Viscous drag and Stokes' lawEdexcel A-Level Physics: Mind map

Exam questions on Viscous drag and Stokes' law

1. A steel ball bearing of radius 1.5 mm moves slowly through glycerol of viscosity 1.4 Pa s. The flow around the ball is laminar.
   The ball is released from rest in a deep tank of glycerol. Explain why it eventually falls at a constant speed.2 marks
2. A student drops identical steel balls through a tall column of oil and measures the terminal velocity. At 20 °C the terminal velocity is 0.048 m s⁻¹ and at 50 °C it is 0.31 m s⁻¹. Assume that the density of the oil is the same at both temperatures and that Stokes' law applies.
   Explain why the balls reach a higher terminal velocity in the oil at 50 °C.2 marks
3. A steel ball of radius 2.0 mm and density 7800 kg m⁻³ falls at its terminal velocity of 0.085 m s⁻¹ through an oil of density 920 kg m⁻³. The flow is laminar. Take g = 9.81 N kg⁻¹. The volume of a sphere is 4πr³/3.
   Calculate the weight of the ball minus the upthrust on the ball.3 marks