Physics - Measurements and Units

These revision notes are for Physics IGCSE Measurements and units and uses the Stephen Pople as guidance, the sub-topics of this topic are:

Numbers and Units          A System of Units          Mass, Volume and Density

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Numbers and Units

The distance 10m is an example of a physical quantity (a number and a unit) and literally means 10 times meters. We can combine units, for example, speed = distance / time

A unit can be made by putting a prefix (symbol) in front of the measurement.

G (giga) 1 000 000 000

M (mega) 1 000 000

k (kilo) 1 000

d (deci) 1 / 10

c (centi) 1 / 100

m (milli) 1 / 1 000

µ (micro) 1 / 1 000 000

n (nano) 1 / 1 000 000 000

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A System of Units

Many scientists use SI units which are the most common units, for example, mass = kilogram, time = seconds and length = metre.

Mass is the amount of substance in an object. The greater the mass, the greater the pull of the Earth's gravitational pull and the greater resistance in the change of motion.

Time is kept by something that goes at a steady rate, old clocks used the swings pendulum but modern clocks use vibrations made by a tiny quartz crystal.

Measuring length is now done based on the speed of light.

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Volume, Density and Mass

If the volume (m³) of an object is a liquid, the volume can be measured by using a measuring cylinder and measuring the height.

If the object is a regular solid, the volume can be measured by measuring the height, width and length and timesing them together.

If the object is a irregular solid, the volume is measured by putting the object into a known amount of water in a measuring cylinder and seeing the increase of height once the object is in the water.

The mass of an object can be measured by a beam balance where you put the unknown object on one pan and known masses of other objects on the opposite pan, when the beam lies horizontal, you know the unknown object has the same mass as the other known side.

However, it is now more common to use a balance. 

Density (g/cm³) can be measured by the equation: density = mass / volume (p = m / V)

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