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 in a liquid increase with depth. Pressure in a liquid acts in all directions. Formula: P = ρgh P = Pressure ρ = density g = acceleration due to gravity h = 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

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 relative densityliquids

How do submarines float and submerge? When floating on the surface a submarine is less dense than the water surrounding it - positive buoyancy The 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 gases This can be expressed using the equation: Initial Pressure x Initial Volume = Final Press. x Final Vol. P I V I = P F V F 1)A gas has a volume of 3m 3 at a pressure of 20N/m 2. What will the pressure be if the volume is reduced to 1.5m 3 ? 2)A gas increases in volume from 10m 3 to 50m 3. If the initial pressure was 10,000N/m 2 what is the new pressure? 3)A gas decreases in pressure from 100,000 Pascals to 50,000 Pascals. The final volume was 3m 3. What was the initial volume? 4)The pressure of a gas changes from 100N/m 2 to 20N/m 2. 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 smaller surface area so pressure is increased

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