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Two bodies of mass 1kg and 3kg have position vectors i^+2j^+k^ and -3i^-2j^+k^, respectively. The centre of mass of this system has a position vector -

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Explanation

The position vector of centre of mass 

r=m1r1+m2r2m1+m2

   =1i^+2 j^+k^+3-3i^-2j^+k^1+3

  =14-8i^-4j^+4k^

 =-2i^-j^+k^

The centre of mass changes its position only under the translatory motion. There is no effect of rotatory motion on centre of mass of the body.

Four identical thin rods each of mass M and length t, form a square frame. Moment of inertia of this frame about an axis through the centre of the square and perpendicular to its plane is 

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Explanation

Apply theorem of parallel axisand the total moment of interia will be the sum of moment of inertia of each rod.

Moment of inertia of rod about an axis through its centre of mass and perpendicular to rod + (mass of rod) × (perpendicular distance between two axes)

                =Mt212+Mt22=Mt23

Moment of inertia of the system = Mt23×4

                =43Mt2

 

A shell of mass 200g is ejected from a gun of mass 4 kg by an explosion that generates 1.05 kJ of energy. The initial velocity of the shell is -

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Explanation

 

In the given problem conservation of linear of momentum yields. 

     m1v1+m2v2=0

 or  4v1+0.2v2=0       ....(i)

Conservation of energy yields. 

12 m1v12 +12m2v22=1050 0r   12 ×4v12+12×0.2×v22=1050or    2v12+0.1 v22=1050 ....(ii)

Solving Eqs. (i) and (ii) , we have 

          v2=100 m/s

 

 

A thin rod of length L and mass M is bent at its midpoint into two halves so that the angle between them is 90°. The moment of inertia of the bent rod about an axis passing through the bending point and perpendicular to the plane defined by the two halves of the rod is 

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Explanation

To find the moment of inertia of a bent rod, we need to consider the moment of inertia of each half of the rod about the bending point and then add them. The moment of inertia of a rod about a perpendicular axis through its midpoint is (ML^2)/12.

A satellite is moving very close to a planet of density ρ. The time period of the satellite is:

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Explanation

T=2πRv=2πRGMR=2πR32GMT=2πR32G43πR3ρ=2πR322π3ρGR32=3πρG

A projectile is fired upwards from the surface of the earth with a velocity kve where ve is the escape velocity and k < 1. If r is the maximum distance from the center of the earth to which it rises and R is the radius of the earth, then r equals

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Explanation

(2)Ki+Ui=Kf+Uf12mkve2-GMmR=0-GMmr12mk2GMR2-GMmR=0-GMmrk2R-1R=-1rr=R1-k2

The gravitational potential difference between the surface of a planet and 10 m above is 5 J/kg. If the gravitational field is supposed to be uniform, the work done in moving a 2 kg mass from the surface of the planet to a height of 8 m is

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Explanation

Potential difference at 10m=5J/kgPotential difference at 8m=510×8=4J/kgWork done=mV=2×4=8J

A planet is moving in an elliptical orbit. If T, V, E, and L stand, respectively, for its kinetic energy, gravitational potential energy, total energy and angular momentum about the center of orbit, then

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Explanation


The direction of angular momentum does not change. It remains same at each and every point of the path. We can find the direction of angular momentum using right hand thumb rule.

In planetary motion the areal velocity of the position vector of a planet depends on the angular velocity (ω) and the distance of the planet from the sun (r). The correct relation for areal velocity is:

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Explanation

3A=πr2dAdt=2πrdrdt=2πrvdAdt=2πr2ω

If A is the areal velocity of a planet of mass M, its angular momentum is

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Explanation

2.

Angular momentum

= 2 x mass x Areal speed = 2 MA

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