6. Measurement of Physical Quantities

Weight is the force with which a planet pulls an object towards its centre. The mass of an object is the same everywhere. But the gravitational force is different on different planets. So the weight of the same object is different on different planets. For example, the gravitational force of the moon is one-sixth of the earth. So an object weighs only one-sixth on the moon.

• Use the proper device for measuring each quantity.
• Use only standard units. Do not use body parts like hand-span.
• Start measuring from the zero mark. Keep the scale straight along the object.
• Keep the eye exactly above the mark while reading.
• Check that the balance has the stamp of the Weights and Measures department.
• Check that the weights are made of metal and the balance is steady.

A physical quantity which can be expressed by its magnitude only is called a scalar quantity. It does not need a direction. Examples: length, mass, volume, temperature, time and speed. For example, the mass of a bag is 5 kg.

A physical quantity which needs both magnitude and direction is called a vector quantity. Examples: displacement, velocity, force and weight. For example, a car moves 20 km towards the north.

The two main causes of errors are not using the proper device and not using the device properly. Some common errors are:

• Using body parts like hand-span as units. Each person gets a different answer.
• Using a wrong device. For example, measuring medicine with a kitchen spoon.
• Not starting from the zero mark of the scale.
• Not keeping the scale straight along the object.
• Reading the scale from the side instead of from straight above.
• Using a balance which is not level, or whose pan is tampered with.
• Using stones or worn-out weights instead of standard weights.

To avoid errors, we should use the right device in the proper way.

The size of body parts like hand-span and foot is different for different people. So everyone gets a different measurement of the same object. A unit must be the same for everyone. So body parts cannot be used as standard units.

Weights and balances wear out with regular use. Some traders may also tamper with them. Then customers get less than what they pay for. Regular checking finds and corrects such errors. So regular standardization protects customers from being cheated.

Some substances are very costly or very important. Some are used in very small quantities. A small error in measuring them can cause a big loss. So accurate measurement is needed. The devices used are:

• Gold and silver – electronic (digital) balance
• Body temperature – clinical or digital thermometer
• Time in running races – digital stopwatch
• Dose of medicine – measuring cup, dropper, syringe
• Chemicals in a laboratory – physical balance, measuring cylinder
• Petrol and diesel – standardized petrol pump meter

• Length (metre) – ruler, measuring tape
• Mass (kilogram) – common balance, electronic balance
• Weight (newton) – spring balance
• Time (second) – clock, stopwatch
• Volume of liquid (litre) – measuring cylinder
• Temperature (degree Celsius) – thermometer
• Speed of a vehicle (km/hr) – speedometer
• Distance travelled by a vehicle (km) – odometer
• Electricity used (unit) – electric meter
• Rainfall (mm) – rain gauge