• Physical Quantity is a quantity that can be measured. It consists of a numerical magnitude and a unit.

SI Units

• Are set systematic international units, for measuring mass, time, and other base units and derived quantities.

Mass

• Is the measure of the amount of substance in an object.

• Has the following effects:

  • all objects are attracted to the Earth. The greater the mass of an object, the stronger is he Earth’s gravitational pull on it.

  • all objects resist attempts to make them go faster, slower, or in a different direction. The greater the mass, the greater is the resistance to change in motion.

• SI base unit is kilogram (kg).

Time

• SI base unit is second (s).

• Following are some of other units for second:

Length

• SI base unit is metre (m).

Measuring Length and Time

Measuring Length

• Ruler can be used to measure small distances of a few centimetres (cm).

  • They are able to measure to the nearest millimetre (mm).

  • Can measure upto one metre.

  • Example of usage:

    • height of the table

• Micrometer is used to measure objects that are too small to be measured with vernier calipers

  • Gives a precision of 0.01 mm

  • Can measure measurements of less than 25 cm.

  • Example of usage:

    • diameter of a wire

• Vernier Calliper is a useful tool for measuring both internal and external diameters.

  • They are able to measure to a precision of 0.01 cm.

Measuring Time

• Time intervals of many seconds or minutes can be measured using a a stopwatch.

  • Some instruments have analogue or a digital display.

Volume and Density

Volume

• It is the quantity of space an object takes up.

• It’s SI unit is cubic metre (m^3).

Density

• It is the quantity of mass per unit volume of a substance.

• Formula:

  • Density= Mass/Volume

  • ρ=m/v

    • ρ is a greek letter ‘rho’ .

  • (both equations are same, written in different ways; i.e first one is the word equation and the other one is symbolic equation.

• SI unit of density is kilogram per cubic metre (kg m^-3).

• Objects that are less dense than water will float on water; and objects that are denser than water will sink.

Measuring Volume and Density

Measuring Volume

Liquid

• Its volume can be measured using a measuring cylinder.

• Most cylinders have scale marked in millilitres (ml) or cubic centimetres (cm^3).

Regular Solid

• If an object has a simple shape, its volume can be calculated through formula’s. For example:

  • volume of a regular block = length x width x height

Irregular Solid

• If the shape of the object is too awkward to calculate its volume, then immerse the object in a measuring cylinder filled with water and then calculate the increase in water level. This value will be the volume of the object.

Measuring Density

• Once volume of the object is obtained through one of the above methods suitable, find the mass of the object using a balance.

• Following is one of the example:

Checking the mass and density of a liquid

• An easy method of finding the density of a liquid is by using a small float called hydrometer.

• The relative density on a hydrometer indicates the relative density of the liquid.

• Checks like these are important in some production processes.