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Chapter: 11th Physics : UNIT 6 : Gravitation

Short Questions and Answer

Physics : Gravitation : Book Back Important Questions, Answers, Solutions : Short Questions and Answer

Gravitation | Physics

Short Answer Questions

1. State Kepler’s three laws.

Law of orbits:

Each planet moves around the Sun in an elliptical orbit with the Sun at one of the foci.

Law of area:

The radial vector (line joining the Sun to a planet) sweeps equal areas in equal intervals of time

Law of period:

The square of the time period of revolution of a planet around the Sun in its Elliptical orbit is directly proportional to the cube of the semi-major axis of The ellipse.

2. State Newton’s Universal law of gravitation.

Newton's law of gravitation states that a particle of mass M1 attracts any other particle of mass M2 in the universe with an attractive force. The strength of this force of attraction was found to be directly proportional to the product of their masses and is inversely proportional to the square of the distance between them.


3. Will the angular momentum of a planet be conserved? Justify your answer.

Yes, Because 

It implies that angular momentum is a constant vector. The angular momentum of the Earth about the Sun is constant throughout the motion.

4. Define the gravitational field. Give its unit.

The gravitational force experienced by unit mass placed at that point.

It is a vector quantity.

5. What is meant by superposition of gravitational field?

Consider ‘n' particles of masses, m1 , m2 ...mn distributed in space at positions r1,r2,r3 ....etc, with respect to point P. The total gravitational field at a point P due to all the masses is given by the vector sum of the gravitational field due to the individual masses. This principle is known as superposition of gravitational fields.

6. Define gravitational potential energy.

● The gravitational potential energy is defined as the work done to bring the mass m2 from infinity to a distance ‘r' in the gravitational field of mass m1.

● Its unit is joule.

7. Is potential energy the property of a single object? Justify.

● Potential energy is a property of a system rather than of a single object due to its physical position.

● Because gravitational potential energy depends on relative position. So, a reference level at which to set the potential energy equal to zero.


8. Define gravitational potential.

The gravitational potential at a distance r due to a mass ‘m' is defined as the amount of work required to bring unit mass from infinity to the distance r.

9. What is the difference between gravitational potential and gravitational potential energy?

Gravitational potential: The amount of work done in bringing a body of unit mass from infinity to that point without acceleration. V = -GM/R. unit J/kg

Gravitational potential Energy:

● The energy stored in the body at that point.

● If the position of the body changes due to force acting on it, then change in its potential energy is equal to the amount of work done on the body by the forces acting on it.

U = −GMm / R unit (J)

10. What is meant by escape speed in the case of the Earth?

The minimum speed required by an object to escape from Earth's gravitational field. Ve = 11.2 kms-1

11. Why is the energy of a satellite (or any other planet) negative?

● The negative sign in the total energy implies that the satellite is bound to the Earth and it cannot escape from the Earth.

● As h approaches ∞, the total energy tends to zero.

● The satellite is completely free from the influence of Earth's gravity and is not bound to Earth at large distances. 

12. What are geostationary and polar satellites?

Geostationary satellites:

● The satellites revolving the Earth at the height of 36000 km above the equator, are appear to be stationary when seen from Earth is called geo-stationary satellites.

Polar satellites:

● The satellites which revolve from north to south of the Earth at the height of 500 to 800 km from the Earth surface are called Polar satellites.


13. Define weight

● The weight of an object is defined as the downward force whose magnitude W is equal to that of upward force that must be applied to the object to hold it at rest or at constant velocity relative to the earth.

● The magnitude of weight of an object is denoted as, W=N=mg.

14. Why is there no lunar eclipse and solar eclipse every month?

● Moon's orbit is tilted 5° with respect to Earth's orbit.

● Due to this 5° tilt only during, certain periods of the year, the Sun, Earth and Moon align in straight line leading to either lunar eclipse or solar eclipse depending on the alignment.

15. How will you prove that Earth itself is spinning?

● The Earth's spinning motion can be proved by observing star's position over a night.

● Due to Earth's spinning motion, the stars in sky appear to move in circular motion about the pole star.

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