Pressure. Science Matters Chapter 38. What is pressure? Force applied by an object over a certain area

Transcription

1 Pressure Science Matters Chapter 38 What is pressure? Force applied by an object over a certain area Applying more force over the same area will increase the pressure

2 Applying the same force over a larger area will decrease the pressure Applying the same force over a smaller area will increase the pressure Pressure is greater when the area been pressed is smaller

3 Definition Pressure is force per unit area Units Pressure is measured in Pascals (Pa) Newtons per square metre (N m-2) Example Question: A Stone block weight N and its Dimensions are 3m x 2m x 1.5m. You may place any side onto the ground. What is the greatest possible pressure that the block can exert on the ground? Answer: The pressure will be greatest when the smallest surface is on the ground

4 = Nm 2 or 75 KPa...and smallest when large surface is on the ground = 37500Nm 2 or 37.5 KPa Pressure in Liquids Objects under water experience pressure. The deeper the object is the greater the pressure. This is because of the weight of the water above them. Pressure increases by 100,000 Pa every 10m down

5 Investigating if the pressure in a liquid depends on depth Atmospheric Pressure Air contains a mixture of gases and Gravity keeps the atmosphere in place. Gravity means that the atmosphere has a weight. The pressure of this weight bearing down on the earth is called atmospheric pressure Atmospheric pressure pressure at sea level is 100,000 N M 2. This equals 10 tonnes of coal. The pressure inside our bodies is equal to sea level atmospheric pressure so it all balances out and we feel normal. As we rise above sea level the pressure decreases Tops of mountains and in aeroplanes our ears pop trying to equalize the pressure. There is no pressure in space because there is no air so astronauts need special suits. Measuring Pressure Atmospheric Pressure is measured with a Barometer There are 2 types Mercury and Anaeroid. Anaeroid barometers are the type we find in homes and offices Barometers used to measure height over sea level (in planes) are called altimeters.

6 Units of Pressure The Unit is the Pascal (Pa). Normal Atmospheric pressure is 76 cm of Mercury (Hg). The correct SI unit is the Pascal (Pa). 76 cm of mercury is equal to x 105 Pa or 1013 hectopascal (hpa) Atmospheric Pressure & the weather The atmosphere acts like a blanket around the world. It blocks too much sun from reaching the earth and keeps the earth warm when the sun is on the other side of our planet. No atmosphere and the earth would have temperatures between 80 o C and 140 o C Meteorology Study of the atmosphere focusing on the weather. Meteorologists are weather scientists Isobars on a weather map indicate the type of weather we will have. When they are Close together the winds are strong. When they are Far apart the winds are light.

7 When there is High Pressure we have nice weather. In summer we get sunshine. In winter we get clear but cold days. High pressure does not allow water vapour to rise...so no clouds Low pressure (below 1013 hpa) the weather is cloudy and unsettled. When there is Very Low pressure we get Stormy weather. Winds raise the air from the ground and sea and form clouds Clouds keep the temperature warmer by preventing heat loss into the atmosphere. High Pressure (over 1013 hpa) means dry settled weather. The high pressure does not allow the air to rise and so there are no clouds. In summer this means bright sunny days. In winter clear but cold nights. Using Pressure

8 Hydraulics Liquids can be used to get a large force from a smaller one If we apply a force to a small area then we get big pressure we can then use this big pressure to lift a big weight Water Supply System The reservoir of a water supply system will exert a greater pressure the higher above the house it is placed We must pump water when their is not enough pressure Flight Medicine Blood pressure is a measure of how good a persons health is