Pressure

What two parameters determine the density of an object?

How does density affect flotation?

• If an object/substance has a greater density it will sink, if it has a lower density it will float.

• Clips:– Peanuts and Beer (Can you explain why this

happens?)– Cannonball in Mercury– Ship floating on air (sulphur hexaflouride)– Monty Python

Pressure

• Which will exert more pressure?

Pressure

Pressure

Pressure

• Pressure in a liquid increase with depth.• Pressure in a liquid acts in all directions.• Formula:

P = ρgh

P = Pressureρ = densityg = acceleration due to gravityh = depth on a liquid

Sample Question• A rectangular wooden block has dimensions

14cm x 12cm x 20cm. It is placed with its upper surface at a depth of 30cm in a basin of water. Find:– The pressure a the upper surface of the block due to

the water– The pressure at the lower surface of the block due to

the water– The force on the upper surface due to the water– The force on the lower surface due to the water– If the weight of the block is 26N determine whether

the block will float or sink

Question• A rectangular block has dimensions 10cm x 20cm

x 30cm. It is placed at a depth with its upper surface at a depth of 1m in water. Find:– The pressure at the upper surface of the block due to

the water– The pressure at the lower surface of the block due to

the water– The force on the upper surface due to the water– The force on the lower surface due to the water– If the weight of the block is 70N, determine whether

the block will float or sink

Upthrust (Buoyancy Force)

• The upward force experienced by a submerged object is known as the upthrust or buoyancy force

• Archimedes discovered that the size of the upthrust is the same as the weight of the liquid displaced by the object.

Archimedes Principle• Archimedes’ Principle

states that when an object is wholly or partially immersed in a fluid, the upthrust it experiences is equal to the weight of the displaced fluid

20N

12N

8N

Floatation

• A floating body displaces its own weight in water.

Floatation

• A floating body displaces its own weight in water.

=10000t 10000t

Law of Floatation

• The Law of Floatation states that the weight of a floating object is equal to the weight of the fluid it displaces.

• Application - Hydrometer– A hydrometer is an instrument used to measure the

relative densityof liquids; that is, the ratio of the density of the liquid to the density of water

How do submarines float and submerge?

When floating on the surface a submarine is less dense than the water surrounding it - positive buoyancyThe ballast tanks are usually full of air.

How do submarines float and submerge?

Negative buoyancy must be achieved to submerge the submarine.Vents are opened and seawater forces air out and begins to fill the ballast tanks.

How do submarines float and submerge?

The depth can be controlled by varying the air to water ration in the ballast tanks.Neutral Buoyancy can be achieved when the weight of the submarine is equal to the weight of the water displaced – the submarine will neither rise nor sink.

How do submarines float and submerge?

For the submarine to surface again it must achieve positive buoyancy and become less dense than the water.Compressed air is blown into the ballast tanks, forcing the seawater out.

Atmospheric Pressure

• Steam in side can forces the air out of the can.• The steam condense when it cools, creating a

vacuum• Atmospheric pressure outside can is greater

than the pressure inside, causing it to collapse

Measuring Atmospheric Pressure(Mercury Barometer)

• As air pressure increases, the level of mercury rises and vice versa.

• High Pressure – good weather• Low Pressure – bad weather

Pressure and Volume in gasesThis can be expressed using the equation:

Initial Pressure x Initial Volume = Final Press. x Final Vol.

PIVI = PFVF

1) A gas has a volume of 3m3 at a pressure of 20N/m2. What will the pressure be if the volume is reduced to 1.5m3?

2) A gas increases in volume from 10m3 to 50m3. If the initial pressure was 10,000N/m2 what is the new pressure?

3) A gas decreases in pressure from 100,000 Pascals to 50,000 Pascals. The final volume was 3m3. What was the initial volume?

4) The pressure of a gas changes from 100N/m2 to 20N/m2. What is the ratio for volume change?

Boyles Law

Definition:

Boyles Law states that at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure

Boyles Law

Boyles Law

What happens the pressure when the volume is decreased?-The pressure increases. Why?

More collisions on a smallersurface area so pressure isincreased

Boyles Law

To verify Boyles Law

• We use Boyles Law Apparatus

• see handout

Measuring Pressure

• Midterm Assignment– Investigate 2 instruments used to measure

pressure– Describe clearly the operation of each instrument

and the principle behind its operation– Include diagrams, pictures where relevant

Pressure and Boiling Points

• As pressure decreases, the boiling point of liquids also decreases