Give reason for the following:
(a) Machine parts get jammed in the winters.
(b) A constant driving force is always required for the maintenance of the flow of oil through pipe lines in the refineries.
(c) Velocity of water in a river is less near the banks and more in the middle.

Explanation

(a) Due to decrease in temperature during winters, the coefficient of viscosity of the lubricating oil becomes more, which results in thickening of the oil. So, machine parts get jammed.
(b) This is due to presence of viscous force between the layers of the oil, which constantly retards the motion of the oil. So, a constant driving force is required for maintaining the flow of oil through pipe lines. (c) There exists a force between the stream of water and bank of the river. So, velocity of water is less near the banks.