Please refer to Gravitation Class 9 Science Notes and important questions below. The Class 9 Science Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 9 Science examination questions given below to obtain better marks in exams

## Gravitation Class 9 Science Notes and Questions

The below Class 9 Gravitation notes have been designed by expert Science teachers. These will help you a lot to understand all the important topics given in your NCERT Class 9 Science textbook.

Refer to Chapter 10 Gravitation Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.

**Gravity i**s one of the most basic forces in the universe. It plays a fundamental role not only in the structure of our solar system but also in the way objects behave on Earth.

**Gravitation**

Gravitation is the force of attraction between two objects in the universe. Gravitation may be the attraction of objects by the earth.

This force is proportional to the product of masses of the objects and inversely proportional to the square of the distance between them. It is independent of medium.

Gravitational force GMmr2GMmr2

Eg :- If a body is dropped from a certain height, it falls downwards due to earth’s gravity. If a body is thrown upwards, it reaches a certain height and then falls downwards due to the earth’s gravity.

- Gravitation may be the attraction between objects in outer space.

Eg :- Attraction between the earth and moon.

Attraction between the sun and planets.

**GRAVITY**

- A natural force that pulls all objects toward the center of the earth
- keeps the moon orbiting
- It holds stars together . . .
- And binds galaxies together for billions of years ….Prevents Planets from losing their atmospheres.

2.**Universal law of gravitation**: The universal law of gravitation states that, ‘Every object in the universe attracts every other object with a force which is directly proportional to product of the masses and inversely proportional to the square of the distance between them.’

- The SI unit of G is N m2 kg−2N m2 kg−2 and its value is 6.673 ×10−11N m2 kg−26.673 ×10−11N m2 kg−2
- The strength of the gravitational attraction between two objects depends on two factors:
- How big the objects are (how much mass they have) and
- How far apart they are.

**3. Free Fall** With negligible air resistance, falling objects can be considered freely falling. objects of different shapes accelerate differently (stone vs

**4 .To calculate the value of” g**“(acceleration due to gravity)

- The acceleration due to gravity is denoted by g.
- The unit of g is same as the unit of acceleration ms−2ms−2
- From the second law of motion, force is the product of mass and acceleration. F = ma
- For free fall, force is the product of mass and acceleration due to gravity.

F=mgormg==GMmr2F=mgormg==GMmr2

orgGMr2orgGMr2

where M is the mass of the Earth and d is the distance between the object and the earth.

- For objects near or on the surface of the earth d is equal to the radius of the earth R
- F=mgormg==GMmr2F=mgormg==GMmr2

orgGMr2orgGMr2

**5. Mass and Weight:**

- Mass is a fundamental, universal property. You have the same amount of mass no matter where you are in the Universe.
- Weight is not fundamental its value depends on circumstances in the Universe. Weight is a force. It is the resultant gravitational force exerted on a body with mass m by all the other bodies on the Universe.

Weight = Fg = G m Me/ R2 = mgFg = G m Me/ R2 = mg

whereMeMe is the mass of the Earth and R is the radius of the Earth.

**6. Weight Of The Object On Moon**

- The mass of the moon is less than the mass of the earth. So the moon exerts lesser force on the objects than the earth.

- The weight of an object on the moon is one sixth (1/6th )(1/6th ) of its weight on the earth.
- The weight of an object on the earth is the force with which the earth attracts the object and the weight of an object on the moon is the force with which the moon attracts the object.

Celestial body | Mass (kg) | Radius (m) |

Earth | 5.98 x 10245.98 x 10245 | 6.37 x 1066.37 x 106 |

Moon | 7.36 x 10227.36 x 1022 | 1.74 x 1061.74 x 106 |

**7. Thrust & Pressure**

- Thrust is the force acting on an object perpendicular to the surface.
- Pressure is the force acting on unit area of a surface

ThrustPressure =−−−−−−−−−−AreaThrustPressure =−−−−−−−−−−Area - The SI unit of thrust is N/m2 orN m−2N/m2 orN m−2. It is called Pascal (Pa).

**8. Pressure In Fluids**

Fluids exert pressure in all direction

Pressure exerted on fluids is transmitted equally in all directions.

**9. Buoyancy**

When an object is immersed in a fluid it experiences an upward force called buoyant force. This property is called buoyancy or upthrust.

**10. Why objects float or sink when placed on the surface of water?**

- Take some water in a beaker. Take a piece of cork and an iron nail of the same mass. Place them on the water. The cork floats and the nail sinks.

- If the density of an object is less than the density of a liquid, it will float on the liquid and if the density of an object is more than the density of a liquid, it will sink in the liquid.

**11. Archeimedes Principle**

Archimedes’ principle states that, When a body is partially or fully immersed in a fluid it experiences an upward force that is equal to the weight of the fluid displaced by it.’

**12 .Relative density**

The relative density of a substance is the ratio of the density of a substance to the density of water. It is a ratio of similar quantities and has no unit.

