PRESSURE AND MOMENTS In This Topic We Will Learn About: Calculating pressure. The link between pressure, force and area. Gas pressures. Atmospheric pressure and pressure in liquids. Moments as turning forces. Pivots and turning forces. Calculating moments.

1 PASCAL = 1 NEWTON PER SQUARE METRE. PRESSURE AND MOMENTS Pressure is a measure of how much force is exerted on a given area. It is about how concentrated the force is. Pressure is calculated by using this equation: Pressure = Force P = F Area A p A F Pressure is measured in Newtons per square metre (N/m²) . Another name for Newtons per square metre is Pascals (Pa). Pascal 1 PASCAL = 1 NEWTON PER SQUARE METRE.

PRESSURE AND MOMENTS Decide which of the examples below show a large pressure and which show a small pressure.

PRESSURE AND MOMENTS answers Decide which of the examples below show a large pressure and which show a small pressure. Large pressure. Small pressure. Small pressure. Large pressure. Large pressure. Small pressure. There is a large pressure when the force is concentrated on a small area.

PRESSURE AND MOMENTS p A F Pressure = Force p = F Area A Use the equation to work out the pressure produced when a man weighing 600 N wears a pair of snowshoes with a total area of 1.5 m². p = F = 600 = 400 Pa A 1.5 Don’t forget to give the correct unit in your answer.

Sometimes we need to re-arrange the equation to answer a question. PRESSURE AND MOMENTS p A F Pressure = Force p = F Area A Sometimes we need to re-arrange the equation to answer a question. A box has a base with an area of 2 m². It exerts a pressure of 300 Pa on the ground. What is the weight of the box? Area of base= 2 m² F = p  A = 300 x 2 = 600 N Don’t forget that weight is a force – it is the force that gravity makes an object exert on the Earth. Because it’s a force, weight is measured in Newtons.

PRESSURE AND MOMENTS Area A F p A Pressure = Force p = F Area A p A F Sometimes we need to use a smaller unit for area to answer a question. This is because 1m² is quite a big area. We are likely to use square centimetres (cm²) for many everyday calculations. A woman stands on the ground. She weighs 450N . Her shoes touch the ground with a total area of 5 cm². What pressure does she exert on the ground? p = F = 450 = 90 N/cm² A 5 The correct unit is now Newtons per square centimetre (N/cm²) as we are dividing Newtons by square centimetres.

PRESSURE AND MOMENTS Gases can exert a pressure on objects because their molecules are always moving around. As the molecules move they hit objects and this causes pressure. When a gas is in a container, its molecules hit the walls of the container and create a pressure. What happens to the pressure of a gas when it is heated? Why? The pressure increases because the heat makes the molecules move faster,so they hit the sides of the container with more energy and more often.

PRESSURE AND MOMENTS What happens to the pressure of a gas when it is squashed into a smaller container? The pressure increases because the molecules haven’t got so far to go before they hit the container. So they hit the sides of the container more often. Divers use gases under pressure so that they can take plenty of supplies with them.

PRESSURE AND MOMENTS The atmosphere exerts a pressure on us. This is due to the weight of the air above us pushing down on the Earth. If you go up a high mountain there will be less air above you so the air pressure will be lower. The weight of lots of air above the ground here gives a large air pressure. There is only a little air above the top of the mountain so the air pressure is low.

PRESSURE AND MOMENTS Liquids exert a pressure because their molecules move around and because of their weight. The deeper below the surface of a liquid you are, the greater the pressure the liquid exerts on you. This is because there is more weight of water above you. Why is the bottom of a dam thicker than the top? Because the pressure of the water is greater at the bottom than the top. Why do deep sea divers have to wear pressurised suits? Because the diver could be injured by the very high water pressure deep under the sea.

PRESSURE AND MOMENTS Levers are very simple machines. They allow you to lift a large load by using a small effort. The longer the lever, the less force (effort) you need to lift the load. Levers work by using a turning force that works around a pivot. Small effort Large load pivot A small force pushing down can move a heavy load if you use a lever. The effort force is much smaller than the load force. This is because the effort is much further from the pivot than the load is.

PRESSURE AND MOMENTS The other name for a pivot is a fulcrum. Which of the examples shown below use the idea of levers? They all do!

The weight of this block makes the see-saw turn anticlockwise. PRESSURE AND MOMENTS Levers use the idea of moments to work. A moment is the turning effect of a force. As the left hand side goes down, the right hand side will go up. The weight of this block makes the see-saw turn anticlockwise.

PRESSURE AND MOMENTS To calculate the moment of a force, you use this equation: moment (Nm) = force (N) x perpendicular distance from pivot (m). The unit is the Newton metre Nm. 10 N 0.8 m The moment produced by the force shown above is: Moment = force  perpendicular distance from pivot. Moment = 10  0.8 = 8 Nm

PRESSURE AND MOMENTS See-saws use the idea of moments. For a see-saw to balance, the moments must be equal on both sides. The example below shows how this works. A B 0.2 m 40 N 10 N 0.8 m The moment due to A is : 10 x 0.8 = 8 Nm The moment due to B is: 40 x 0.2 = 8 Nm These moments are equal so the see-saw balances.

Check your learning Do these see-saws balance? If not, what will they do? 3N, 5m from the pivot 5N, 3m from the pivot 4N, 4m from the pivot 3N, 5m from the pivot 12N, 2m from the pivot 4N, 6m from the pivot

Check your learning- answers Balances 3N, 5m from the pivot 5N, 3m from the pivot Right hand side goes down 4N, 4m from the pivot 3N, 5m from the pivot Balances 12N, 2m from the pivot 4N, 6m from the pivot

Check your learning How many ways can you think of to balance the see-saw below? 3N, 8m from the pivot On the right hand side you could have: 12N at 2m 6N at 4m 24N at 1m 4N at 6m And many other combinations …anything that multiplies up to 24.