3A7 Basic Fluid Mechanics

Question 1

What are the main characteristics of a fluid?

Answer 1

• No fixed shape
• Can flow
• Particles move freely

A fluid includes liquids and gases. Key properties of fluids include viscosity, density, compressibility, and pressure.

Question 2

Define

Fluid mechanics

Answer 2

Branch of physics that studies the behavior of fluids (liquids and gases) in motion and at rest.

Fluid mechanics combines principles of mechanics, thermodynamics, and material science.

Question 3

True or False

Both liquids and gases are considered fluids.

Answer 3

True

Fluids are substances that flow and adapt to the shape of their container.

Question 4

Fill in the blanks:

Fluid ______ studies fluids at rest, while fluid ______ studies fluids in motion.

Answer 4

statics; dynamics

Fluid statics focuses on pressure and buoyancy, while dynamics examines flow behavior.

Question 5

Explain why fluid mechanics is important in engineering.

Answer 5

They help design systems like pipelines, hydraulic machines, and aircraft, ensuring they function efficiently and safely.

Applications include aerodynamics, hydraulics, and even weather prediction.

Question 6

True or False:

An object floats if its weight is less than the buoyant force.

Answer 6

True

Floating objects displace an amount of fluid equal to their weight.

Question 7

Fill in the blank:

The pressure exerted by a fluid increases with ______.

Answer 7

depth

Hydrostatic pressure increases due to the weight of the fluid above.

Question 8

Why does a dam have a thicker base than top?

Answer 8

To withstand higher pressure at greater depths.

Hydrostatic pressure is proportional to depth and density of the fluid.

Question 9

Define:

hydrostatic pressure

Answer 9

It is the pressure exerted by a fluid at rest due to its weight.

Calculated as P=ρgh, where ρ is density, g is gravity, and h is depth.

Question 10

What is the hydrostatic pressure equation?

Answer 10

P = ρgh

This equation can also be written as P = ρgd + Patm, where ρ is the density, g is the gravitational constant, and h is the depth.

Question 11

What is the SI unit for hydrostatic pressure?

Answer 11

Pascals (Pa), equivalent to N/m².

Pressure is defined as force divided by area.

Question 12

Define:

gauge pressure

Answer 12

The additional pressure exerted on an object or fluid above atmospheric pressure.

Absolute pressure is the sum of gauge pressure and atmospheric pressure.

Question 13

True or False:

Atmospheric pressure decreases with altitude.

Answer 13

True

Air density decreases at higher altitudes, reducing pressure.

Question 14

Define:

Pascal’s principle

Answer 14

Pressure applied to a confined fluid is transmitted equally in all directions.

This principle is the basis for hydraulic systems.

Question 15

What is the principle behind hydraulic brakes?

Answer 15

Pascal’s Principle

The principle states that pressure applied to a confined fluid is transmitted equally in all directions.This allows small forces, such as those present in brakes, to be amplified to stop a car.

Question 16

Fill in the blank:

A fluid is considered ______ if its density is constant.

Answer 16

incompressible

Most liquids are nearly incompressible under normal conditions.

Question 17

Define:

Surface tension

Answer 17

Cohesive force at the surface of a liquid allowing it to resist an external force.
## Footnote

It is responsible for phenomena like water droplets forming spherical shapes.

Question 18

Why do non-Newtonian fluids behave differently than Newtonian fluids?

Answer 18

Their viscosity changes with applied force.

Examples include ketchup, cornstarch mixtures, and toothpaste.

Question 19

True or False:

A fluid with higher density will exert more pressure at the same depth.

Answer 19

True

Pressure depends on both depth and fluid density.

Question 20

Define:

specific gravity

Answer 20

Ratio of the density of a substance to the density of a reference substance, usually water for liquids and solids.

Specific gravity is dimensionless because it compares two densities without involving units. The reference substance is usually water for a liquid or solid, and air for a gas.

Question 21

True or False:

A specific gravity greater than 1 means the object will float in water.

Answer 21

False

A specific gravity greater than 1 means the object will sink in water.

Floating occurs when specific gravity is less than 1, as the object is less dense than water.

Question 22

What does Archimedes’ principle state?

Answer 22

A body submerged in a fluid experiences an upward buoyant force equal to the weight of the displaced fluid.