**Questions for Class 9 Science Chapter 10 Gravitation**

**Question. . The 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 :** Acceleration due to gravity does not depend on mass of object. Hence, all bodies fall with the same acceleration provided there is no air or other resistance.

**Question. . How can a person lie on a bed of nails without getting hurt?****Ans :** Weight is spread out over them all making the pressure on each individual nail just about equal to a pinch if there are many nails. The greater the number of nails the lesser will be the pressure per square inch. If there are a large number of nails, it will be almost like lying on a solid surface.

**Question. . Why is G called ‘a universal gravitational constant’?****Ans :** The value of G is same for any pair of objects in the universe. Also its value does not depend on the nature of the intervening medium. That is why constant G is called ‘universal gravitational constant’.**Question. . Why should we be sent flying in space if the force of gravity somehow vanishes today?****Ans :** The centripetal force required to keep us rotating along with the Earth would not be available in the absence of force of gravity. We would then fly off along the tangent to the Earth into the space.

**Question. . Explain : Centrifugal force and Centripetal force.****Ans :** A force which is required to move a body uniformly in a circle is known as centripetal force. This force acts along the radius and towards the centre 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 centre of the circle.

**Question. . Discuss the variation of weight of a body with the Latitude of Earth and Altitude of Earth.****Ans :**

(i) Earth is not a perfect sphere.

(ii) Its radius at equator is longer than at the poles.

(iii) The acceleration due to gravity is inversely proportional to the square of the radius.

(iv) Therefore, the value of “g” at the poles is higher than that at the equator.

(v) As a result, the weight of an object increases if it moves from the equator to the poles.

**Question. . Two objects of masses M1 and M2 are dropped in vacuum from a height above the surface of Earth (M1 is greater than M2). Which one will reach the ground first and why?****Ans :** Since acceleration is the rate of change of velocity, and velocity is the rate of change of position, objects dropped from the same height will feel the same acceleration, and so move with the same rate of increasing velocity, and so hit the ground at the same time, independent of their masses.**Question. . How does the force of gravitation between two objects change when the distance between them is reduced to half?****Ans :** When the distance between the objects is reduced to half the gravitational force increases by four times the original force.

**Question. . What is Gravitation?****Ans :** Gravitation is the force of attraction between two objects in the universe :

(i) Gravitation may be the attraction of objects by the Earth.**Example :** If a body is dropped from a certain height, it falls downwards due to Earth’s gravity.

If a body is thrown upwards, it reaches a certain height and then falls downwards due to the Earth’s gravity.

(ii) Gravitation may be the attraction between objects in outer space : Attraction between the Earth and Moon. Attraction between the Sun and planets.

**Question. . Why are the ends of tools like knives, pins and nails pointed?****Ans :** The ends of tools such as pins, knives and nails are made pointed to increase the effect of the force. Because the pointed ends have much smaller areas. As result for a certain amount of force applied, the pressure becomes very large. That is why sharp edges needs very small force to cut fruits or vegetables.

**Question. . Why are the ends of tools like knives, pins and nails pointed?****Ans :** The ends of tools such as pins, knives and nails are made pointed to increase the effect of the force. Because the pointed ends have much smaller areas. As result for a certain amount of force applied, the pressure becomes very large. That is why sharp edges needs very small force to cut fruits or vegetables.

**Question. . The 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 :** Acceleration due to gravity does not depend on mass of object. Hence, all bodies fall with the same acceleration provided there is no air or other resistance.

**Question. . Is value of “g” same at all places on the Earth? Give reason for your answer.****Ans :** No, the value of “g” is maximum at the poles and minimum at equator. This variation is because to the oblong shape of the Earth and its rotation about its own axis.**Question. . Define the weight of an object on Moon.****Ans :** The weight of an object on the Moon is the force with which the Moon attracts that object.

**Question. . Why does a mug full of water feel lighter inside water?****Ans :** A mug of water appears lighter inside the water because a buoyant force acts on the mug when placed inside the water.

**Question. . What is the importance of universal law of gravitation?****or****Write four phenomenons which were successfully explained using universal law of gravitation.****Ans :** Many unconnected phenomenon can be explained by gravitational law successfully.

(i) Force bind us with Earth

(ii) Motion of Moon around Earth

(iii) Motion of planet around Sun

(iv) Tides due to the Moon and Sun

**Question. . What is the relation between gravitational force of the Moon with the Earth.****Ans :** The gravitational force of the Moon is about one-sixth of what it is on the Earth.**Question. . Name the scientist in whose honor the SI unit of pressure is named.****Ans :** The SI unit of pressure is named after Blaise Pascal.

**Question. . Name the force which accelerates a body in free fall.****Ans :** Gravitational force of Earth.**Question. . What is the unit of “g”?****Ans :** The unit of “g” is ms^{–2}.

**Question. . Why value of “g” more or less constant on or near the Earth?****Ans :** Radius of Earth does not change much; “g” is more or less constant on or near the Earth.

**Question. . What is weightlessness?****Ans :** A body is said to be in a state of weightlessness when the reaction of the supporting surface is zero or its apparent weight is zero.