Gravitation

MCQs (Part - V)

Question 21: A thin spherical shell of mass M and radius R has a small hole. A particle of mass m is released at the mougth of the hole. Then:

  1. the particle will execute simple harmonic motion inside the shell
  2. the particle will oscillate inside the shell, but the oscillations are not simple harmonic
  3. the particle will not oscillate, but the speed of the particle will go on increasing
  4. none of these

Answer: (d) none of these

Question 22: A particle of mass m and charge q is projected vertically upwards. A uniform electric field is acted vertically downwards. The most appropriate graph between potential energy U (gravitational plus electrostatic) and height h(<< radius of earth) is: (assume U to be zero on surface of earth)

Answer: (a)

Explantion: Potential energy at height h

U = -[work done by gravity + work done by electrostatic force]

= mgh + qEh

= (mg + qE) h

or, U = kh

where, k = mg + qE

i.e., U-h graphs is straight line passing through orgin.

Question 23: A point mass of 10 kg is placed at the centre of earth. The weight of the point mass is:

  1. zero
  2. 98 N
  3. 49 N
  4. none of these

Answer: (a) zero

Explantion: At the centre of earth, the value of g is zero.

w = mg = 0

Question 24: The time period of a simple pendulum at the centre of earth is:

  1. zero
  2. infinite
  3. less than zero
  4. none of these

Answer: (b) infinite

Explantion:

Question 25: A particle hanging from a massless spring stretches it by 2 cm at earth's surface. How much will the same particle stretch the spring at height 2624 km from the surface of earth ? (Radius of earth = 6400 km)

  1. 1 cm
  2. 2 cm
  3. 3 cm
  4. 4 cm

Answer: (a) 1 cm


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