Pressure

Pressure Why do predators often have SHARP teeth and claws? Why does wearing skis or snow shoes stop you SINKING into the snow?

Pressure Pressure is exerted whenever a force is applied over an area. 1. 2. Which one exerts the biggest pressure, 1 or 2?

Application of pressure A force concentrated on a small area means high pressure, e.g. razor blades, needles, high heeled shoes, spurs, ice skates, sharp knives. The high pressure on the cutting edge of an ice-skate melts the ice and helps the skater slide across the surface. On the cutting edge of a knife a very high pressure is exerted - this makes it easier to cut.

Exercise 1: Pressure 1. Why do tractors have such large wheels? 2. Why do camels have such large feet? The large wheels have a large surface area in contact with the ground. Tractors are usually quite heavy and often have to work on soft ground like mud and soil. The large area of the tyres means that they put less pressure on the ground and they are therefore less likely to sink. Camels are often found in deserts. The camels are also often quite heavy. The large surface area of their feet on the ground means that they put less pressure on the sand and are less likely to sink into the sand. Camels have large feet

greater area pressure force smaller Exercise 2: Pressure Pressure tells us over what sort of area a _____ is acting. Pressure depends upon two factors: the force that is being applied and the ____ over which the force is acting. The larger the force, the greater the ________. The ______ the force, the lower the pressure. The smaller the area, the _______ the pressure. The larger the area, the smaller the pressure. force area pressure smaller greater greater area pressure force smaller

Force Pressure = Area Pressure F P x A Pressure is the force per unit area so is calculated using the expression shown below: Pressure = Area Force Pressure is measured in: Newtons per metre squared (N/m2) which is called a PASCAL (Pa) Pressure can also be measured in: Newtons per millimetre squared (N/mm2); Newtons per centimetre squared (N/cm2).

Exercise 3: Hammer and Nail A hammer is used to drive a nail into a wooden floor. The hammer is brought down with a force of 200 N. The area of the top of the nail is 0.5 cm2. What pressure is put upon the top of the nail by the hammer blow? Pressure = Force ÷ Area Pressure = 200 N ÷ 0.5 cm2 Pressure = 400 N/cm2

Exercise 4: Pressure Calculations 1 1. What are the units of pressure, force and area? 2. A force of 500 N acts over an area of 10 m2. What pressure is created by this force? 3. A girl of weight 400 N has feet of area 100 cm2. What pressure does she put on the ground? 4. A car of weight 6 400 N has four wheels. Each wheel has an area of 80 cm2. What pressure does the car put on the ground? Pressure measured in Pascals. Force measured in Newtons. Area measured in metres squared. 50 Pa. 4 N/cm2. 20 N/cm2.

Exercise 5: Pressure Calculations 2 1. A force of 300 N creates a pressure of 4 N/m2. Over what area is the force acting? 2. A pressure of 200 Pa is created over an area of 5m2. What force acts to create this pressure? 3. A tractor has tyres of area 3 m2. It has a weight of 12 000 N. What pressure does the tractor put on the ground? 4. A truck has tyres of area 5 m2. It puts a pressure of 15 000 N/m2 on the road. What is the weight of the truck? 75 m2. 1 000 N. 4 000 Pa. 75 000 N.

Fluid Pressure The pressure in SOLIDS always acts DOWNWARDS because of the PULL of GRAVITY. In FLUIDS (LIQUIDS and GASES) the pressure acts IN ALL DIRECTIONS. Pressure in fluids can be used to TRANSFER FORCES. Also, in fluids, the PRESSURE INCREASES WITH DEPTH. Less pressure DAMS are WIDER AT THE BOTTOM than at the top because pressure increases with depth in water. Higher pressure

Pressure in Solids, liquids and gases. Pressure in solids only acts downwards Pressure in liquids and gases acts in all directions. In liquids, pressure increases with depth.

Pressure in liquids In a liquid: Pressure acts in all directions and pressure increases with depth.

Pressure (N/m2) = 10 N/Kg x depth (m) x density (Kg/m3) The relationship between pressure and depth is shown by a water bottle with holes along its length. low pressure High pressure Pressure (N/m2) = 10 N/Kg x depth (m) x density (Kg/m3) The pull of gravity The deeper you go, the higher the pressure The denser the liquid, the heavier it is!

Pressure inside all parts of the hydraulic system is the same Hydraulic systems use the principle that pressure is transmitted throughout a liquid. They are used to transfer movement from one part of a machine to another without linking them mechanically. All hydraulic systems use two pistons linked via a pipe carrying a special oil called hydraulic fluid. Force applied here Force transferred here Pressure inside all parts of the hydraulic system is the same

Hydraulics activity

Hydraulics brakes All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave piston. The master piston is used to apply a force. This puts the liquid under pressure. Pressure = Force applied Area master piston The pressure is transmitted to the pistons on all four wheels.

Air Pressure: Does the person at the top or the bottom of the mountain feel the most air pressure? Who has the biggest column of air above them?

The larger air pressure in the balloon means there are more air particles moving around and hitting the sides. If you undo the end of the balloon the air particles rush out so the pressure inside and outside the balloon become equal. Aerosols work in the same way – the gas it at a high pressure in the can.

Which diver feels the greatest pressure? Height of column of water above diver:

Barometer used to measure pressure

Investigate the link between pressure and volume. Results: Conclusion: as volume decreses pressure increses. Volume (ml) Pressure (kPa) 20 19 18 17 16 15

Barometers - altimeter Type of barometer … measures altitude based on pressure

Pressure, Wind and Weather Systems Air stops rising when it meets air of equal density, then diverges at high level to produce more wind which eventually sinks elsewhere to complete the circulation cell Pressure, Wind and Weather Systems WINDS are horizontal flows of air; winds blow from areas of high pressure to areas of low pressure (nature tries to equalise pressure) PRESSURE describes the tendency of the air to rise or to sink at any given place or time. Air tends to rise or sink as a result of its density. Air density varies with altitude but, at the ground level, air density is governed by its temperature. Thus, variations in radiation and temperature control pressure and wind. Insolation Air heated by contact with ground expands; becomes less dense and rises Denser air drawn in at low level to replace rising, less dense air LOW PRESSURE Denser air drawn in at low level to replace rising, less dense air Sun heats up ground

Pressure effect on Weather Low pressure will bring stronger winds and rains High pressure will bring settled dry weather

Measuring Pressure Barometers – detect & measure pressure changes barometric pressure Units measure pressure over area Millibars (mb) Hectopascal (hPa) Inches of mercury (Hg) 1013.25 mb = 1013.25 hPa = 29.92 in. Hg Standard Atmosphere