Forces density and pressure

Flashcards from Physics AS Level lecture notes to help students review and prepare for exams.

What is Archimedes' principle?

Buoyancy force is equal to the weight of the fluid displaced.

What is upthrust?

A force that pushes upwards on an object submerged in a fluid (liquid or gas), also known as buoyancy force. It's due to the difference in hydrostatic pressure.

Why is upthrust larger in liquids than in gases?

Because liquids are much denser than gases.

What affects hydrostatic pressure?

Depth (h).

Why do objects appear to weigh less when immersed in a liquid?

Due to upthrust.

What is necessary for an object to float?

It must have a density less than the density of the fluid it is immersed in.

What influences upthrust?

Pressure, not the density of the object.

What are drag forces?

Forces acting in the opposite direction to an object moving through a fluid (gas or liquid).

What is a key component of drag forces?

It increases with the speed of the object.

What is viscous force?

A frictional force experienced by an object in a fluid, caused by the viscosity of the fluid.

What is the center of gravity of an object?

The point at which the weight of the object may be considered to act.

Where is the center of gravity located for symmetrical objects with uniform density?

At the point of symmetry.

How does the position of the center of gravity affect stability?

An object is stable when its center of gravity lies above its base.

How do base width and center of gravity relate to stability?

A wider base and lower center of gravity make an object more stable.

What is a moment?

The turning effect of a force.

When do moments occur?

When forces cause objects to rotate about some pivot.

What is the formula for the moment of a force?

moment = Force x perpendicular distance

What are couples?

A pair of forces that acts to produce rotation only.

How does the moment of a couple differ from the moment of a single force?

The moment of a couple doesn't depend on a pivot, only on the perpendicular distance between the two forces.

What criteria must be met for a pair of forces to be considered a 'couple'?

Equal in magnitude, opposite in direction, perpendicular to the distance between them, zero resultant force.

What is the torque of a couple?

The total moment of the couple.

How is torque calculated?

Either force x perpendicular distance between forces

What is the principle of moments?

For a system to be balanced (in equilibrium), the sum of clockwise moments about a point must be equal to the sum of anticlockwise moments about the same point.

What does it mean for a system to be in equilibrium?

All the forces are balanced; there is no resultant force or torque/moment.

What are the implications of a system being in equilibrium?

The object will remain at rest or at a constant velocity and not rotate.

How are coplanar forces represented in equilibrium?

By closed vector triangles.

What are common forces acting on objects?

Weight, normal reaction force, tension, friction.

If an object acted on by three forces is in equilibrium, what is the resultant of the three forces?

Zero.

What are the components of weight on an inclined plane?

mg sinθ (parallel to the plane) and mg cosθ (perpendicular to the plane).

Define Density?

Density is the mass per unit volume of an object.

What is the formula of density

Rho= Mass/Volume

What are the units of density?

g cm-3 or kg m-3

What is the volume equation for a Sphere

V = 4/3πr^3

What is the volume equation for a Cube

V = d^3

What is the volume equation for Cylinder

V = πr^2h

What is the general formula for prisms?

V = A of base x h

What are the rules for unit conversion?

When converting a larger unit to a smaller one, you multiply, when converting a smaller unit to a larger one, you divide.

Define Pressure

Pressure tells us how concentrated a force is, it is defined as the force per unit area.

What is the formula of density

P = F/A

What happens when a force is spread over a large area or a small area in terms of pressure?

If a force is spread over a large area it will result in a small pressure. If it is spread over a small area it will result in a large pressure.

What units are pressure measured in?

Pressure is measured in pascals, Pa. 1 Pa is the same as 1N m^-2

Is pressure a scalar or vector quantity?

Pressure, unlike force, is a scalar. Therefore pressure does not have a specific direction.

What area is referred to in terms of pressure?

The area referred to is the 'cross-sectional' area of a 3D object. This is the area of the base that the force is applied on.

What is hydrostatic pressure?

Hydrostatic pressure is the pressure that is exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity.

What is the equation for hydrostatic pressure

P=hpg

What factors does the pressure of a liquid depend on?

The size of this pressure depends upon the density (p) of the liquid, the depth (h) of the object and the gravitational field strength / gravitational acceleration (g).

What equation needs to be used to workout total pressue?

Total pressure = Hydrostatic pressure + Atmospheric pressure

What is Atmospheric pressure (also known as barometric pressure)?

101 325 Pa / 100kPa