Determination of distance
For measuring large distances such as the distance of moon or a planet from the Earth, special methods are adopted. Radio-echo method, laser pulse method and parallax method are used to determine very large distances.
Laser pulse method
The distance of moon from the Earth can be determined using laser pulses. The laser pulses are beamed towards the moon from a powerful transmitter. These pulses are reflected back from the surface of the moon. The time interval between sending and receiving of the signal is determined very accurately.
If t is the time interval and c the velocity of the laser pulses, then the distance of the moon from the Earth is d = ct/2
Determination of mass
The conventional method of finding the mass of a body in the laboratory is by physical balance. The mass can be determined to an accuracy of 1 mg. Now−a−days, digital balances are used to find the mass very accurately. The advantage of digital balance is that the mass of the object is determined at once.
Measurement of time
We need a clock to measure any time interval. Atomic clocks provide
better standard for time. Some techniques to measure time interval are given below.
The piezo−electric property* of a crystal is the principle of quartz clock. These clocks have an accuracy of one second in every 109 seconds.
These clocks make use of periodic vibration taking place within the atom. Atomic clocks have an accuracy of 1 part in 1013 seconds.
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