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�NAVODAYA VIDYALAYA SAMITI�(An Autonomous Body Under MHRD) Government Of India �

Presents

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E-Content for Class VII (Science)

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Chapter 13

MOTION AND TIME

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-Developed by

Mr. Vinod Kumar

T.G.T. Science

J. N. V. Dibrugarh, Assam

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Learning Objectives-

1.Knowledge of Time

2.Understanding of the Simple Pendulum and time period

3.Knowledge of units of time and speed

4.Knowledge of common modern and ancient time-measuring devices

5. Skills of measurement of Speed

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Measurement of Time-�The interval between two instances is called time.

  • For measuring shorter time intervals second, minute and hour are used.
  • Time is measured by devices like watch and wall clock. Both these use periodic motion to measure the time. In a wall clock the pendulum shows periodic motion to measure time.

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Modern Time measuring Devices

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Ancient Time measuring Devices

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Simple Pendulum- �

  • A simple pendulum is a small metallic ball suspended by a thread from a rigid point. The small metallic ball is called bob of the pendulum.
  • When the bob is released from after pulling it slightly to one side, it starts moving to other side. The to and fro motion of the simple pendulum which repeats itself after a fixed interval of time is called periodic or oscillatory motion.

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The time taken by simple pendulum to complete one oscillation is called ‘time period’. �

A Simple Pendulum Different Positions of the bob of

an oscillating simple pendulum

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Units of Time and Speed-

The basic unit of time is called second.

Its symbol is ‘s’ or ‘sec’. Minute (m)

and hour (h /hr) are larger units of

time. Centuries, millenniums, millions

and billions of years are also used to

express historical and evolutionary

events.

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The basic unit of speed is m/s. It can also be written in

m/min or km/hr. The symbol of any unit is always

written in singular. e.g.- 21 km and not 21 kms.

Measuring speed

If the value of distance covered by an object and time taken is known to us, the value of speed can be deduced by the formula-

Speed = Distance/Time

We can find distance if the value of speed and time is known to us.

Distance= speed X Time

Similarly we can find time if the value of speed and distance is known to us.

Time= Distance/Speed

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Animal

Speed in km/h

Speed in m/s

Falcon

320

320 x1000/60 x 60

88.8 m/s

Cheetah

112

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31.1 m/s

Rabbit

56

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15.5 m/s

Squirrel

19

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5.2 m/s

Human

40

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11.1 m/s

Speedometer records the speed in km/h and odometer records the distance travelled in km.

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Speedometer Odometer

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Points to remember-

* The interval between two instances is called time. S.I. unit of time is ‘s’.

* Modern time pieces work on working of simple pendulum.

*The thread and bob form a simple pendulum that remains hanged from a point. The time taken by simple pendulum to complete one oscillation is called ‘time period’.

* Speed = Distance/Time

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THANK YOU