(a) Explain the following terms: (i) viscosity; (ii) terminal velocity. (b)(i) Describe an experiment to determine the terminal velocity of a steel ball fal...

Answer Details

(a)(i) Viscosity is the property of a fluid by which it opposes relative motion between its adjacent layers; it is a kind of internal friction in the fluid.

(ii) Terminal velocity is the constant (maximum) velocity attained by a body falling through a fluid when the net force on it is zero, i.e. when its weight is balanced by the upthrust plus the viscous drag.

(b)(i) Experiment (terminal velocity of a steel ball in glycerine):

• Fill a tall glass jar with glycerine and fix two rubber bands (marks) round it, well below the surface and a measured distance \(d\) apart.
• Gently release a small steel ball into the centre of the glycerine and let it fall.
• Using a stopwatch, time the ball \((t)\) as it travels between the two marks. In that lower region the velocity is constant.
• Terminal velocity \( v = \dfrac{d}{t} \). Repeat and average.

(ii) Two precautions: release the ball centrally so it falls far from the walls of the jar; take timings only in the lower region where the velocity has become steady, and avoid parallax when reading the marks.

(c)(i) Two effects of viscosity: it opposes/retards the motion of bodies moving through a fluid (drag); it reduces the rate of flow of the fluid and generates heat.

(ii) Two applications of viscosity: lubrication of machine parts by oils; use of viscous fluids in shock absorbers (dashpots) and hydraulic brakes.