Physics Chapter on Buoyancy and Pressure

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Questions and Answers

What is the definition of pressure in a fluid?

Force added to area

Force divided by area (correct)

Force multiplied by area

Area divided by force

What volume characteristic does pressure in a liquid depend on?

The volume of the liquid

The temperature of the liquid

The vertical height of the liquid column (correct)

The shape of the container

Which of the following correctly describes the relationship between temperature and gas pressure?

Higher temperature increases gas pressure (correct)

Temperature does not affect gas pressure

Higher temperature decreases gas pressure

Gas pressure is independent of the molecular motion

How does the compressibility of liquids compare to that of gases?

Liquids are less compressible than gases Signup and view all the answers

What does density measure about a substance?

The mass per unit volume of the substance Signup and view all the answers

Which of the following factors does NOT affect the pressure of a fluid in an open container?

Volume of the fluid Signup and view all the answers

In the formula for pressure, what does the symbol 'P' represent?

Pressure Signup and view all the answers

What causes gas pressure within a container?

The collisions of gas molecules with container walls Signup and view all the answers

What is the state of an object when the resultant of all forces acting on it is zero?

In equilibrium Signup and view all the answers

When resolving a tension force, which components are derived from it?

Horizontal and vertical components Signup and view all the answers

What occurs when a car is pushed at one end with a force that is exactly opposed by a similar force at the other end?

The car remains stationary Signup and view all the answers

What is the formula for calculating a load moment?

Load x Load arm Signup and view all the answers

Which statement is true regarding the balance of a lever system?

Effort moment equals load moment Signup and view all the answers

What is a primary function of a simple machine, like a lever?

To change the direction of a force Signup and view all the answers

In the context of equilibrium, what does it mean for scales to be balanced but not in equilibrium?

Forces are equal but still causing motion Signup and view all the answers

How does a smaller effort force contribute to moving a heavier load using a lever?

By moving through a larger arc distance Signup and view all the answers

What is the main purpose of a lever?

To perform work for a load to be lifted Signup and view all the answers

In a first-class lever, where is the fulcrum located in relation to the load and effort?

Between the load and effort Signup and view all the answers

What is the formula for calculating mechanical advantage (MA) in a lever?

MA = Load / Effort Signup and view all the answers

If the load (L) is 100 kg and the effort (E) is 10 kg, what is the mechanical advantage (MA)?

10 Signup and view all the answers

Which of the following is an example of a second-class lever?

Wheelbarrow Signup and view all the answers

What is meant by 'leverage' in the context of a first-class lever?

The ratio of distance moved by load to effort Signup and view all the answers

In a first-class lever, if the lifting arm is twice the length of the load arm, how much less effort is needed to lift the load?

2 times less effort Signup and view all the answers

What characteristic of a first-class lever allows a load to be moved with less effort?

The distance ratio between effort and load Signup and view all the answers

What type of stress develops in the direction opposite to a twist?

Tension stress Signup and view all the answers

Which aspect can cause internal stress during heat treatment of metals?

Abrupt or uneven temperature changes Signup and view all the answers

What is commonly referred to as 'Locked In Stress'?

Residual stress built into a material Signup and view all the answers

What initiates a crack that may lead to fatigue failure?

Point of highest tensile stress Signup and view all the answers

Which of the following can be considered a 'stress raiser' contributing to fatigue failure?

Nicks, cuts, and scratches Signup and view all the answers

In which types of vehicles are fatigue failures reported to be quite common?

Aircraft and motor cars Signup and view all the answers

What is the effect of temperature changes during operation on moving parts?

They may lead to fatigue failure Signup and view all the answers

What formation within materials is likely to act as stress raisers over time?

Flaws beneath the surface Signup and view all the answers

What directly influences atmospheric pressure at a given location?

The weight of the column of air above that location Signup and view all the answers

How is Absolute Pressure calculated in relation to Gauge Pressure?

Absolute Pressure is Gauge Pressure plus Atmospheric Pressure Signup and view all the answers

At an altitude of 29,000 ft, if the outside pressure is 4.4 psi, what is the differential pressure experienced by the aircraft cabin set at 11 psi?

6.6 psi Signup and view all the answers

Which of the following best describes a solid?

Is characterized by strong intermolecular forces Signup and view all the answers

Which statements accurately describe fluids?

Fluids exert force perpendicular to the surface of submerged objects Signup and view all the answers

What is a characteristic of Gauge Pressure as compared to Absolute Pressure?

Gauge Pressure measures pressure above atmospheric pressure Signup and view all the answers

Which of these must occur for an aircraft to maintain structural integrity at cruising altitude?

Differential pressure must be managed effectively to limit stress Signup and view all the answers

What is true regarding the properties of metals as solids?

Metals are usually hard, strong, and malleable Signup and view all the answers

Study Notes

Buoyancy and Pressure in Liquids

Liquids and gases are both fluids, therefore the theory behind buoyancy and pressure in liquids and gases is similar.
Liquids have a constant density, while gases are compressible.

