(a) A quantity which has only magnitude and no direction is known as scalar quantity. Distance, work, speed, time, energy, etc. are all scalar quantities. These quantities can be expressed completely by their magnitude alone.

(b) A quantity which has magnitude as well as direction is known as vector quantity. Force, velocity, displacement, etc. are all vector quantities. These quantities are expressed completely only when their directions are defined along with their magnitude.

Various types of errors can occur while taking measurement. These are as follows:

• Instrumental errors: These errors arise when the instrument used for measurement has become faulty. For eg, ruler with rough and broken edge can cause instrumental error.
• Observational errors: These errors arise when the reading in the instrument is noted incorrectly. These can also arise if the instrument is used in incorrect method. For example, if the tip of pencil does not coincide with zero of the scale, then we will get error in measurement of length of pencil.
• Theoretical errors: These are caused by simplification of the model system. For example, a theory states that the temperature of the system surrounding will not change the readings taken when it actually does, then this factor will begin a source of error in measurement.
• Environmental errors: These errors arises due to the instruments working getting affected due to some environmental factors such as temperature, pressure, etc.

(b) Because of difference in physical conditions from place to place, the reference chosen as units for weights and measures varies. So, it becomes necessary to get the weights and measures standardized at regular intervals.

We need not be very accurate every time we take a measurement. The degree of accuracy in measurements vary according to the situations. For example,

• when someone asks about the time of the day, we do not have to be specific up to the seconds of time, because there is no need to be that accurate in the given situation. On the other hand, for laboratory purposes or for scientific experiments, we have to measure the time even up to milliseconds accurately.
• while measuring mass of precious substances, such as gold, silver, etc., we need to be accurate in our measurement.
• in competitions and events, a difference of second can make you lose or win the game. Thus, accurate time measurement in such cases is very important.
• shopping in our day to day life, like for groceries, milk, flour etc. requires lot of alertness at our part so that we do not get cheated. We should always check whether the measurement is done properly or not by the vendors selling these products.

The devices used for accurate measurement are:

• For time measurement, stop watch can be an accurate device
• For mass measurement, electronic balance can be an accurate device.
• For length measurement, ruler or measuring tape can be the accurate devices.
• For temperature measurement, thermometers can be an accurate device.