1.2.6 Density Objectives  = m/V Correctly use the equation for density  = m/V. p = F/A and pressure p = F/A

The density of an object is a measure of how tightly packed the matter is in the substance. As you will remember from Key Stage 3 there are 3 basics states of matter. However what can you remember about their spacing and their motion? SolidLiquidGas

© Pearson Education Ltd 2008 This document may have been altered from the original Week 7 Most Solids in the 1000’s Liquids in the hundreds Gases less than 10 Substance Density Kgm -3 Platinum (3 rd densest element)21,500 Lead11,000 Mercury13, 600 Iron7,870 Aluminium2,710 Glass2,500 Water1,000 Ice917 at -4 o C Ethanol789 Hardwood700 Balsa wood200 CO Air1.29 Methane0.7 Hydrogen These values are to give you a general idea NOT to be memorised!!

Measuring Density There are no set practicals for measuring the density of solids, liquids and gases but here are methods for each. Solids Record the mass in Kg of the object using an accurate set of scales. Record it’s volume either by calculation or measuring using the method shown. You may use a eureka can for a different approach.

Liquids Simply measure the mass of the measuring cylinder, add the liquid up to a convenient volume, calculate the mass of the liquid and calculate the density. This can be carried out for an ice cube by dropping the ice cube into a known volume of water, watching as it melts and recording the new volume after it has melted fully.

Air pumped out Re-weigh it, then allow the flask to refill with water by putting the tube into water and measuring the change in the volume of water. You can then calculate the density of the gas. Gases Slightly harder since the mass of a gas is very small. Have the gas in a flask and record mass of the flask and gas, remove the gas either by heating or pumping it out, seal the tube with a clamp.

Volume of air in cylinder © Pearson Education Ltd 2008 This document may have been altered from the original Week 7 Page 43 example question Air flowing past the blades of a wind turbine in 1 second has a volume of 16,000 m 3. The density of air is 1.29 kgm -3. What is the mass of the volume of air?

Meteorological balloon © Pearson Education Ltd 2008 This document may have been altered from the original Week 7 Page 43 Question 4 A Meteorological balloon expands as it rises through the atmosphere. At take-off its volume is 1.4 * 10 4 m 3. It is filled with Helium gas of density 0.18 kgm -3

1.2.7 Pressure We all experience pressure in may areas of our life. The first experience is probably being a small kid and getting a puncture on your first bike! The reason for using pneumatic tyres (air filled) is that they are still compressible even at the pressures in a racing bicycle tyre which is 4 times as high as a car or mountain bike! However this is not the case in systems relying on fluids or solids! Normal Air pressure is around Pa or 1.01 * 10 5 Pa This is sometimes referred to as 1bar. A bottle of champagne can withstand 6bar.

In order to calculate the pressure exerted we need to use the following equation… Pressure = force/area P = F/A P = F/A PressureN/m -2 Pascal (Pa) The units for Pressure are N/m -2 or Pascal (Pa) The Pascal is the pressure exerted when a force of ! Newton acts over and area of 1m 2

Make an approximate calculation of the pressure exerted by these two objects!

© Pearson Education Ltd 2008 This document may have been altered from the original Week 7

© Pearson Education Ltd 2008 This document may have been altered from the original Week 7 A manometer The manometer is used to measure a person’s blood pressure The air that is pumped in is slowly let out until the blood starts flowing again The pressures in the apparatus and your arm are equal! This is measured in mmHg the same as for weather measurements. Standard air pressure is 760 mmHg

Column of fluid You are aware of feeling pressure in a swimming pool as you feel it on your ears as you sit on the bottom This is caused by the weight of the water above you Calculated as Pressure = h  g © Pearson Education Ltd 2008 This document may have been altered from the original Week 7

Construction period: April 20, 1931 – March 1, 1936 Construction cost: $49 million ($676 million if built now) Deaths attributed to construction: 96 Dam height: m, second highest dam in the USA Dam length: m Dam thickness: 200 m at its base 15 m thick at its crest. Built to withstand a lot of pressure!

© Pearson Education Ltd 2008 This document may have been altered from the original Week 7 Forces acting upon a ship It is the pressure exerted in an upwards direction that causes a boat to float This pressure must be equal to the weight of the boat This causes the boat to float at different depths depending on it’s weight If you have ever tried to force a football or swimming float under water, you will have noticed that the deeper you push it down the greater the force needed.