Gravitation Class 9th Physics Notes

## Gravitation Class 9th Physics Notes

**Q.1** **state and explain Newton’s law of gravitation?**

**Ans. Newton’s of universal gravitation :- ** this law states that in this universe Avery two bodies attract each other with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance b/w them .**Explanation**:- consider two bodies a and b as shown in figure let their masses are m_{1} and m_{2} respectively and the distance b/w them is “r” then according to Newton’s law of gravitation the force of attraction :f: is given as:-

Eq (iii) is the mathematical form of Newton’s law of gravitation. Where “G” is gravitational constant and its value is equal to 6.673 x 10^{-11=} N.m^{2} /kg^{2}

**Q.2 using law of universal gravitation find mass of earth?**

Ans. we can find mass of earth “Me” by using universal gravitation by following method suppose we have a body of mass “m” laying on the on the surface m of earth as shown in figure.

Let “m” is the mass of body “Me” is the mass of earth “Re” is the radius of earth which is the distance b/w the centers of body and earth. Now according to law of universal gravitation the force b/w earth and object is given by:-

Gravitation Class 9th Physics Notes

**Q.3 Explain variation of “g” with altitude?**

Ans. considers an object laying on the surface of earth as shown in figure. Where me is mass of earth Re is radius of earth and “h” is the height of object from the surface of earth then according to law of universal gravitation the formula of mass of earth is given by:.

**Q.4 what is satellite? Derive the formula of orbital speed of an arbitral speed?**

A**ns. satellite:-** A satellite is an object which can move around a planet. Suppose a satellite of mass “m” is revolving around the earth in a circular orbit of radius “r” as shown then according to Newton’s law of gravitation the force of attraction b/w them is given by following:-

**CONCEPTUAL QUESTIONS**

**Q.2
moon is attracted by earth why it does not fall on ground?**

Ans. moon is attracted by earth toward its center due to its tangential speed, but it does not fall on ground. Because the gravitational forces of earth on moon in this case is acting as centripetal force.

**Q.3
why the water does not fall out of a bucket when it is whirled in a vertical
center?**

Ans. when water whirled in a vertical circle in a bucket it does not fall down because of centripetal force.

**Q.4
why is it not easy to whirl a hammer by a longer chain?**

**Ans.** it is not easy to whirl a hammer by a longer chain because the moment
of inertia is greater and the moment of inertia depends upon mass of the body
and radius of the circle. That’s why we can’t whirl a hammer easily by using
longer chain.

**Q.5
explains if a stone held in our hands released it falls tower earth center?**

Ans. we know that everybody is attracted by earth toward it’s center because of force of gravity so that’s why when a stone held in our hands is released it falls toward the earth center.

**Q.6
what is the value of “G” on moving?**

Ans. the value of “G” is constant every where it does not depends upon the nature and size of the masses and nor it depends upon the nature of medium b/w bodies But it is same in all universe.

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