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3 FORCES AND PRESSURE 3.1 UNDERSTANDING PRESSURE 1. Define Pressure Pressure is force per unit area 2. State pressure’s formula Pressure = Force Area A F P = The SI unit : Nm -2 = Pascal = Pa 3. State relationship between pressure and area. The pressure of a given force increases as the surface area decreases. 4. Describe applications involving High Pressure Increasing the pressure by reducing the area A sharp knife has a very small surface area on its cutting edge so that high pressure can be exerted to cut the meat. The studs on a football boot have only a small area of contact with the ground. The pressure under the studs is high enough for them to sink into the ground, which gives extra grip. Nails, needles and pins have very sharp ends with very small surface areas. When a force is applied to the head of a nail, the pressure will drive its sharp end into a piece of wood easily. The sole of an ice is fixed a narrow metal bar. The high pressure on surface of the ice so that the ice melts and allowing the ice skater to glide smoothly . Racing bicycles need very high air pressure inside the tyres, because the narrow tyres have a very small contact area with the road. The hard road surface can support the high pressure under the wheels. 37

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5. Application involving Low Pressure Reducing the pressure by increasing the area Skis have a large area to reduce the pressure on the snow so that they do not sink in too far. A tractor moving on soft ground has wide tires to reduce the pressure on the ground so that they will not sink into the ground. A wide shoulder pad of a heavy bag will reduce the pressure exerted on the shoulder of the person carrying the bag. Example 1 A block of metal of dimensions 0.5 m x 0.6 m x 1.0 m has a mass of 300 kg. Calculate the maximum pressure acting on the ground. A student pressing a thumbtack into a piece of wood with a force of 20 N. The surface area of the head of the thumbtack is 1 cm 2 and the cross-sectional area of the tip of the thumbtack is 0.01 cm 2 . Calculate: (a) the pressure exerted by the student’s thumb on the head of the thumbtack (b) the pressure of the tip of the thumbtack on the wood. (c) What conclusion can be drawn from your answers to (a) and (b)? 38
3.2 UNDERSTANDING PRESSURE IN LIQUIDS 39 What is pressure in liquid? A liquid in a container exerts pressure because of its weight . For example, if you try to put your finger over the end of a tap when it is turned on, you can feel the pressure of the water in the pipe. Characteristics of Pressure in a liquid. The pressure at any point in a liquid, at a particular depth, acts equally in all directions. The pressure in a liquid does not depend on the area of its surface. The pressure in a liquid

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