7th Class Science Time and Motion Motion and time

Motion and Time

Synopsis

• Change in the position of a particle with respect to time and a fixed place (observer) is known as motion.

• The rate of change of position in unit time interval is known as speed. Speed is also the distance covered by an object in unit time. Speed is relative and the same body can be said to be moving fast or slow when compared to different things.

• The S.I. unit of distance is metre (m) and that of time is second(s), therefore the unit of speed is metre per second (m/s or m s'1). The C.G.S. unit of speed is cm/s or cm s1.

• A body is said to be in non - uniform motion, if its speed or direction keeps changing.

• A body is said to be in uniform motion, if it moves with a constant speed in a straight line.

• Time is measured in seconds(s), minutes(min), hours(h), days, weeks, months, years and so on.

• Time is measured using clocks, watches, sundials, sand clocks, etc.

• Devices to measure time make use of some kind of periodic motion.

• The simple pendulum is one of the simplest instruments used to measure time. A simple pendulum consists of a heavy metallic ball (called the bob) which is suspended from a rigid surface by a thread.

• One complete ‘to’ and ‘fro’ motion is known as an oscillation.

• The time taken by the pendulum to complete one complete oscillation is known as the time period of the pendulum. The time period of a pendulum at a place depends only on the length of the pendulum and not on the weight or material of the bob.

• The symbols of all the units are represented by a single letter. To measure the speed of an object, the distance travelled by it and the time taken must be known. If the speed and time taken by the body is known, then the distance travelled can be determined.

• Total distance travelled = Speed Total time taken
• If the speed and the distance travelled is known, then the time taken can be found

• Different graphs are used to represent different kinds of data (or information).

• A speedometer measures speed while an odometer measures the distance covered.