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:
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:
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:
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)
Answer: (a) 1 cm
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