This explains why ships float and balloons rise in air.

Question 23

Explain why oil floats on water.

Answer 23

Oil has a lower density than water.

Objects or substances with lower density than the fluid they are in will float.

Question 24

Fill in the blank:

Hydrostatic pressure depends on fluid ______, gravitational acceleration, and depth.

Answer 24

density

This relationship is fundamental in understanding underwater pressures.

Question 25

What factor influences whether an **object sinks or floats** due to buoyancy?

Answer 25

The object sinks when its weight exceeds the buoyant force. ## Footnote A stone sinks because its density is greater than water’s.

Question 26

# Define: Compressibility

Answer 26

Measure of how much a **substance’s volume decreases** under applied pressure. ## Footnote Gases are highly compressible, while liquids are nearly incompressible.

Question 27

Why is **pressure** independent of container **shape**?

Answer 27

Pressure is **transmitted uniformly in all direction**, so shape does not matter. ## Footnote Pressure depends only on fluid height, density, and gravity, but not shape.

Question 28

How does **atmospheric pressure** allow us to drink through a **straw**?

Answer 28

Removing air creates **lower pressure inside the straw**, and atmospheric pressure pushes the liquid up. ## Footnote This principle is related to **fluid statics and pressure gradients**.

Question 29

# Define: Ideal fluid

Answer 29

Fluid that **cannot be compressed** and has zero viscosity, meaning it flows without resistance. ## Footnote Ideal fluids do not exist in reality but are a useful concept in fluid dynamics.

Question 30

# Fill in the blank: The **ideal gas law** is written as \_\_\_\_\_\_.

Answer 30

PV=nRT ## Footnote It relates pressure, volume, temperature, and the number of moles of gas.

Question 31

# True or False: **Boyle’s Law** states that **pressure and volume** of a gas are inversely proportional at constant temperature.

Answer 31

True ## Footnote For a gas, P₁V₁ = P₂V₂ when the temperature is constant.

Question 32

Explain the relation between **Boyle’s Law** and **scuba diving**.

Answer 32

Boyle’s Law predicts how gas volume changes with pressure as divers descend or ascend. ## Footnote Divers must adjust to avoid lung overexpansion or compression injuries.

Question 33

# Define: Charles’s Law

Answer 33

**Volume** of a gas is **directly proportional to its temperature** at constant pressure. ## Footnote As temperature increases, gas particles move faster, causing expansion. This is represented by the equation where V₁/T₁ = V₂/T₂, with V₁ representing initial volume and V₂ representing final volume.

Question 34

How is **Charles’s Law** applied in hot air balloons?

Answer 34

Heating the air inside a hot air balloon **increases its volume**, making it less dense than the surrounding air, allowing the balloon to rise.

Question 35

# Define: Gay-Lussac’s Law

Answer 35

**Pressure** of a gas is **directly proportional to its temperature** at constant volume. ## Footnote Increasing temperature causes gas particles to collide more forcefully, raising pressure. In the equation P/T = k, P represents pressure of the gas, T is the absolute temperature of the gas, and k is a constant.

Question 36

How does **Gay-Lussac’s Law** explain the risk of aerosol cans in **high heat**?

Answer 36

When an aerosol can is exposed to heat, the gas pressure inside increases due to the rise in temperature, potentially causing it to explode. ## Footnote The relationship between pressure and temperature is crucial for understanding safety measures in pressurized containers.

Question 37

# Define: Flow rate

Answer 37

Volume of fluid **passing through a point** per unit time. ## Footnote Measured in units like m³/s or liters per second.

Question 38

What is the standard unit for volumetric **flow rate**?

Answer 38

Cubic meter per second (m³/s). ## Footnote This unit measures the volume of fluid passing through a cross-section per second.

Question 39

Explain the relationship between **flow rate** and **velocity**.

Answer 39

Flow rate is directly proportional to velocity; a higher velocity leads to a greater flow rate if the cross-sectional area is constant. ## Footnote Flow rate (Q) is calculated as Q=A⋅v, where A is the cross-sectional area and v is velocity.

Question 40

# Define: Streamline flow

Answer 40

Fluid particles moving along **smooth paths** without turbulence. ## Footnote Also called **laminar flow**, it occurs at lower velocities.

Question 41

How does **turbulence** occur in **fluid flow**?