Pressure in Solids

Pressure is defined as Force per unit area.
Pressure = Force / Area.
Using g = 10 m/s per s.

Pressure in Gases

The molecules in a substance move around more rapidly when heated, as its thermal energy increases.
There is a direct relationship between the temperature of a substance and the motion energy of its molecules.
Gas pressure is the result of the continual bombardment of the walls of the container.

Pressure in Fluids

Pressure is still defined as force per unit area, but in a fluid it is caused by the continual bombardment of the molecules against the inside of the container.
The pressure exerted by a column of fluid in an open container is determined by:
- the vertical height of the column
- gravity
- density of the fluid
The pressure is not affected by the volume of the liquid.

Density and Specific Gravity

Density is defined as the mass per unit volume of a substance.
A given volume of lead has many times the mass of the same volume of water.
When the density of other solids or liquids are compared to water, a table of comparative densities or specific gravities can be determined.

Resolution of a Vector

A tension force can be resolved into two components:
- horizontal
- vertical
This can be done using standard trigonometric ratios.

Resultant Force and Equilibrium

Sometimes, forces act at different directions on a body.
In such cases, forces must be resolved to calculate a resultant net force.
When an object does not change its state of motion or rest, the resultant of all the forces acting on it is zero, and it is said to be in a state of equilibrium.

Moments

Load Moment = (load x load arm)
Effort Moment = (force x effort arm)
Either side of the lever has a moment which is the force multiplied by the distance, called the arm from the fulcrum.
The system is balanced when the load moment and the effort moment are equal.
The smaller effort force moves through a larger arc to raise the heavier load a small distance.

Levers

A lever is a simple machine.
A simple machine is a device which provides a mechanical advantage.
The purpose of a lever is to perform work:
- for a load (L) to be lifted
- by an effort (E)
- pivoting around a fulcrum (F)
If the load moved is greater than the effort used, the machine has a positive mechanical advantage.

First Class Lever

An example of a first-class lever is a crowbar.
The fulcrum is situated between the load and the effort.
The load is greater than the effort.
MA = Load / Effort
The lid only needs to be raised a short distance but your hand travels a larger distance.

First Class Lever (cont'd)

If L is 100 kg and E is 10, MA = 10.
The ratio of the distance moved by each end of the lever will also equal 10. Distance Ratio = MA.

Mechanical Advantage

The mechanical advantage of a first-class lever depends on the distance moved by effort compared to load.
The load can be lifted with 7 times less effort if the lifting arm is 7 times the length of the load arm.

Residual Stress

Abrupt or uneven temperature changes tend to cause internal stress.
This often occurs when heat-treating metals.
This effect often explains why a component fails in service even though its externally applied stress levels are low.
Can be beneficial, such as in car windshields.
Also called “Locked In Stress”.

Fatigue

During operation, moving parts experience a variety of loadings, such as vibration, changes of load, and temperature changes.
Repeated applications of small loads may eventually result in fatigue failure.
A crack can originate at the point of highest tensile stress in the part.
Fatigue failures are quite common in aircraft and motor cars.
Fatigue failures are at least as common as overload failures.

Fatigue (cont'd)

Surface damage like nicks, cuts, scratches, gouges, cracks, or corrosion pits can cause a fatigue failure at that point.
These so called ‘stress raisers’ also occur where a part has an abrupt change in shape or section thickness.
For this reason sharp edges should be avoided.
Flaws beneath the surface of materials are also stress raisers.
Fatigue failures happen chiefly with metals.

Atmospheric Pressure

Atmospheric pressure at a location depends on the weight of the column of air above that location.

Absolute Pressure vs. Gauge Pressure

Gauge pressure reads pressure above (or below) atmospheric.
Absolute Pressure is Gauge Pressure plus Atmospheric Pressure.

Differential Pressure

Modern aircraft retain a cabin altitude equivalent to 8000’ or 11 psi.
Cruising at 29,000 ft, the outside pressure is 4.4 psi.
Therefore, the structure of the aircraft is experiencing a differential pressure of 11 - 4.4 = 6.6.psi
This is a significant component of the total stress on the airframe.

Solids

Solids have a definite shape and definite volume which is independent of its container.
The forces (bonds) that keep the atoms or molecules together are strong.
Therefore, a solid does not require outside support to maintain its shape.
Most metals are solids and as such are usually hard and strong and capable of being shaped mechanically (malleable and ductile).

Fluids

Both liquids and gases are classified as fluids.
The force exerted by a fluid at any point on the surface of a submerged object is perpendicular to the surface of the object.

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Description

Test your understanding of buoyancy and pressure in liquids, gases, and solids. This quiz covers the fundamental principles that govern how pressure operates in different states of matter and the relationships between temperature, force, and area. Challenge yourself to apply these concepts accurately.