A body hanging from a massless spring stretches it by 3 cm on Ear

Subject

Physics

Class

JEE Class 12

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 Multiple Choice QuestionsMultiple Choice Questions

1.

The one which does not represent a force in any context is

  • Friction

  • Impulse

  • Tension 

  • Weight


2.

The work-energy theorem states that the change in

  • kinetic energy of a particle is equal to the work done on it by the net force

  • kinetic energy of a particle is equal to the work done by one of the forces acting on it.

  • potential energy of a particle is equal to the work done on it by the net force

  • potential energy of a particle is equal to the work done by one of the forces acting on it


3.

A car of masses 1500 kg is lifted up a distance of 30 m by crane A in 0.5 minutes. The second crane B does the same job in 1 minute. The ratio of their powers is

  • 1 : 2

  • 2 : 1

  • 1 : 1

  • 1 : 4


4.

Water from a hose pipe of radius 5 cm strikes a wall normally at a speed of 5 ms-1. The force exerted on the wall in newton is

  • 13.5 π

  • 6.25 π

  • 62.5π

  • 27π


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5.

The position vectors of two identical particles with respect to the origin in three-dimensional coordinate system are r1 and r2. The position vector of centre of mass of the system is given by

  • r1 + r2

  • 2(r1 + r2)

  • r1 − r2

  • r1 + r22


6.

If a body of moment of inertia 2 kg m2 revolves about its own axis making 2 rotations per second, then its angular momentum (in Js) is

  • 2 π

  • 4 π

  • 6 π

  • 8 π


7.

A rigid body is the one in which

  • it can have only rotational motion

  • it can have only translational motion

  • the distances between all pairs of particles of the body do not change

  • its centre of mass always lies inside the material of the body


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8.

A body hanging from a massless spring stretches it by 3 cm on Earth's surface. At a place 800 km above the Earth's surface, the same body will stretch the spring by (Radius of Earth = 6400 km)

  • 3427 cm

  • 6427 cm

  • 2764 cm

  • 2734 cm


B.

6427 cm

Acceleration due to gravity,

               g = GMr2    g  1r2

When the body is hanged on a spring

              F = − kx = mg       ..... (i)

acts on it, where x is extension in spring. Let 800 km above the Earth's surface, the stretch in the length of spring is x' and value of g is g'.

                     g = GMR2 and g' = GM(R + h)2So, g' = g1 + hR2            ...... (ii)

where, R = radius of Earth

From Eqs.(i), we get

                 kx = mg ⇒ x ∝ g         ...... (iii)

Therefore,  x'x = g'g

From Eqs. (ii) and (iii), we get

          x'x = g1 + hR2/ g          = gg 1 + 800 × 1036400 × 1032          = gg 1 + 182          = gg 982 = g × 64g × 81 = 6481     x'  = 6481 x            [ x = 3 cm] x' = 6481 × 3 = 6427 x' = 6427 cm


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9.

The acceleration due to gravity on the surface of a planet is one-fourth of the value on Earth. When a brass ball is brought to this planet, its

  • mass is halved

  • weight is halved

  • mass becomes one-fourth

  • weight becomes one-fourth


10.

Polar satellites

  • are high altitude satellite

  • are widely used for telecommunication

  • are used for environmental studies

  • go around the Earth in a East-West direction


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