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If `l_(1), m_(1), n_(1)` and `l_(2), m_(2), n_(2)` are the direction cosines of two vectors and `theta` is the angle between them, then the value of cos `theta` is

A

`l_(1)l_(2) + m_(1)m_(2) + n_(1)n_(2)`

B

`l_(1)m_(1) + m_(1)n_(1) + n_(1)l_(1)`

C

`l_(2)m_(2)+m_(2)n_(2) + n_(2)l_(2)`

D

`m_(1)l_(2) + l_(2)m_(2) + n_(1)m_(2)`

Text Solution

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The correct Answer is:
A
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AAKASH SERIES-WORK, POWER & ENERGY -EXERCISE - I
  1. If l(1), m(1), n(1) and l(2), m(2), n(2) are the direction cosines of ...

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  2. If vec(A) + vec(B) = vec(C ), then magnitude of vec(B) is

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  3. If vec(a)=m vec(b)+vec(c ). The scalar m is

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  4. Choose the false statement

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  5. Three vectors satisfy the relation vec(A).vec(B)=0 and vec(A).vec(C )=...

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  6. Let vec(F) be the force acting on a particle having position vector ve...

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  7. The area under a .force - displacement. curve gives :

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  8. The work done by gravity in lifting a body to a certain height depends...

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  9. When a body is falling freely from some height work done by gravity is

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  10. A man is waiting at a bus stop by holding a box on his head. Work done...

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  11. A block of mass .m. is pulled by a force .F. on a level surface throug...

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  12. body against friction is mgS 12.

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  13. Two identical bodies of same mass are raised to same heights by two pe...

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  14. Two bodies of different masses are dropped similtaneously from the top...

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  15. Three spheres of different materials are in motion their radii, densit...

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  16. Two bodies of masses m, and m, have equal momentum. Their K.E. are in ...

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  17. The total workdone on a particle is equal to change in it.s kinetic en...

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  18. (A): No work is done by the centripetal force acting on a body moving ...

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  19. Two springs with stiffness constants K(1) and K(2) (K(2) gt K(1)) are ...

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  20. . A lorry and a car moving with same KE are brought to rest by applyin...

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