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What is pressure?. Why would a lady in high heels standing on your foot hurt more than an elephant standing on your foot?. The elephant has a larger weight than the lady, but the contact area between its feet and the floor is far larger. High pressure and low pressure.
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What is pressure? Why would a lady in high heels standing on your foot hurt more than an elephant standing on your foot? The elephant has a larger weight than the lady, but the contact area between its feet and the floor is far larger.
High pressure and low pressure A force spread over a larger area produces a lower pressure. Which type of footwear would be better for standing on a muddy field – the flat-soled shoes or high-heeled boots?
High pressure and low pressure The shoes have flat soles, so they spread the person’s weight over a large surface area. These boots exert a low pressure on the ground. In contrast, the soles of the high-heeled boots have a smaller surface area and so exert a higher pressure. These shoes are likely to sink into soft ground.
Calculating pressure f force pressure= p a area Pressure is the force per unit area and is calculated using this formula: This can also be represented in a formula triangle. Pressure is measured in newtons per square metre (N/m2), which are also called pascals (Pa). Pressure can also be measured in newtons per square centimetre (N/cm2).
Pressure calculation How much pressure is the woman exerting on the floor? Weight of woman = 500 N Area of contact with floor = 20 cm2 Force Pressure = Area 500 = 20 = 25 N/cm2
Using low pressure A force spread over a large area produces low pressure. This is used in skis and snowboards. The large surface area of the board means the snowboarder exerts very little pressure on the snow. This means he slides over the top of the snow and does not sink into it.
Using high pressure A force concentrated on a small area produces high pressure. This is used in things like pins and sharp knives. The force applied to the tiny surface area of the end of a pin creates a high enough pressure to puncture even wood. The narrow blade of a knife means that it exerts a high pressure, making it easier to cut fruit and vegetables.
Pressure in a liquid Pressure in a liquid acts in all directions and increases with depth. You can investigate this by using a needle to make holes at different heights in a plastic bottle, before filling it with water. The water escaping from the lower holes will travel further because it is at a higher pressure. low pressure high pressure
Overcoming pressure underwater When entering deep water, a diver will experience pressure from all sides. At greater depths, this pressure becomes too much for the body to handle. So how do we overcome this pressure? Submarines use extremely strong materials to withstand the huge forces. This means some can dive to the very deepest points of the ocean.
Hydraulics 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 the parts mechanically. All hydraulic systems use two pistons linked via a pipe which carries special oil called hydraulic fluid. force applied here force transferred here Pressure inside all parts of the hydraulic system is the same.
Hydraulic brakes All hydraulic brake systems (like those used in cars) use a small master piston and bigger slave pistons. The master piston is used to apply a force. This puts the liquid under pressure. The pressure is transmitted to the pistons on all four wheels of the car.
Hydraulic brake – pressure equations The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation: force applied pressure = area of master piston The pressure is transferred to the slave pistons, so the force exerted by the slave piston can be calculated using: force exerted pressure = area of slave piston force exerted = pressure × area of slave piston The slave pistonhas a larger areathan themaster piston, so the force exerted by the slave pistons is greaterthan the force exerted by the driver on the brake pedal.
Force and rotation A force acting on an object can cause it to turn about a pivot. 100N pivot What happens to the see-saw when a force is applied on the left-hand side? Does the see-saw turn? If so, clockwise or anticlockwise?
Force and rotation – a moment The left-hand side of the see-saw moves downwards when a force is applied to it – this is an anticlockwise turn. 100N The turning effect of a force is called amoment.
Using moments A spanner is a lever that can be used to unscrew a nut. The spanner exerts a moment or turning force on the nut. pivot distance from force to pivot force force If the moment is big enough, it will unscrew the nut. If not, there are two ways of increasing the moment.
Using moments – increasing the moment 1.Increase the distance from the force to the pivot – apply the force at the end or use a longer spanner. pivot distance from force to pivot force If the same force is applied over a greater distance, a larger moment is produced.
Using moments – increasing the moment 2. Increase the force applied – push/pull harder or get someone stronger to do it! pivot distance from force to pivot force If a greater force is applied over the same distance, a larger moment is produced.
Moment equation moment f d The moment of a force is given by the formula: moment = force (N) × distance from pivot (cm or m) This can also be represented in a formula triangle: Moments are measured in newton centimetres (Ncm) or newton metres (Nm).
Moment calculation The counterweight on the trebuchet weighs 300N and is attached to the short arm. It is 1m from the pivot. It exerts a clockwise moment. What is the size of this moment? moment = 300 × 1 = 300Nm 3m 1m 100N 300N pivot
Principle of moments The lead shot on the trebuchet weighs 100N and is attached to the long arm. It is 3m from the pivot. It exerts a clockwise moment. What is the size of this moment? moment = 100 x 3 = 300Nm 3m 1m 100N 300N pivot
Principle of moments If the anticlockwise moment and clockwise moment are equal, then the trebuchet is balanced. This is known as the principle of moments. When something is balanced about a pivot: total clockwise moment = total anticlockwise moment 3m 1m 100N 300N pivot
Principle of moments What happens if the counterweight is increased to 1000N? The moments will no longer be balanced, so the trebuchet will be able to fire. 3m 1m 1m 100N 1000N
Principle of moments The principle of moments can be investigated using 10g masses with this balance. 10g exerts a force of 0.1N. Anticlockwise moment Clockwise moment = 0.1 × 7 = 0.7Ncm = (0.1 × 3) + (0.1 × 4) = 0.7Ncm Both moments are equal, so the see-saw is balanced.
Principle of moments – calculation Two girls are sitting on opposite sides of a see-saw. One girl weighs 200N and is 1.5m from the pivot. How far from the pivot must her 250N friend sit if the see-saw is to balance? 200N 250N 1.5m ?m total clockwise moment = total anticlockwise moment 200N × 1.5m = 250N × distance = 1.2m 200 × 1.5 = distance 250
How do tower cranes work? Tower cranes are essential at any major construction site. trolley load arm counterweight loading platform tower Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?
Moment calculation – crane If the crane below is balanced, how heavy is the load? 3m 6m 10,000N ? moment of = moment of load counterweight
Moment calculation – crane Moment of load = load× distance from tower = ?× 6 = counterweight× distance Counterweightmoment = 10,000 × 3 = 30,000 Nm 6m 3m = moment of counterweight Moment of load 10,000N = 30,000 ? × 6 5,000N ? ? = 30,000 6 ? = 5,000N
