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A vector has both magnitude and direction. Does it mean that anything that has magnitude and direction is necessarily a vector? The rotation of a body can be specified by the direction of the axis of rotation, and the angle of rotation about the axis. Does that make any rotation a vector?

For a quantity to be a vector, magnitude and direction are not the only sufficient conditions. Rotation of a body is specified by direction. But, it is not necessary that it is always a vector quantity. Finite rotation of a body about an axis is not a vector because finite rotation does not obey the laws of vector additions. Only infinitesimally small rotation is a vector quantity.
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Can you associate vectors with: 
(a) length of a wire bent into a loop
(b) a plane area
(c) a sphere? Explain.


(a) Yes, the direction of the length of a wire bent in the form of a loop is along the tangent to the loop.

(b) Yes, area is a vector whose direction is perpendicular to the plane of area.

(c) No, we cannot associate vector with the volume of the sphere.

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A fighter plane flying horizontally at an altitude of 1.5 km with speed 720 km/h passes directly overhead an anti-aircraft gun. At what angle from the vertical should the gun be fired for the shell with muzzle speed 600 m/s to hit the plane? At what minimum altitude should the pilot fly the plane to avoid being hit? (Take g = 10 m/s2)

Velocity of shell is,   u  = 600 m/s
Velocity of fighter plane is,
                 v = 720 km/hr = 200 m/s

Velocity of shell is,   u  = 600 m/sVelocity of fighter plane is, 
Let the shell be fired at angle θ with vertical. To hit the fighter plane, the horizontal component of muzzle speed should be equal to the velocity of plane.
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or                             200 space equals space 600 space sin space straight theta
or                            sinθ space equals space 1 divided by 3
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     The maximum height attained by the shell is,
              straight H equals fraction numerator straight u squared sin squared left parenthesis 90 minus straight theta right parenthesis over denominator 2 straight g end fraction
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or         space space straight H asymptotically equal to 16 space km
     Therefore the minimum altitude for safe flight is 16km.


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For any arbitrary motion in space, which of the following relations are true :

A.

 aaverage = [ v (t2) - v (t1 ) ] / ( t2 – t1)


A. TRUE

Tips: -

 The arbitrary motion of the particle can be represented by this equation.
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A vector has magnitude and direction. Does it have a location in space ? Can it vary with time ? Will two equal vectors a and b at different locations in space necessarily have identical physical effects ? Give examples in support of your answer.

i) A vector in general has no definite location in space because a vector remains unaffected whenever it is displaced anywhere in space provided its magnitude and direction do not change. However, a position vector has a definite location in space.

(ii) A vector can vary with time as example the velocity vector o an unaffected particle varies with time.

(iii) Two equal vectors at different locations in space do not necessarily have same physical effects. For example, two equal forces acting at two different points on a body which can cause the rotation of a body about an axis will not produce equal turning effect.
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