Density, upthrust and viscosity

Sample questions

Original practice questions written in the style of the exam.

1. Question 13 marks

   (a) Describe the difference between laminar flow and turbulent flow.

   (b) Engine oil is much easier to pour when it is hot. State how the viscosity of a liquid changes with temperature.

   Mark scheme

   • Laminar flow: layers of fluid move smoothly past each other with no mixing; the velocity at any point is constant (1)
   • Turbulent flow: the fluid mixes, forming eddies; velocity at a point changes in magnitude and direction (1)
   • The viscosity of a liquid decreases as its temperature increases (1)
2. Question 25 marks

   A steel ball bearing of radius $1.5\,\text{mm}$ falls through glycerol and reaches terminal velocity.

   Quantity	Value
   density of steel	$7800\,\text{kg m}^{-3}$
   density of glycerol	$1260\,\text{kg m}^{-3}$
   viscosity of glycerol	$1.4\,\text{Pa s}$

   Calculate the terminal velocity of the ball bearing.

   Mark scheme

   • Volume $V = \tfrac{4}{3}\pi r^3 = 1.41 \times 10^{-8}\,\text{m}^3$ (1)