# Worksheet: Stokes' Law

In this worksheet, we will practice calculating the friction force acting on a sphere as it travels through a viscous fluid.

**Q1: **

What is the magnitude of the force due to fluid friction exerted on a sphere of radius 15 cm that moves horizontally at 15 cm/s through water of dynamic viscosity Pa⋅s?

**Q2: **

A sphere of radius 12 cm moves horizontally at 20 cm/s through a liquid that produces a fluid friction force of magnitude 240 μN. What is the dynamic viscosity of the liquid?

- A Pa⋅s
- B Pa⋅s
- C Pa⋅s
- D Pa⋅s
- E Pa⋅s

**Q3: **

Which of the following formulas correctly relates , the fluid friction force; , the dynamic viscosity; , the radius of a sphere; and , the velocity with which the sphere moves through the fluid?

- A
- B
- C
- D
- E

**Q4: **

A solid sphere moves vertically downward through water at a constant velocity , as shown in the diagram. The forces acting on the sphere are its weight, , the upthrust from the water, , and the fluid friction due to the water, . Which of the following equations correctly shows the relation of these forces to each other?

- A
- B
- C
- D
- E

**Q5: **

A solid sphere of radius 1.5 cm
and density 1,050 kg/m^{3}
moves vertically downward through a liquid at a constant velocity , as shown in the diagram. The density of the liquid is 975 kg/m^{3}.
Find the dynamic viscosity of the liquid.

**Q6: **

A sphere of radius 7.5 cm and
density 1,500 kg/m^{3} moves
horizontally through water of dynamic viscosity
Pa⋅s. The
sphere is decelerating at m/s^{2}.
What is the magnitude
of the velocity of the sphere?

**Q7: **

A solid sphere of radius 1.1 cm and
density 1,095 kg/m^{3}
moves vertically downward through a liquid of dynamic viscosity
Pa⋅s
and density 1,025 kg/m^{3},
as shown in the diagram. Find the velocity of the sphere.

**Q8: **

A solid sphere of radius 6.200 mm moves vertically downward at a constant velocity cm/s
through a liquid of dynamic viscosity Pa⋅s and density
1,025 kg/m^{3}, as shown in the diagram. Find the density of the sphere.

**Q9: **

A positively charged solid sphere is placed within a fluid. The fluid is within a
cylindrical container that has oppositely charged circular faces, as shown in the diagram.
The sphere moves vertically upward with a constant velocity
cm/s. The sphere has a radius of 2.2 cm and a density of 1,500 kg/m^{3}. The fluid has a density of 1,450 kg/m^{3} and a dynamic viscosity
of Pa⋅s. What is the ratio of the magnitude of the electric force
acting on the sphere to the magnitude of the drag force acting on the sphere?