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NCERT Class 11 Physics Textbook Chapter 8 With Answer PDF Free Download
Chapter 8: Gravitation
8.1 Introduction
Early in our lives, we become aware of the tendency of all material objects to be attracted to the earth.
Anything thrown up falls down towards the earth, going uphill is a lot more tiring than going downhill, raindrops from the clouds above fall towards the earth and there are many other such
phenomena.
Historically it was the Italian Physicist Galileo (1564-1642) who recognized the fact that all bodies, irrespective of their masses are accelerated towards the earth with a constant acceleration. It is said that he made a public demonstration of this fact.
To find the truth, he certainly did experiments with bodies rolling down inclined planes and arrived at a value of the acceleration due to gravity which is close to the more accurate value obtained later.
A seemingly unrelated phenomenon, the observation of stars, planets, and their motion have been the subject of attention in many countries since the earliest of times.
Observations since early times recognized stars that appeared in the sky with positions unchanged year after year.
The more interesting objects are the planets which seem to have regular motions against the background of stars.
The earliest recorded model for planetary motions proposed by Ptolemy about 2000 years ago was a ‘geocentric’ model in which all celestial objects, stars, the sun, and the planets, all revolved around the earth.
The only motion that was thought to be possible for celestial objects was motion in a circle. Complicated schemes of motion were put forward by Ptolemy in order to describe the observed
motion of the planets.
The planets were described as moving in circles with the center of the circles themselves moving in larger circles. Similar theories were also advanced by Indian astronomers some 400 years later.
However, a more elegant model in which the Sun was the center around which the planets revolved – the ‘heliocentric’ model – was already mentioned by Aryabhatta (5th century A.D.) in his treatise.
A thousand years later, a Polish monk named Nicolas Copernicus (1473-1543) proposed a definitive model in which the planets moved in circles around a fixed central sun.
His theory was discredited by the church, but notable amongst its supporter was Galileo who had to face prosecution from the state for his beliefs.
It was around the same time as Galileo, a nobleman called Tycho Brahe (1546-1601)
hailing from Denmark spent his entire lifetime recording observations of the planets with the
naked eye.
His compiled data were analyzed later by his assistant Johannes Kepler (1571- 1640). He could extract from the data three elegant laws that now go by the name of Kepler’s laws.
These laws were known to Newton and enabled him to make a great scientific leap in proposing his universal law of gravitation.
| Author | NCERT |
| Language | English |
| No. of Pages | 20 |
| PDF Size | 391 KB |
| Category | Physics |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 11 Physics Chapter 8 Gravitation
Q.1: Answer the following :
(a) You can shield a charge from electrical forces by putting it inside a hollow conductor. Can you shield a body from the gravitational influence of nearby matter by putting it inside a hollow sphere or by some other means?
(b) An astronaut inside a small space ship orbiting around the earth cannot detect gravity. If the space station orbiting around the earth has a large size, can he hope to detect gravity?
(c) If you compare the gravitational force on the earth due to the sun to that due to the moon, you would find that the Sun’s pull is greater than the moon’s pull. (you can check this yourself using the data available in the succeeding exercises). However, the tidal effect of the moon’s pull is greater than the tidal effect of the sun. Why?
Solution:
(a). No, as of now, no method has been devised to shield a body from gravity because gravity is independent of medium and it is the virtue of each and every matter. So the shield would exert the gravitational forces.
(b). Yes, if the spaceship is large enough then the astronaut will definitely detect the Mars gravity.
(c). Gravitational force is inversely proportional to the square of the distance whereas, Tidal effects are inversely proportional to the cube of the distance. So as the distance between the earth and the moon is smaller than the distance between the earth and the sun, the moon will have a greater influence on the earth’s tidal waves.
Q.2. Choose the correct alternative :
(a) Acceleration due to gravity increases/decreases with increasing altitude.
(b) Acceleration due to gravity increases/decreases with increasing depth (assume the earth to be a sphere of uniform density).
(c) Acceleration due to gravity is independent of the mass of the earth/mass of the body.
(d) The formula –G M m(1/r 2 – 1/r1) is more/less accurate than the formula mg(r2 – r1) for the difference of potential energy between two points r2 and r1 distance away from the centre of the earth.
Solution:
(a) decreases
(b) decreases
(c) mass of the body
(d) more
Q.3. Choose the correct alternative:
(a) If the zero of potential energy is at infinity, the total energy of an orbiting satellite is negative of its kinetic/potential energy.
(b) The energy required to launch an orbiting satellite out of earth’s gravitational influence is more/less than the energy required to project a stationary object at the same height (as the satellite) out of earth’s influence.
Solution:
(a) If the zero potential energy is at infinity, the total energy of an orbiting satellite is negative of its kinetic energy.
(b) The energy required to launch an orbiting satellite out of Earth’s gravitational influence is less than the energy required to project a stationary object at the same height (as the satellite) out of Earth’s influence.
Q. 4. Does the escape speed of a body from the earth depend on (a) the mass of the body, (b) the location from where it is projected, (c) the direction of projection, (d) the height of
the location from where the body is launched?
Solution:
The escape speed is given by the expressionv= \sqrt{\frac{2GM}{R}}=\sqrt{2gR}v=R2GM=2gR
(a) The escape speed of a body from the Earth does not depend on the mass of the body.
(b) The escape speed of a body from the Earth does not depend on the location from where a body is projected.
(c) The escape speed does not depend on the direction of projection of a body.
(d) The escape speed of a body depends upon the height of the location from where the body is launched since the escape velocity depends on the gravitational potential at the point from which it is launched. This potential in turn depends on the height.
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