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# Ch06 pressure o com_pwb

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### Ch06 pressure o com_pwb

1. 1. 43 CHAPTER 6 Pressure This chapter is organised into the sections as follows: A. Pressure B. Challenging Questions SECTION A Pressure (Take g = 10 N/kg) 1 A rectangular container measures 0.5 m by 0.4 m by 0.8 m, and has a mass of 91.8 kg. What is the minimum pressure it can exert? A 229.5 N/m2 B 459 N/m2 C 2 295 N/m2 D 4 590 N/m2 [ ] 2 Eskimo Joe usually wears snow shoes when he goes out of his igloo. The main reason he needs to do this is because A snow shoes are in fashion. B snow shoes are not easily soaked. C snow shoes make larger area of contact with the ground. D snow shoes do not get dirty so quickly. [ ] 3 A strong wind exerts an average pressure of 1.2  105 Pa on a sail of dimension 2.0 m by 2.25 m. What is the force of the wind? A 5.3  104 N B 2.4  105 N C 2.7  105 N D 5.4  105 N [ ] C D C 06 OPtwb 0193 p43-46.indd 43 8/29/07 9:56:14 AM
2. 2. 44 Pressure 6 4 A circus elephant of mass 3 000 kg balances on one of its legs on a red ball, such that the total surface area in contact with the ball is 0.075 m2 . Determine the pressure it exerts on the ball. A 3 000 Pa B 30 000 Pa C 40 000 Pa D 400 000 Pa [ ] 5 A baby elephant, a 10-wheeler truck, a lady in high-heel shoes and a giant are positioned on a sandy beach. Which of them will most likely be able to make the deepest impression on the sandy beach? A baby elephant B 10-wheeler truck C lady in high-heel shoes D giant [ ] 6 Explain: (a) why walking barefooted on pebbles is more painful than walking on sand. [2] (b) why nails must have sharp point to be hammered into wall easily. [2] 7 (a) (i) Complete the formula that follows: Force =  area [1] (ii) Identify the SI unit for the missing quantity. [1] D C As one walks on sandy ground, there is an increased contact points, such that the area of contact of the bare feet with the ground is much larger than that with the pebbles alone. A smaller pressure is produced. Hence sand will not cut deep into the ﬂesh. With a sharp point, the area of contact of nail with wall is much smaller. This increases the pressure, making it easier to penetrate into the wall. pressure Its SI unit is N/m2 (or Pa). 06 OPtwb 0193 p43-46.indd 44 8/4/07 3:53:48 PM
3. 3. 45 Pressure 6 (b) Shirley is trying to push a thumb tack into a pin-board, such that a pressure of 105 Pa can be produced. The sharpest point on the thumb tack has an area of 0.1 cm2 . (i) Convert 0.1 cm2 to m2 . [1] (ii) Hence calculate the pushing force Shirley needs to exert. [2] 8 A rectangular concrete block of dimension 0.15 m by 0.45 m by 1.24 m is resting on a layer of plasticine. The weight of the concrete block is 3 000 N. The pressure it exerts on the plasticine is different, depending on which way up the block is positioned. (a) Complete the table below to ﬁnd out which position the concrete block exerts the greatest pressure. [2] Position Force/N Area of contact base/m2 Pressure exerted/N m–2 A 3 000 1.24 0.45 = 0.558 5.38 103 B 3 000 C 3 000 (b) From the table, what can you infer about the relationship between the pressure exerted and the area of contact? [1] (c) Hence deduce at which position will the concrete block sink the deepest into the plasticine. [1] 0.15 m 0.45 m 1.24 m plasticine layer 0.1 cm2 = 0.1  1 cm2 = 0.1  (0.01 m)2 = 0.000 01 m2 Force = pressure  area = 105  0.000 01 = 1 N 1.24  0.15 = 0.186 1.61  104 0.45  0.15 = 0.067 5 4.44  104 The smaller the surface area of contact, the larger is the pressure exerted. Position C 06 OPtwb 0193 p43-46.indd 45 8/4/07 3:53:49 PM
4. 4. 46 Pressure 6 SECTION B Challenging Questions 1 The diagram shows Mr Brown applying a force on a cement block with his friend Mr Johnson, who is bravely sandwiched between beds of sharp nails, beneath the cement block. CAUTION – DO NOT ATTEMPT THIS AT ALL! (a) Explain whether it is (i) wise for Mr Brown to begin the demonstration by getting Mr Johnson to sit on a few nails and work upwards to more nails. [2] (ii) safer to use a cement block with smaller mass or greater mass. [2] (b) Hence explain why it is possible to lie on a bed of nails. [2] With fewer nails, the area of contact Mr Johnson has with the bed of nail is much reduced. This will increase the pressure the nail would exert on him, causing much pain and damage to his skin. The greater the mass of the cement block, the smaller the acceleration of the block and the bed of nails towards Mr Johnson. This will cause less damage to Mr Johnson. However, the cement block should not be too heavy as to cause direct damage to Mr Johnson. The many nails increase the area of contact with Mr Johnson’s skin. Hence much of the applied force is distributed over the many nails that make contact with his skin. This results in a tolerable pressure that does not puncture his skin. 06 OPtwb 0193 p43-46.indd 46 8/4/07 3:53:49 PM