Determining Density

In order to be able to determine densities we should learn:

Units Measurement of Length Measurement of Volume Measuring Mass and Weight

Units SI Units

The following table gives SI units for the basic physical quantities (things that can be measured). All scientists throughout the world use these units. (SI from the French “Le Systeme International d'Unites”.)

SI Units

Physical Quantity cal

SSI Unit nit

Symbol

    m

Mass    

  second  

    A

Temperature Kelvin  

Amount of substance Mole mol

Luminous intensity Candela cd

Length

Time

Electric Current

metre

kilogram kg

s

ampere

K

Units Prefixes

Used to express physical quantities that are very big or very small.

Although metres are the SI unit for length we use other units based on the metre.

Small objects will be measured in centimetres millimetres or micrometres.

Large objects will be measured in kilometres.

Prefix Meaning Symbol

Micro 1,000,000

Milli 1,000 m

Centi 100 c

Deci 10 d

Kilo 1,000 k

Mega 1,000,000 M

Prefixes

Units conversions Express 5600 m/s in units of km/h

km 6.5

1000

5600 km 1 x

5600mx

1000m 1km

m

m

km 6.5

1000

5600 km 1 x

5600mx

1000m 1km

m

m h

ss

h

000278.03600

1h 1 x

1s x

3600s s 6060 1

skm

h

km

s

m20160

000278.0

6.5

1

5600 Thus,

…. and a tricky one!

Reading measuring cylinders

PrecautionsAlways take the following precautions when reading the volume of

a liquid:

1. Place the container on a flat horizontal surface.2. The eye must be positioned perpendicularly at the mark of the scale. This is

to avoid errors in measurement due to parallax.  Q. What are the readings on the following measuring cylinders? (Scales in

cm³.)

10

15

35

40

20

30

Measuring Densities: Direct MethodRegular SolidsVolumes can be calculated by taking measurements then using formulae.

2 cm

3 cm2 cm

3 cm

2 cm

Volume of rectangular block =

Volume of cylinder =

Measuring densities:

Archimedes way or the high-way

Irregular Solids

1. Volume of a small irregular solid that sinks

Irregular Solids

2. Volume of a small irregular solid that floats

Measuring densities: Archimedes way or the high-wayIrregular Solids3. Volume of a larger irregular solid