Lecture 5 1_mass_and_density

Mass and Density

Have Fun !!!

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?

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

A beam balance

- gives very accurate measurements

An electronic balance

- easier to use

- gives more accurate measurements

- gives digital readings

Instruments for Measuring Mass

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?

Measuring weight

A compression spring balance

An extension spring balance

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

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.

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.

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.

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.

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:

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.

Finding the density of a liquid

Step 1: Measure the mass and the

volume of the substance

Mass

Volume Density =

Step 2:

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:

Finding the density of an irregularly shaped

object

Finding the density of an irregularly shaped

object

Finding the density of an irregularly shaped

object

Mass

Volume Density =

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

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

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.

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.

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.

olive oil 0.92 g/cm3

water 1.0 g/cm3

glycerine 1.3 g/cm3

cork 0.24 g/cm3

wood 0.65 g/cm3

aluminium

2.5 g/cm3

What will happen to a solid when immersed in

a liquid of the same density?

The solid will be freely suspended in the liquid.

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