7th Class Science Time and Motion Motion and Time

Motion and Time

Motion

If a body changes its position with time it is said to be in motion.

Different Types of Motion

Motion has been broadly classified into two groups. They are uniform motion, non-uniform motion.

Uniform Motion: If a body travels equal distance in equal interval of time then it is said to be uniform motion. For example, a car travels first 30 kilometres in one hour and next 30 kilometres again in one hour and so on then we say that car has uniform motion.

Non-Uniform Motion: If a body travels unequal distance in equal interval of time then it is called non-uniform motion. For example, a car travels first 30 kilometres in one hour and next 30 kilometres in half an hour then we say that, the car has non-uniform motion.

Speed

The speed of a moving body is the distance travelled per unit time. The speed of a moving body can be expressed as:

\[\text{Distance}\,\,\text{=}\,\,\text{Speed}\,\,\text{ }\!\!\times\!\!\text{ }\,\,\text{Time}\]

The SI unit of distance is metre (m) and of time is second (s), and the unit of speed is metre/second (m/s).

Time

It important to measure time. If the time is not measured, one would not know small things such as when a train or plane will arrive. The unit of measurement of time is the time interval between two events. An event that occurs at regular interval of time is called periodic. For example, the rising of sun, changing of seasons, the waxing and waning of the moon are periodic. The rising of sun is used to measure day and cycle of seasons are used to mark a year.

Time Measuring Instruments

In ancient times, instruments such as sundial, hourglass and candle clock were used for measuring time.

Sundial

Sundial consists of a triangular metal plate fixed vertically at the centre of a circular plate. To measure time, sundial was placed in the sun with the triangular plate known as gnomon along the north-south direction. The position of the shadows formed by the gnomon at different times of day were used to mark the time on the circular plate or dial. Sundial told quite accurate time, but the limitation was that it could neither be used after sunset nor carried around.

Hourglass

Another instrument for measuring time was hourglass. Hourglass consisted of two glass bulbs connected through small opening. One of the glass bulb was filled with sand. Sane passed from one bulb to another through small opening. When all the sand from one bulb had passed to another, the instrument can be inverted to measure the time again

Candle Clock

Another instrument for measuring time was candle clock. The candle was marked with numbers. As the candle burnt, the marking indicated time.

Pendulum Clock

The modern time measuring instrument is pendulum clock. An Italian astronomer Galileo Galilei discovered that the motion of a pendulum is periodic and can be used for measuring time.

The time taken by pendulum to complete one oscillation is called its time period. The time period of pendulum of the clock is 1 second.