Physical quantities
 Physical quantity is a property of an object or substance that can be measured using an appropriate measuring instrument.
SI units
 All quantities in science consist of a number and a unit

There is a system of units used throughout the scientific world known as SI units. The SI units are based on the units of six base quantities:
 Length metre (m)
 Mass kilogram (kg)
 Time second (s)
 Temperature Kelvin (K)
 Electric current ampere (A)
 Amount of substance mole (mol)
Derived Units
 The units of all other quantities are derived from these base units. For example, speed is found by dividing the distance travelled by the time taken. Therefore the unit of speed is metres per second which can be written as m/s or ms1
 Each derived quantity has units which show how it is related to the base quantities.
 Example The unit of force is the Newton. What is this in derived SI units?
Force(N) = mass(kg) × acceleration(ms−2)
Unit of mass = kg
Unit of acceleration = ms2
Therefore units of Newtons = kgms2
Measuring length
 Length is a straightline distance between two points along an object
 SI unit for length is the metre (m)
 Instruments used to measure length include
 Ruler, measuring tape
 Vernier callipers
 Screw gauge micrometre
Ruler
 Have markings as small as a centimetre. Each centimetre is divided into 10 divisions which are millimetre.
 When taking measurements your eye must be directly above the reading to avoid parallax error
 Place the object to be measured in line with the zero mark to avoid zero error.
 Rulers can measure to the nearest millimetre
Vernier callipers
 Is used to accurately measure the thickness or internal diameter of small objects.

The callipers use a vernier scale and the simplest type enables a length to be measured to 0.01cm. it is a small sliding scale which is 9mm long but divided into 10 equal divisions so;
 One end of length to be measured is made to coincide with the zero of the millimetre scale and the other end with the zero of the vernier scale
 To measure internal diameter of a narrow tube, place the internal jaws inside the tube. Move the jaws apart until they touch the inner sides of the object. Once the jaws are in position, tighten the screw clamp to ensure the vernier scale does not move out of place while measurement is read.
How to read a vernier

To measure the internal or external diameter
 Take the millimetres from the man scale marking before the zero on the vernier scale.
 Take the next reading from the first vernier mark to coincide with a main scale mark
 Add the two readings
Example : What is the reading the instrument shown on the diagram above
Main scale reading = 11mm
Vernier reading = 0.4 mm
Final reading = 11.4mm
EXERCISE
Read the following vernier scales
Screw gauge micrometer
 Can measure smaller lengths than the vernier callipers.
 It can measure very small objects to 0.001cm. one revolution of the thimble opens the accurately flat, parallel jaws by one division on the scale on the shaft of the gauge; this is usually , i.e. 0.05cm
 If the thimble has a scale of 50 divisions round it, then rotation of the drum by one division opens the jaws by
 A friction clutch ensures that the jaws exert the same force when the object is gripped.
To read the micrometer
 Take the reading of millimetres and half millimetres from the sleeve 3.50mm
 Take the reading from the thimble 0.11mm
 Add the readings together 3.61mm