Using the formula for pressure in a fluid, making sure all values are in their standard unit: p gh (13.6 103)(9.8)(760 103) 101 292.8 The pressure due to the mercury at a depth of 760 mm is 101.3 kPa.
SAMPLE PROBLEM 4G
A cylinder of height 20 cm is submerged in water. Given that the density of water is 1 000 kg m3, calculate the difference in pressure between the top and bottom of the cylinder.
0.20 m Fig 4.8
SAMPLE ANSWER 4G
Using the formulae for pressure in a fluid, making sure all values are in their standard unit: ptop pbottom ght ghb, where ht depth to the top of the cylinder, hb depth to the bottom of the cylinder, density of water and g acceleration due to gravity
g (ht hb) gh,
h the difference in height between the top and the bottom of the cylinder
p (1 000)(9.8)(0.20) 1 960 Pa The pressure it exerts at the table is 1 960 Pa.
SCIENCE,TECHNOLOGY & SOCIETY
Atmospheric pressure and weather We live at the bottom of an ocean of air, called the atmosphere. This fluid exerts a pressure, just as any other fluid would. At sea level this is roughly 101.3 kPa (or 760 mm Hg). Variations in this pressure have an effect on the weather. Although weather prediction is far more complicated than this, we can generalise by saying high pressure results in fine, sunny weather, while low pressure results in cloudy, windy, wet weather. Atmospheric pressure is often expressed in hectopascals (hPa) on weather forecasts, 1 hPa 100 Pa.
