MEASUREMENT OF LENGTH

• MEASUREMENT OF LENGTH

Length can be measured by (1) Direct Methods or by (2) Indirect Method.
Direct Methods:
These methods involves the use of
i. metre scale ( 10 RISE TO POWER-3 m to 10 RISE TO POWER 2m)
ii. vernier calipers (up to 10 RISE TO POWER -4m)
iii. screw gauge ( up to 10 RISE TO POWER -5m)
Indirect Method:
Beyond the range 10 RISE TO POWER -5 to 10 RISE TO POWER 2 we use indirect methods.

• REFLECTION METHODS

1. Echo Method: It is method used to find the distance (x) of a hill from a given point. In this method sound wave sent from the point of observation P to hill. The sound wave is reflected back by hill. By finding the time difference (t) between transmissions of sound and heard of echo.
The distance travelled by sound wave = x + x = v × t
⇒ 2x = v × t
⇒ x =        

Hence we can find x using x =        

, where v is velocity of sound.

2. Laser Method: It is used to find the distance of moon from earth. The laser beam is transmitted from earth is received back on earth after reflection from moon. The time interval (t) between transmission and reception of beam is measured.
If c = velocity of Laser
Then distance = velocity × time
x +x = c × t
2x = c × t
Or x =        

Where x = distance between earth and moon.
3. Sonar Method: (Sound Navigations and Ranging): This method is used to find the depth of ocean and to locate submarine inside the water. In this method ultrasonic waves are sent through ocean from transmitter to submarine. These ultrasonic wav are reflected from the submarine. By finding the time difference (t) between
transmitted and reflected waves, the distance x of submarine can be find using the formula

x =  c * t/ 2