5.5 Pressure & Pressure Differences in Fluids

What is pressure?

Force exerted on a surface per unit of area that force is distributed over

What are the 2 states of matter that a fluid can be?

• Liquid

• Gas

When a force acts on a surface it causes _________.

pressure

What are 2 ways to increase pressure?

• Increase force

• Smaller SA

What are 2 ways to decrease pressure?

• Decrease force

• Larger SA

As you increase the size of the force you ___________ the size of the pressure. 

increase

As you increase the size of the area of which the force is spread acts, you ___________ the pressure.

decrease

The pressure in fluids causes a force normal (at ______ ______) to any surface.

right angles

How can the pressure at the surface of a fluid be calculated? State the units.

pressure = force normal to that surface ÷ area of that surface

$$\rho=\frac{F}{A}$$

• pressure — pascals (Pa)

• force — newtons (N)

• area — metres squared (m²)

What is pressure measured in?

Pascals (Pa)

How can the pressure due to a column of liquid be calculated? State the units.

pressure = height of the column × density of the liquid × gfs

$$p=h\rho g$$

• pressure — pascals (Pa)

• density — kilograms per metres cubed (kg/m³)

• gfs — newtons per kilogram (N/kg)

The pressure in a liquid acts in ____ ____________.

all directions

The pressure in a liquid is ___________ at greater depths. 

greater

What is the equation for density? State the units.

density = mass ÷ volume

$$\rho=\frac{m}{v}$$

• density — kilograms per metre³ (kg/m³)

• mass — kilograms (kg)

• volume — metres cubed (m³)

What is density measured in?

kg/m³

A partially (or totally) submerged object experiences a greater pressure on the ___________ surface than on the _________ surface. This creates a resultant force upwards.

What is this force called?

• bottom

• top

• upthrust

When we submerge an object, an _______ volume of fluid is _________. The weight of the fluid that is __________ is equal to the force of the ________. 

• equal

• displaced

• displaced

• upthrust

An object that is more dense than the fluid around it will ______.

sink

Why does an object that is more dense than the fluid around it sink?

• Object weight > weight of displaced fluid (= upthrust)

• Weight (pulls object downwards) > upthrust pushing upwards

An object that is less dense than the fluid around it will ______.

float

Why does an object that is less dense than the fluid around it float?

• Partially submerged object’s weight (acting downwards) = weight of displaced fluid — e.g. upthrust (acting upwards)

• Resultant force = 0

What is the atmosphere?

Layer of air around Earth

The atmosphere gets less dense with _____________ _________.

increasing altitude

What creates atmospheric pressure?

Air molecules colliding with surface

What is altitude?

Height above sea level

The number of air molecules (and so the ________ __ ______) above a surface ___________ as the height of the surface above _________ ________ increases.

• weight of air

• decreases

• ground level

As height increases there is always ______ air above a surface than there is at a ________ height.

• less

• lower

Atmospheric pressure _____________ with an increase in height.

decreases