# A mountaineer shouts his name in valley . Half a sec later , he hears the echo . How far away is the rock which is reflecting his voice ?

1 ) A mountaineer shouts his name in valley . Half a sec later , he hears the echo . How far away is the rock which is reflecting his voice ?

A ) 80 m , B ) 86 m ,

C ) 82 m , D ) None of these ,

Correct Answer : — B ) 86 m ,

Solution : -

Given data : -

· t = 0.5/2 sec = 0.25 sec ( we divide time by 2 because voice took 0.5 sec ( half sec ) for both going towards rock and reflecting back to person . And we just have to find distance between rock and person in single direction ) ,

· Speed of sound in air = 343 m/s ,

· Distance = ? ( between rock and person ) .

Formula : -

v = d / t ,

Ø d = v x t ,

Ø d = 343 x 0.25

Ø d = 85.75 m , Answer .

2 ) A wire is bent into a ring of radius R and given a charge q . The magnitude of electric field at the center of ring is ……. ,

A ) Infinite , B ) finite ,

C ) zero , D ) None of these ,

Correct Answer : — C ) zero ,

3 ) The electric field due to an infinity long thin wire at a distance R varies as ……….. ( 1/R ) .

4 ) A particle is displaced from a position 2i — j + k to another position 3i — 2j +2k under action of a force ( F = 2i — j + k ) . The work done by force is ?

A ) 2J , B ) 4J ,

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