Answer 41

It occurs when a fluid moves at **high velocities** or around obstacles, causing chaotic, irregular flow patterns instead of smooth, laminar flow. ## Footnote Turbulence is influenced by factors like velocity, fluid viscosity, and surface roughness, and it is characterized by swirling eddies and vortices.

Question 42

# Define: Viscosity

Answer 42

Measure of a fluid’s **resistance to flow**. ## Footnote Honey has high viscosity compared to water, which has low viscosity.

Question 43

What are the standard **units** used to measure **viscosity**?

Answer 43

* Pascal-seconds (Pa·s) in the SI system * Centipoise (cP) in the CGS system. ## Footnote 1 pascal-second (Pa·s) = 1000 centipoise (cP), and viscosity quantifies a fluid’s resistance to flow.

Question 44

What does **Bernoulli’s Principle** state?

Answer 44

In a fluid flow, as the velocity of the fluid increases, the **pressure within the fluid decreases**. ## Footnote This principle explains why **airplanes achieve lift** and how carburetors work.

Question 45

What assumptions are made in **Bernoulli's principle**?

Answer 45

1. The fluid is ideal. 2. The flow is steady and laminar. 3. The fluid is incompressible. 4. Viscosity effects are ignored. ## Footnote Laminar flow means that each particle of the fluid follows a smooth path called streamline.

Question 46

# Fill in the blank: In a **Venturi tube**, the _____-_________ area is narrowest where the velocity is highest.

Answer 46

Cross-sectional ## Footnote This demonstrates the inverse relationship between velocity and pressure.

Question 47

What physical quantities are combined in the **Bernoulli equation**?

Answer 47

It combines **pressure, kinetic energy per unit volume, and potential energy** per unit volume in a flowing fluid. ## Footnote The equation is written as P+1/2ρv²+ρgh=constant.

Question 48

# Fill in the blank: The **Continuity Equation** applies to fluids with constant \_\_\_\_\_\_.

Answer 48

density ## Footnote This condition is typically assumed for incompressible fluids.

Question 49

# Fill in the blank: The **continuity equation** describes the \_\_\_\_\_\_\_ \_\_\_ \_\_\_\_\_\_ in a **steady fluid flow**.

Answer 49

conservation of mass ## Footnote The Continuity Equation states that the product of cross-sectional area and velocity is constant along a streamline.

Question 50

What happens to the velocity of a fluid as the **cross-sectional area** decreases?

Answer 50

The **velocity increases** to maintain a constant flow rate. ## Footnote This follows from the **Continuity Equation, 𝐴1𝑣1=𝐴2𝑣2**

Question 51

Why is the **Continuity Equation** essential in **pipe design**?

Answer 51

It ensures that **flow rates are maintained** when pipe diameters change. ## Footnote Engineers use this principle to avoid pressure drops and inefficiencies.

Question 52

Why does a **shower curtain** move inward when the water is flowing?

Answer 52

The fast-moving water reduces air pressure inside the shower due to **Bernoulli’s Principle**. ## Footnote This is a real-world example of the relationship between velocity and pressure.

Question 53

# True or False: Faster-moving fluids exert higher lateral **pressure**.

Answer 53

False ## Footnote Faster-moving fluids exert lower lateral pressure according to Bernoulli’s Principle.

Question 54

How do **airplane wings** utilize Bernoulli's principle?

Answer 54

The wing shape causes **faster air above and slower air below**, creating lift. ## Footnote This pressure difference results in an upward net force on the wing.

Question 55

How does **Bernoulli's equation** explain the stability of a ball in a water jet?

Answer 55

Pressure differences due to **varying velocities of water** keep the ball in the stream. ## Footnote Faster water movement on one side creates lower pressure, pushing the ball back into the stream.

Question 56

What is the **Magnus effect**?

Answer 56

Phenomenon where a spinning object moving through a fluid **creates a pressure difference**, causing it to curve in its path. ## Footnote This effect is commonly observed in sports like soccer, baseball, and tennis, where spin influences ball trajectory.

Question 57

# True or False: The **Magnus effect** occurs because the spinning object alters the velocity of fluid around it, creating lift.

Answer 57

True ## Footnote The Magnus effect arises from the interaction between the object's spin and the surrounding fluid, causing **asymmetric pressure distribution**.