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Time and Speed

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Time and Speed - Lesson Summary


The distance covered by an object in unit time is called its speed. Objects are said to be in fast or slow motion depending upon the speed of their motion.  Speed is calculated using the expression

Speed =  Distance Time

The SI unit of time is second (s) and that for distance is metre (m). So the SI unit of speed is metre per second (m/s). Speed is also measured in kilometre per hour (km/h). The average speed of a moving object is defined as the total distance covered by it divided by the total time taken.

The instrument used in vehicles to indicate speed is called the speedometer. The distance covered by a vehicle is indicated by the odometer. 

An object moving along a straight line with constant speed is said to be in uniform motion. The body in uniform motion travels equal distances in equal intervals of time. An object is in non-uniform motion if its speed keeps varying, i.e. it travels unequal distances in equal intervals of time.

Measurement of Time

There are certain events in nature that can help us track time. For example, the phases of the moon indicate the time of the month. There are man-made structures that were constructed to measure time. For example, the Jantar Mantar in Jaipur, which is basically a sun clock, is used to measure time. Water clocks and sand clocks were also used hundreds of years ago to measure time. 

Mechanical clocks use oscillatory motion to measure time. The time period of a pendulum is the time taken for one complete oscillation. The working of a pendulum clock is based on the time period of its pendulum. Winding clocks and wrist watches were developed from modifications to the pendulum clock. A quartz clock is more accurate than other clocks because its oscillations are regulated by a quartz crystal.

Simple Pendulum

A small metallic bob suspended by a light, inextensible string from a rigid support, such that it is free to oscillate without friction about a point, is called a simple pendulum. The bob of the pendulum moves to and fro along the same path and passes through the mean position. This type of motion is called oscillatory motion. The maximum displacement of the pendulum, i.e., the displacement between the mean position and the extreme position, is called its amplitude. The time taken by an oscillating pendulum to make one complete oscillation is called its time period. The number of oscillations made by the pendulum in one second is called its frequency of oscillation. The SI unit of  frequency is hertz (Hz).

There are three laws that govern the time period of a simple pendulum.

  • The first law states that the period of oscillation of a simple pendulum of constant length is independent of its amplitude, provided the amplitude is small.
  • The second law states that the period of oscillation of a simple pendulum of constant length is independent of the size, shape, mass and material of the bob, provided it is not very light.
  • The third law states that the time period of oscillation of a simple pendulum is directly proportional to the square root of the length of the pendulum, for a given place.

It has been found experimentlly that the time period of a freely oscillating pendulum is given by

T = 2π( l g )½,

where l is the length of the pendulum and g the acceleration due to gravity.



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