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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