coefficient of viscosity.

When a metallic sphere falls through castor oil,its velocity becomes uniform,called terminal velocity. Rain drops falling under gravity do not acquire very high velocity.Why?

Interstitial compounds are formed when small atoms are trapped inside the crystal lattice of metals. Which of the following is not the characteristic property of interstitial compounds?