Something that can be measured. An appropriate unit is required for measurement. eg.

Time - Seconds

Speed - m/s

mass - KG

Mass - Kilogram (kg)

Length - metre(m)

Time - Seconds(s)

Electric current - ampere(A)

Absolute temperature - Kelvin (K)

Amount of substance - mole (mol)

Luminous intensity - Candela (cd)

A unit made of a combination of the fundamental units.

eg. 

Newtons:
 kgms^-2

0.000 000 456 m

in scientific notation 

1. convert number to scientific notation

2. find the closest multiple of 10³ (below the original)

3. replace the 10^x with the metric prefix

eg1.

123 000m = 1.23 x 10⁵ m
=1.23 x 10² x 10³ m

=123 x 10³ m

=123 km

eg2.

0.00000036g =3.6 x 10^-7 g

=3.6 x 10² x 10^-9 g

=360 x 10^-9 g

=360 ng

An error that will create precise, but not necessarily accurate, results.

eg. If scales have 1g of substance on them the result will always be 1g off what it should be.

add the uncertainty:

eg1.

24±1 + 53±6

=77±7

eg2.

34±5 - 12±2

=22±7

add the percentage uncertainties:

eg1.

100±10 x 50±1

10/100 = 10%, 1/50 = 2%

10% + 2% = 12%

100 x 50 = 5000

5000 x 12% = 600

answer: 5000±600

eg2.

50±2  / 25±5

2/50 = 4%, 5/25 = 20%

20% + 4% = 24%

50/25 = 2

2 x 24% = 0.48

answer: 2±0.48

significant figures:

a.       1050 m to the nearest 10 m

b.      12 000 m to the nearest 100 m

c.   225 000 000 g to the nearest 10 000 m

a - 3

b - 3

c - 5

Work to the lowest precision. Use the least number of decimal points.

eg1.

1505g + 12.578g
=1517.578g
=  1518g because 1505 has no digits after decimal

Work to the least number of significant figures.

eg1.

1505 x 15.578 = 18929.89

= 1.893 x 10⁴ because the 1505 has only 4 sig. figures so the answer must too.