Basic Facts 1

This is the first of a weekly set of web pages that sumarize the material of the course. Material appearing on these pages is likely to appear in the weekly quizzes. You are responsible for commiting the material on these pages to memory.

Physical Measurements and Units

We shall work with the following physical quantites in terms of these units.

 

Quantity

Unit

Dimensions

Meaning

Mass

kilogrammes (kg)

kg

amount of material

Length

metres (m)

m

Time

seconds (s)

s

Temperature

Kelvin (K)

1K = 1°Celsius

Velocity

metres per second

m s-1

how fast something goes

velocity = distance travelled
                     time taken

Acceleration

metres per second squared

m s-2

how fast velocity alters

acceleration = change in velocity
                           time taken

Force

Newtons (N)

1N = 1 kg m s-2

a push or pull, what it takes to make something accelerate

acceleration = total applied force
                      mass of the object

Work

Joules (J)

1J = 1 kg m2 s-2

force times distance, when you do physical work you get tired

Work=Force × Distance moved in direction of force.

Energy

Joules (J)

1J = 1 kg m2 s-2

Energy is a measure of the ability to do work.

Work done = Energy after - Energy before

Pressure

Pascals (Pa)

1 Pa = 1 N m-2

Force per unit area. Used to describe forces that are applied over some large area.

Pressure =               Total Force             
                  area to which force is applied

Force = Pressure × Area
NOTE. If a unit is named after a famous physicist then the unit symbol is capitalized otherwise it is written in lower case.

Remember that distance, velocity, acceleration, and force are all quantities that have both a size and direction associated with them. We call quantities with both a size and a direction VECTOR quantities.

The other quantities, mass, time, temperature, work, energy, and pressure all have only a size. Quantities that have only a size are called SCALAR quantities.

Number Representations and Power Names

We shall find it useful to use Scientific Notation to write out numbers. The significant digits will be written starting in the tens or units place and with as many decimal places as are waranted by the precision of the number. The actual magnitude will then be added by showing a power of ten multiplier. For example

125,536 = 1.2536×105, 100,000,000 = 108, 0.013 = 1.3×10-2, 0.000001156 = 1.1156×10-6.

We often simplify our use of scientific notation by the use of power names. Thus we say 1000m = 1kilometre and 0.001m = 1millimetre. Here are the common prefixes in use

Prefix

Symbol

Power

Example

giga

G

109

1,000,000,000 Pascals = 1 giga Pascal = 1 GPa

mega

M

106

1,000,000 Joules = 1 mega Joule = 1 MJ

kilo

k

103

1,000 metres = 1 kilometer = 1 km

centi

c

10-2

0.01 metres = 1 centimetre = 1cm

milli

m

10-3

0.001 grammes = 1 milligramme = 1mg

micro

µ

10-6

0.000001 Newtons = 1 micro Newton = 1µN

nano

n

10-9

0.000000001 Joules = 1 nano Joule = 1nJ

NOTE. No matter what Christopher Loyd ("Doc" in Back to the Future) thinks, giga is pronounced geega not djiga!

Periodic Motion

Periodic motion is any motion that repeats itself after a time = P, called the Period.
We define the Frequency of motion by f = 1/P so that P = 1 /F.
We measure the frequency in Hz. 1 Hz = 1 repeat per second.

Physics 175