Notes – Pressure & Buoyant Force Chapter 3, Lesson 2

Notes – Pressure & Buoyant Notes – Pressure & Buoyant ForceForce

Chapter 3, Lesson 2Chapter 3, Lesson 2

Pressure in a FluidPressure in a Fluid

• A fluid is a material that can flow and A fluid is a material that can flow and has no definite shape.has no definite shape.

• Liquids and gases are fluids.Liquids and gases are fluids.

• Fluids can exert forces.Fluids can exert forces.


• Pressure is the Pressure is the force per unit of force per unit of area applied on area applied on the surface of an the surface of an object.object.

Pressure in a FluidPressure in a Fluid• Pressure depends Pressure depends

on the area over on the area over which a force is which a force is exerted.exerted.

• If the area If the area increases, increases, pressure pressure decreases.decreases.


• Pressure depends on the size of the Pressure depends on the size of the force.force.

• If the force increases, pressure If the force increases, pressure increases.increases.


• The equation for pressure is: P = F ÷ The equation for pressure is: P = F ÷ AA

Pressure = force (newtons)Pressure = force (newtons)

area (meters squared)area (meters squared)


• The SI unit of pressure is the pascal, The SI unit of pressure is the pascal, abbreviated Pa. A pressure of 1 Pa is abbreviated Pa. A pressure of 1 Pa is equal to a force of 1 N applied over equal to a force of 1 N applied over an area of 1 man area of 1 m22

• 1 Pa = 1 N / m1 Pa = 1 N / m22


• For any liquid in a For any liquid in a container of any shape, container of any shape, the pressure depends the pressure depends only on the height of only on the height of the column of liquid the column of liquid aboveabove the surface the surface where you measure the where you measure the pressure.pressure.


• The deeper The deeper underwater you underwater you go, the greater go, the greater the pressure is on the pressure is on you. This is you. This is because as you because as you go deeper, the go deeper, the column of water column of water above you gets above you gets taller.taller.

Pressure in a FluidPressure in a Fluid• On April 10, 1963, the USS Thresher On April 10, 1963, the USS Thresher

nuclear-powered attack submarine nuclear-powered attack submarine was lost at sea during deep-diving was lost at sea during deep-diving tests. All 129 aboard died. (8400 ft / tests. All 129 aboard died. (8400 ft / 2600 m below sea level).2600 m below sea level).


• In a fluid, pressure is exerted on all In a fluid, pressure is exerted on all points on the outside of an object in points on the outside of an object in a direction a direction perpendicularperpendicular to the to the surface of the object at that point.surface of the object at that point.


• A gas, like any fluid, exerts a A gas, like any fluid, exerts a pressure on an object. That pressure pressure on an object. That pressure depends on the height of the gas depends on the height of the gas above the object.above the object.


• Atmospheric pressure is the force Atmospheric pressure is the force exerted per unit area by air particles.exerted per unit area by air particles.

• As you climb, or increase your As you climb, or increase your elevation, atmospheric pressure elevation, atmospheric pressure decreasesdecreases..

mb = millibar (100 Pa)


• Atmospheric Atmospheric pressure at sea level pressure at sea level is caused by a large is caused by a large weight of gas, but is weight of gas, but is difficult to notice difficult to notice because there is an because there is an equal, internal equal, internal pressure pushing out pressure pushing out from the inside of from the inside of your body.your body.

Buoyant ForceBuoyant Force

• The buoyant force is The buoyant force is the the netnet upward force upward force exerted on an object exerted on an object in a fluid by the in a fluid by the surrounding fluid. It surrounding fluid. It acts against gravity.acts against gravity.

• Buoyant force is the Buoyant force is the DIFFERENCE between DIFFERENCE between the pressure above the pressure above and the pressure and the pressure below.below.


• In a fluid, there is In a fluid, there is nono net horizontal net horizontal force on an object caused by force on an object caused by pressure from the fluid.pressure from the fluid.

No net horizontal force (equals 0).Only vertical forces


• In a fluid, pressure increases with In a fluid, pressure increases with depth.depth.

• The force of the fluid pushing The force of the fluid pushing upup on on the bottom of an object suspended in the bottom of an object suspended in water is always water is always greatergreater than the than the force of the fluid pushing down on force of the fluid pushing down on the top of the same object.the top of the same object.


The horizontal forces cancel (no net force).

Water pressure at the top surface is less than water pressure at the bottom surface. Thus, the fish swims around happily in the water.

The boxfish experiences a buoyant force resulting from increasing pressure at increasing depth.


• The size of the buoyant force does The size of the buoyant force does NOTNOT depend on depth. depend on depth.


• The buoyant force does NOT change The buoyant force does NOT change as an object goes deeper because as an object goes deeper because it’s a DIFFERENCE between the it’s a DIFFERENCE between the pressure above and the pressure pressure above and the pressure below. Both those increase so the below. Both those increase so the buoyant force stays the same. buoyant force stays the same.


5 N

10 N10 N

15 N


At the fish goes deeper, the force on its back increases.

However, the force on its belly also increases to match and keep the fish suspended in the water.

The net buoyant force is always the same, no matter what the depth.

Buoyant ForceBuoyant Force• Archimedes’ Principle states that the Archimedes’ Principle states that the

buoyant force on an object is equal buoyant force on an object is equal to the to the weightweight of the fluid the object of the fluid the object displaces.displaces.

This is why objectsfeel lighter in water!

Pressure is the ____ per unit area.

A mass

B volume

C force

D pascal

3.2 Pressure and the Buoyant Forces

Which direction does the buoyancy force act on an object?

A upward

B downward

C to the left of the object

D to the right of the object


Where is the pressure the greatest on an object in a fluid?

A top

B bottom

C sides

D none of the above


What does the weight of the fluid displaced by an object held under water equal?

A the normal force on the object

B the mass of the object

C the buoyant force on the object

D the volume of the object

Which exerts the greatest pressure at the bottom of the column?

A a column of water 5 m high

B a column of water 3 m high

C a column of water 1 m high

D cannot be determined

A column of fluid 5 m high weighs 100 N. The column covers an area of 0.25 m2. What is the pressure exerted by the fluid at the top of the column?

A 25 pascals

B 20 pascals

C 5 pascals

D 0 pascals

A toy boat weighs 120 N. If the boat floats in water, which describes the buoyant force acting on the boat?

A 60 N upward

B 240 N upward

C upward and equal to the weight of water displaced by the boat

D upward and equal to the total volume of the boat

SCI 8.c

3-2 Vocab – Pressure & Buoyant 3-2 Vocab – Pressure & Buoyant ForceForce

text p. 140-148text p. 140-1481.1. forceforce

2.2. fluidfluid

3.3. areaarea

4.4. pressurepressure

5.5. atmospheric pressureatmospheric pressure

6.6. buoyantbuoyant

7.7. buoyant forcebuoyant force

8.8. Archimedes’ principleArchimedes’ principle

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Notes – Pressure & Buoyant Force Chapter 3, Lesson 2