Fluids

Select the correct definition of fluid.

Fluid is a medium that can flow. Generally, fluid means gas or liquid, but some solids made up with small particles can behave like fluids (for example, sand in an hourglass).

Archimedes’ Law

Select the correct definition of upthrust.

Upthrust is the upward force exerted on an object immersed in a fluid, equal to the weight of the fluid displaced.

Archimedes’ Law - 2

Select the correct statement of Archimedes’s Law.

Archimedes's principle indicates that the upward force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and it acts in the upward direction at the centre of mass of the displaced fluid.

Archimedes’ Law - 3

A boy dropped a ball with a radius of 10cm into a pond. Calculate the upthrust  force acting on it when it is completely submerged.

$$W = m\times g = V\times \rho \times g= {4\over3} \pi \times R^3\times \rho \times g={4\over3} \pi \times 0.1^3\times 1000 \times 9.8\approx 41.1N$$

Flow

Select the correct definition of laminar flow.

Laminar flow occurs when a fluid flows in parallel layers, with no disruption between the layers.

Flow - 2

Select the correct definition of turbulent flow.

Turbulent flow is a flow regime characterized by chaotic property changes. The velocity of the fluid is never constant during tubular flow.

Flow - 3

Which type of fluid flow is shown on the picture?

The fluid flow on the image is chaotic, no laminar fluid flow with streamlines and layers of constant velocities can be distinguished.

Flow - 4

Which of the statements about the nature of streamlines are wrong?

Select the incorrect options.

Streamlines are the lines of laminar fluid flow. At any point on a streamline the velocity of the fluid will be constant over time.

Viscosity

Select the correct definition of viscosity.

Viscosity is the measure of the resistance of a fluid to flow.

Viscosity - 2

Select the correct equation for viscous drag on a small sphere at low speeds (Stokes' law).

F is the viscous drag

r is the radius of the sphere (m)

v is the velocity of the spere (m/s)

$$\nu$$ is the coefficient of viscosity of the fluid (Pas)

The drag force of a sphere is proportional to its velocity and radius. $$F = 6 \pi r\nu v$$.

Viscosity - 3

Select the correct definition of the coefficient of viscosity.

A measure of the resistance to flow of a fluid.

Viscosity - 4

How is the viscosity of the fluid related to the ambient temperature?

The viscosity of the fluid decreases with increasing ambient temperature.

Density

Calculate the density of a pine bar with a mass of 6kg, a length of 1m, a width of 20cm and a depth of 10cm.

$$m=\rho\times V$$, therefore,. $$\rho = m/V={6\over1\times 0.2\times 0.1}=300kg/m^3$$

Archimedes’ Law - 4

A golden ring with a mass of 10g was dropped into a vessel with mercury. Calculate the upthrust  force acting on the ring.

Archimedes' rule: $$F =\rho \times g\times V$$, where $$\rho$$ is the density of mercury and V is the volume of the ring.
$$V = m/{\rho_{gold}}={10g\over19.3g/cm^3}\approx0.52cm^3$$
$$F =13.5\times 9.8 \times 0.52\times 0.001\approx0.069N$$