# NCERT Physics Class 11 Exemplar Ch 8 Gravitation Part 4

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## MCQ Type Questions

Q. 25 The gravitational force between a hollow spherical shell (of radius R and uniform density) and a point mass is F. Show the nature of F vs r graph where r is the distance of the point from the centre of the hollow spherical shell of uniform density.

Answer:

Q.26 Out of aphelion and perihelion, where is the speed of the earth more and why?

Answer: At perihelion because the earth has to cover greater linear distance to keep the areal velocity constant.

Q.27 What is the angle between the equatorial plane and the orbital plane of

(a) Polar satellite?

Answer:

(b) Geostationary satellite?

Answer:

## Short Answer

Q. 28 Mean solar day is the time interval between two successive noon when sun passes through zenith point (meridian). Sidereal day is the time interval between two successive transit of a distant star through the zenith point (meridian). By drawing appropriate diagram showing earth’s spin and orbital motion, show that mean solar day is four minutes longer than the sidereal day. In other words, distant stars would rise 4 minutes early every successive day.

(Hint: you may assume circular orbit for the earth).

Answer: Every day the earth advances in the orbit by approximately . Then, it will have to rotate by (which we define as 1 day) to have sun at zenith point again. Since corresponds to 24 hours; extra corresponds to approximately 4minute [3 min 59 seconds]

Q.29 Two identical heavy spheres are separated by a distance 10 times their radius. Will an object placed at the mid-point of the line joining their centers be in stable equilibrium or unstable equilibrium? Give reason for your answer.

Answer: Consider moving the mass at the middle by a small amount

to the right. Then the forces on it are: to the right and

to the left. The first is larger than the second. Hence the net force will also be towards the right. Hence the equilibrium is unstable.