2. Mass - the amount of matter in a substance
-depends on the number and size of particles making up that substance
-the same wherever it is measured
Mass
What is mass?
3. What is the SI unit of mass?
SI unit for mass – the kilogram (kg)
Large masses (e.g. a car) - measured in tonnes
1 tonne
= 1 000 kg
Small masses (e.g. a watch) – measured in grams (g)
1 gram (g)
= 0.001 kg
4. A beam balance - gives very accurate measurements
An electronic balance - easier to use - gives more accurate measurements - gives digital readings
Instruments for Measuring Mass
5. Weight - the gravitational force acting on a body
The weight of a body is greater on Earth than on the moon. Why? The earth exerts a greater gravitational force on it than the moon.
Weight
What is weight?
6. Measuring weight
A compression spring balance
An extension spring balance
7. What is the SI unit of weight?
SI unit for weight - the newton (N)
A body of mass 1 kg has a weight of about 10 N.
Hence an apple of mass 100 g will have a weight of about 1 N
8. In science, such statements are not accurate
Why?
There is no mention of the volume of the materials being compared.
We sometimes hear people saying that expanded polystyrene is lighter than wood or that water is heavier than oil or that aluminium is lighter than iron.
9. We can correctly say iron is heavier than aluminium,
expanded polystyrene is lighter than wood
or that water is heavier than oil
only when comparing equal volume of them.
10. Iron has a greater mass than an equal volume of aluminium and so we say ‘Iron is denser than aluminium.’ Expanded polystyrene has a smaller mass than an equal volume of wood and so we say ‘Expanded polystyrene is less dense than wood.’
In scIence, the term ‘density’ Is used.
11. Density
1 cm3 of aluminium has a mass of 2.7 g, so we say
the density of aluminium is 2.7 g/cm3.
1 m3 of air has a mass of 1.29 kg, so we say
the density of air is 1.29 kg/m3.
12. Definition of Density: The density of a substance is its mass per unit volume.
Mass
Volume
Density =
SI unit for density is the kilogram per cubic metre (kg/m3). Another common unit for density is the gram per cubic centimetre (g/cm3).
Formula:
13. Mass
Volume
Density =
Finding the density of a substance
Step 1: Measure the mass and the volume of the substance
Step 2: Divide the mass of the substance by its volume.
14. Finding the density of a liquid
Step 1: Measure the mass and the volume of the substance
Mass
Volume
Density =
Step 2:
15. Finding the density of a regularly shaped object Step 1: Measure the mass and calculate from dimensions, the volume of the object
l
h
b
l
l
l
r
r
h
Mass
Volume
Density =
Step 2:
19. Substance Density in g/cm3
Platinum 21.5
Gold 19.3
Lead 11.3
Silver 10.5
Copper 8.9
Iron 7.9
Zinc 7.1
Aluminium 2.7
Glass 2.5
Ice 0.92
Pinewood 0.50
Cork 0.24
20. Substance Density in g/cm3
Mercury 13.6
Glycerine 1.26
Sea water 1.03
Pure water 1.00 (at 4OC)
Olive oil 0.92
Turpentine 0.87
Petrol 0.80
Alcohol 0.79
Air 0.001 29
Hydrogen 0.000 09
21. Substances, e.g. a piece of metal or a volume of gas become less dense when they are hotter.
Why is this so?
They expand when they become hotter. With a bigger volume, their densities decrease, since density is mass per unit volume.
22. Density of ice is 0.92 g/cm3. Density of sea water is 1.03 g/cm3.
Hence ice is less dense than water and so it floats.
23. Objects such as pebbles or ball bearings sink in water because they have a higher density/are denser than water.
Floating and sinking
Some objects float and some others sink.
A piece of cork or wood will float on water because it has a lower density/is less dense than water.
25. What will happen to a solid when immersed in a liquid of the same density?
The solid will be freely suspended in the liquid.
26. Visit the following websites to learn about
1. How a submarine can either float or sink depending on changes in its density
http://science.howstuffworks.com/submarine1.htm
2. How a hot air balloon works – has to do with density again
http://science.howstuffworks.com/hot-air-balloon.htm
