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Force and Pressure
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3. APPLICATION INVOLVING HIGH PRESSURE A sharp knife has a very small surface area on its cutting edge so that high pressure can be exerted to cut the meat. 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 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.
4. APPLICATION INVOLVING LOW PRESSURE 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.
18. At sea level , the atmospheric pressure is about 100 000 Pa (1×10 5 Pa) 1 atmosphere (1 atm) = 10 5 Pa / 10 5 Nm -2 At 5600 m above sea level, the atmospheric pressure is about 50 000 Pa (5×10 4 Pa) The pressure in the space around satellite is almost zero . An aircraft travelling at an altitude of 11000 m will experience an atmospheric pressure of 25 000 Pa (2.5×10 4 Pa) Atmospheric pressure varies with height
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21. 1) The fan sucks out the air when it is switched on. 2) Space in x become partially vacuum. 3) The higher atmospheric pressure forces the air and dust particles into the dust bag 4) Dust particles are trapped but the air still can flow through the exit. x c) Vacuum cleaner
28. Archimedes’ Principle Archimedes’ Principle state that “ When an object is wholly or partially immersed in a fluid, the buoyant force is equal to the weight of fluid displaced by the object.
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30. Apparent weight, W 2 Actual weight, W 1 0.40 N 0.67 N Buoyant force, F b //Upthrust = Actual weight, W 1 – Apparent weight, W 2 = Weight of water displaced
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32. Examples 35 N 35 N 3.5×10 -3 m 3 3.5×10 -3 m 3
41. Pascal principle is widely used in a hydraulic system. Hydraulic system Output force = output piston area Input force input piston area F 2 = A 2 F 1 A 1
![UNDERSTANDING PRESSURE ,[object Object],P = pressure, Pascal (Pa) F = forces, N A = area, m 2 ( Unit: N m -2 ) State relationship between pressure and area. ,[object Object],[object Object],[object Object],[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)