A Level Physics Quiz. Materials, Fluids: Viscosity

What is the unit of viscosity η in the SI system?

• Pascal-second

• Newton-meter

• Joule

• Watt

Answer explanation

Pascal-second as it is the SI unit of dynamic viscosity. Same as N s m^-2.

What force is responsible for an object's buoyancy when submerged in a fluid?

• Gravitational Force

• Electrostatic Force

• Magnetic Force

• Upthrust

Answer explanation

Upthrust, also known as buoyancy, is the force that pushes an object up when it's submerged in a fluid.

Which of the following objects would experience the most viscous drag when moving through the same fluid at the same speed?

• A small pebble

• A large pebble

• A feather

• They would all experience the same viscous drag

Answer explanation

A large pebble, as viscous drag, or drag force due to viscosity, directly depends on the object's size. The larger the object, the more viscous drag it experiences.

Stokes' Law relates to which three quantities?

• Diameter of the sphere, viscosity of the fluid, and terminal velocity.

• Diameter of the sphere, pressure, and terminal velocity.

• Pressure, viscosity of the fluid, and density of the fluid.

• Terminal velocity, density of the fluid, and temperature.

Answer explanation

Stokes' Law directly relates the diameter (radius) of a small sphere moving through a viscous liquid, the viscosity of the liquid, and the terminal velocity the sphere reaches.

Thixotropy is a characteristic of certain types of fluids that...

• ...solidify when pressure is applied.

• ...become less viscous when disturbed or agitated.

• ...become more viscous when disturbed or agitated.

• ...have constant viscosity despite being disturbed.

Answer explanation

Thixotropic fluids are those that experience a decrease in viscosity when subjected to shear stress or agitation, and recover their original viscosity when at rest.

What is the upthrust experienced by a body immersed in a fluid dependent on?

• the density of the fluid

• the volume of the fluid displaced

• the acceleration due to gravity

• all of the above

Answer explanation

The upthrust (or buoyant force) experienced by an object in a fluid depends on the density of that fluid, the volume of fluid displaced by the object, and the acceleration due to gravity.

According to Stokes' law, if the radius of a small spherical object moving in a viscous medium doubles, how will this affect the viscosity η?

• It will halve.

• It will double.

• It will quadruple.

• It will remain the same.

Answer explanation

Stokes' law states that the drag force is directly proportional to the radius squared. Therefore, if the radius doubles, the drag force quadruples.

How does an increase in temperature generally affect the viscosity of a liquid?

• increases viscosity

• decreases viscosity

• no effect on viscosity

• depends on the type of liquid

Answer explanation

Generally, an increase in temperature decreases the viscosity in a liquid as the particles gain more kinetic energy and can move more freely.

How does speed affect viscous drag on an object moving through a fluid?

• viscous drag increases as speed increases

• viscous drag decreases as speed increases

• viscous drag remains constant regardless of speed

• viscous drag is not related to speed

Answer explanation

Viscous drag or the resistance faced by an object moving in a fluid increases as the speed of the object increases.

Archimedes' principle is closely related to which concept in fluid mechanics?

• viscosity

• stokes' law

• thixotropy

• upthrust

Answer explanation

Archimedes' principle states that the upward buoyant force (upthrust) exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces.