Tiêu đề TOPIC 7 PRESSURE - WazaElimu.com.pdf ✅

54 TOPIC 7: PRESSURE PRESSURE  This is the force per unit area. Pressure = Force Area SI-Unit of Pressure is N⁄M2 or Pascal (Pa)  Other units of pressure are: atmosphere (atm), millimeters of mercury (mmHg) and torr bar. 1 N⁄M2 = 1 Pa 1 atm = 760mmHg 1 atm = 101325 N⁄M2 1 atm = 1 bar 1 bar = 100000 Pa 1 atm = 101325 Pa NOTE:  For a given amount of force, the smaller the area of application, the greater the pressure exerted. PRESSURE DUE TO SOLIDS  The pressure in solids depends on the surface area of contact. A force (F) applied by a small area exert a higher pressure as compared to when it is applied by a large surface. Thus: Pressure = Force applied Area of contact Increase in area cause the decrease in pressure for the same amount of force. That is why:  Feet of elephant cannot sink into soft soil even though it is very heavy. This is due to large surface area over elephant feet.  Sharp edges of knife cut easily than blunt knife. This is because a sharp knife has a small surface area.  A tractor and other heavy truck works on soft ground cannot sink due to large surface area of the tyres (wide tyres).  We feel great pain if we carry a bucket of water with a thin handle than when we use a wide handle (enhanced handle). Questions 1. Explain why hitting an inflated balloon with a hammer will not cause it to burst but sticking it with a pin will burst. 2. Why are dams constructed thicker at the bottom than at the top? 3. A hole at the bottom of a ship is more dangerous than one near the surface. Explain. 4. The mass of a cuboid is 60kg. if it measures 50cm by 30cm by 20 cm. Calculate its maximum pressure. P = F A P= F A


57 VARIATION OF PRESSURE WITH DEPTH The pressure in liquid increase with depth, Consider below  Thus the holes of the container (can) that are at the same level will experience the same pressure and the liquid will spurt to the same distance. This is because pressure in a liquid acts equally in all directions. It is also equal at the same depth. Consider also the figure below:  The holes punched at different levels on the container will spurt the water to different distances.  The holes at the top have the least height of liquid above it. This means that the water spurt the shorter distance. This distance increases with an increase in the height above the hole.  Therefore, the hole at the bottom of the container will spurt water to furthest distance.  When water or other liquid is poured into a communicating vessel, it will attain the same level in all its tubes, no matter the different shapes of the tubes. This proves that for a given liquid, the pressure at a point within it varies only with depth. Therefore, Pressure at A, B, C and D is the same.