IX PHYSICS : Gravitation
CBSE chapterwise MCQ Multiple Choice Questions,Test Paper,Sample paper based on CCE pattern for class 9 science Gravitation. Gravitation; universal law of gravitation, force of gravitation of the earth (gravity), acceleration due to gravity; mass and weight; free fall. Gravitation and Free Fall Test paper1 Download File Gravitation and Free Fall ENotes Full Download File 9th gravitation Extra score solved Questions Download File 

Question: Two balls of different masses are thrown vertically upwards with same initial speed. Which one of them will rise to the greater height?
Answer: Both will reach the same maximum height at the same instant, because they will have the same acceleration due to gravity
Question: What are Laws of Planetary Motion?
Ans: Kepler's three laws of planetary motion can be described as follows:
The path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus. (The Law of Orbit)
An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time. (The Law of Equal Areas)
The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. (The Law of periods) T2/R3 ratio is the same
Question: Derive the inverse square of Newton.
We can derive Newton's law of gravitation from Kepler's third law.
Kepler's Third Law can be written as
T2 = kr3
or T2 = (4π2/GM).r3 ..............(1)
now, we know that the time period, T = 2πr/v, so (1) becomes
(2πr/v)2 = (4π2/GM).r3
or 4π2 (r/v)2 = (4π2/GM).r3
or (r/v)2 = (1/GM).r3 = r3/GM .......................(2)
we know that,centripetal acceleration a = v2/r [from ma = mv2/r]
Thus, equation (2) becomes
r/a = r3/GM
or 1/a = r2/GM
or a = GM/r2 .......................... (3)
and we also know that F = ma, thus using equation (3) we would get
F = ma = m(GM/r2)
Thus, we have derived the Newton's Law of Gravitation, which is F = GMm/r 2 from Kepler's Third Law
Question: When we move from the poles to the equator. Hence, the value of ‘ g ’ decreases. Why
Ans: The shape of earth is an ellipse so when we move from the poles to the equator the radius of the earth R increases. Hence, the value of ‘ g ’ decreases because value 'g' is inversely proportional to the radius of earth. g = GM/R2
Question: What is the difference between centrifugal force and centripetal force?
Ans: Centripetal Force
(i) It is the force that keeps a body in circular path.
(ii) It acts toward the center.
Centrifugal Force
(i) It is the pseudo force that tries to make a body fly off the circular path.
(ii) It acts outward the center.
Question: Explain :Centrifugal force and Centripetal force?
Ans: A force which is required to move a body uniformly in a circle is known ascentripetal force. This force acts along the radius and towards the center of the circle,
Centrifugal force arises when a body is moving actually along a circular path, by virtue of tendency of the body to regain its natural straight line path. This force acts along the radius and away from the center of the circle.
The Centripetal force formula is given by Fc = mv^2/r
Question: Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object?
Ans : This is because acceleration is inversely proportional to mass. Since mass of heavy of is greater it accelerates slower than lighter object. This is why a heavy object does not fall faster than a light object?
Question: The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?
Ans: The earth attract the moon with equal force with which the moon attracts the earth. This because , according to Newton’s third law of motion “ to every action there is equal reaction
Question: If the moon attracts the earth, why does the earth not move towards the moon?
Ans: This is because acceleration is inversely proportional to mass. The earth is much larger than the moon so, the acceleration produced on the earth surface cannot be noticed.
Question: What is aphelion and perihelion?
Ans: The aphelion is the point in the orbit of an object where it is farthest from the Sun. The point in orbit where an object is nearest to the sun is called the perihelion. The word aphelion derives from the Greek words, apo meaning away, off, apart and Helios (the Greek god of the sun).
Answer: Both will reach the same maximum height at the same instant, because they will have the same acceleration due to gravity
Question: What are Laws of Planetary Motion?
Ans: Kepler's three laws of planetary motion can be described as follows:
The path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus. (The Law of Orbit)
An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time. (The Law of Equal Areas)
The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. (The Law of periods) T2/R3 ratio is the same
Question: Derive the inverse square of Newton.
We can derive Newton's law of gravitation from Kepler's third law.
Kepler's Third Law can be written as
T2 = kr3
or T2 = (4π2/GM).r3 ..............(1)
now, we know that the time period, T = 2πr/v, so (1) becomes
(2πr/v)2 = (4π2/GM).r3
or 4π2 (r/v)2 = (4π2/GM).r3
or (r/v)2 = (1/GM).r3 = r3/GM .......................(2)
we know that,centripetal acceleration a = v2/r [from ma = mv2/r]
Thus, equation (2) becomes
r/a = r3/GM
or 1/a = r2/GM
or a = GM/r2 .......................... (3)
and we also know that F = ma, thus using equation (3) we would get
F = ma = m(GM/r2)
Thus, we have derived the Newton's Law of Gravitation, which is F = GMm/r 2 from Kepler's Third Law
Question: When we move from the poles to the equator. Hence, the value of ‘ g ’ decreases. Why
Ans: The shape of earth is an ellipse so when we move from the poles to the equator the radius of the earth R increases. Hence, the value of ‘ g ’ decreases because value 'g' is inversely proportional to the radius of earth. g = GM/R2
Question: What is the difference between centrifugal force and centripetal force?
Ans: Centripetal Force
(i) It is the force that keeps a body in circular path.
(ii) It acts toward the center.
Centrifugal Force
(i) It is the pseudo force that tries to make a body fly off the circular path.
(ii) It acts outward the center.
Question: Explain :Centrifugal force and Centripetal force?
Ans: A force which is required to move a body uniformly in a circle is known ascentripetal force. This force acts along the radius and towards the center of the circle,
Centrifugal force arises when a body is moving actually along a circular path, by virtue of tendency of the body to regain its natural straight line path. This force acts along the radius and away from the center of the circle.
The Centripetal force formula is given by Fc = mv^2/r
Question: Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object?
Ans : This is because acceleration is inversely proportional to mass. Since mass of heavy of is greater it accelerates slower than lighter object. This is why a heavy object does not fall faster than a light object?
Question: The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?
Ans: The earth attract the moon with equal force with which the moon attracts the earth. This because , according to Newton’s third law of motion “ to every action there is equal reaction
Question: If the moon attracts the earth, why does the earth not move towards the moon?
Ans: This is because acceleration is inversely proportional to mass. The earth is much larger than the moon so, the acceleration produced on the earth surface cannot be noticed.
Question: What is aphelion and perihelion?
Ans: The aphelion is the point in the orbit of an object where it is farthest from the Sun. The point in orbit where an object is nearest to the sun is called the perihelion. The word aphelion derives from the Greek words, apo meaning away, off, apart and Helios (the Greek god of the sun).
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