Define Pressure 2. State pressure’s formula = Force Area 3. State relationship between pressure and area. 4. Describe applications involving High Pressure Increasing the pressure by reducing the area A sharp knife has a very The studs on a football small surface area on its boot have only a small cutting edge so that high area of contact with the pressure can be exerted ground. The pressure to cut the meat. Ender the studs is high enough for them to sink into the ground, which gives extra grip. The SSL unit : Nm-2 = Pascal = Pa The pressure of a given force increases as the surface area decreases. 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 trees, because the narrow trees have a very small contact area with the road. The hard road surface can support the high pressure under the wheels. 37 www. Physicist’s. Com 5. Application involving Low Pressure Reducing the pressure by increasing the area Skis have a large area to A tractor moving on soft ground has wide reduce the pressure on the snow so that they do tires to reduce the pressure on the not sink in too far. 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 keg. 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 com and the cross-sectional area of the tip of the thumbtack is 0. 01 com. 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)? Www. Physicist’s.

A liquid in a container exerts pressure because of its weight. For example, if you try o 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 The pressure in a liquid The pressure in a liquid point in a liquid, at a does not depend on the acts equally in all particular depth, acts area of its surface. Directions and does not equally in all depend on the shape of directions. The container. Relate depth The pressure in a liquid is directly to pressure proportional to the depth. In a liquid .

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The pressure in a liquid increases with depth. Relate density to pressure in a liquid The pressure in a liquid is directly proportional to the density of the liquid. Derive the formula for the pressure in a liquid. The container on the right has a base area A. It is filled to a depth h with a liquid of density p. Calculate: (a) volume of liquid = Ah (b)mass of liquid = iv= pap (c) weight of liquid = MGM = pasha (d)force on base (e) pressure www. Physicist’s. Com = pasha -? F pasha = = Pugh A A Pressure in liquid = Pugh 39 1 1 50 km-3, calculate the pressure below mm of sea water due to the water alone.

Example 2 The figure shows a cylinder containing liquid mercury. What is the pressure caused by the liquid mercury at the point P? [ Density of liquid mercury is 1. 36 x 105 keg m-3 ] Describe applications of pressure in liquids . 1. Dam holds water at high altitude. The wall of the dam has to be thicker at the base. Explain. The wall of a dam is much thicker at the bottom than at the top because it must withstand the increased lateral pressure in depths of the water. 2. Water is stored in water tank at higher level. Why? Normally a water tank is placed at higher level so as to supply water at greater pressure. 3.

Submarine is built with thick wall. Explain. Submarine is built with thick wall so as to withstand enormous pressure at greater depth. 4. A patient receiving intravenous drips of a certain fluid from a bottle. In order for the fluid to flow into the vein, the bottle must be placed at a height above the injection site. Why? The liquid solution is at a higher pressure so it h Explain gas pressure Gas pressure is the force per unit area exerted by the gas molecules as they collide with the walls of their container.

Explain ; Atmospheric pressure is the pressure caused by the atmospheric weight of the air above us. Pressure . ; Atmospheric pressure acts equally in all directions. ; Atmospheric pressure varies with the height of the object above sea level. It decreases with the altitude or the height above sea level. At higher altitudes, the density and the temperature of the air are lower. As a result, the frequency of collisions of the molecules is lower. Hence, atmospheric pressure is lower. Describe activities to show existence of atmospheric pressure 41 of the glass.

Hold the card firmly in place and turn the glass over. Take away your hand. The cardboard does not fall and the water remains in the glass. The explanation for this phenomenon is that the resultant force caused by the atmospheric pressure acts on the surface of the cardboard is greater than the weight of the water in the glass. Describe (I) Drinking straw applications of ATM. Pressure A metal can containing water is heated until the water in it vaporizes. Allow the steam to exit from the mouth of the can. The can is then capped and cooled down with tap water. As the result , the can is crushed and crumpled.

The explanation for this phenomenon is that the pressure inside the metal can decrease and the external atmospheric pressure ,which is higher compresses the metal can. (it) Rubber sucker When we suck through a straw, the air pressure in the straw is lowered. Then the pressure of the atmosphere acting on the surface of the drink in the glass pushes the water up the straw and into our mouth. . When the sucker is pressed into place, most of the air behind it is squeezed out. The sucker is held in position by the pressure of atmosphere on the outside surface of the rubber.

If the seal between the sucker and the surface is airtight , the sucker will stick permanently. 42 (iii) Syringe (iv)Vacuum cleaner Pulling up the piston reduces the atmospheric pressure inside the cylinder. The atmospheric pressure on the liquid surface then pushes the liquid up into the syringe. If we then hold the plunger in place and lift the syringe out of liquid , none will fall out. This is again due to atmospheric pressure . Measuring (I)The Simple Portion atmospheric barometer pressure The simple barometer Portion By using is along glassiest that has Portion been filled with mercury and Barometer the inverted onto a dish of mercury.

The mercury column rises or falls according to the pressure of air on the mercury in the dish. The space above the mercury column sis vacuum so it exerts no pressure on the top of the mercury column. A vacuum cleaner produces only a partial vacuum. The fan inside the cylinder blows atmospheric pressure outside then pushes air up the cleaner hose ,carrying dust and dirt with it. If the vertical height of the mercury is h CM , toffee the atmospheric pressure reading is ” h CM mercury “. How does the height ,h of the Portion barometer vary?

The height, h will remains unchanged when (I) the diameter of the glass tube increases (it) the glass tube is tilted (iii) the glass tube is lowered further into the dish (v) the glass tube is lifted up from the dish (v) the quantity of mercury in the dish is increased The height, h will increases when the barometer is slowly submerged in water. The height, h will decreases when (I) the vacuum space in the glass tube is filled with gas (it) the barometer is carried out to a mountain Example 4 The figure shows a mercury barometer is placed in a school laboratory where the atmospheric pressure is 75 CM Hog. ) What is the value of h (b) What is the length of the vacuum space when the glass tube is (I) uplifted at height of 5 CM (it) lowered further into the dish at a depth of 4 CM (c) If the density of mercury is 1. 36 x 104 km-3 and the density of water is 1 x 103 km-3, determine (I) the atmospheric pressure in the units Pa (it) the value of h if the mercury is replaced by water. Example 5 The figure shows a barometer. The vacuum space is filled with a gas X. What is the pressure of the gas X? [ Take atmospheric pressure = 76 CM Hog ] Measuring Aneroid Barometer atmospheric pressure By using Aneroid Barometer

x

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