## Extra Questions for Class 9th: Ch 10 Gravitation (Science) Important Questions Answer Included

**Very Short Answer Questions (VSAQs): 1 Mark**

Q1. Who discovered that force is the cause of motion?

**Answer**

Galileo Galilei.

Q2. Who formulated the universal law of gravitation?

Q2. Who formulated the universal law of gravitation?

**Answer**
Sir Issac Newton.

Q3. Which force causes the things to fall towards the earth?

Q3. Which force causes the things to fall towards the earth?

**Answer**
Gravitational force.

Q4. Who found out the value of gravitational constant (G)?

**Answer**
Henry Cavendish.

Q5. What is the unit of gravitational constant (G)?

Q5. What is the unit of gravitational constant (G)?

**Answer**
The unit of gravitational constant is Nm

Q1. Give reason-stone falls towards the earth but earth do not rise towards the stone.

Q1. Explain :

(i) Universal gravitational constant

(ii) Free fall

(i) Frictional force: Backwards

Example: If a book slides across the surface of a desk, then the desk exerts a frictional force in opposite (i.e. backwards) direction of its motion.

(ii) Gravitational force: Downwards

Example: When we throw a ball in the air, it returns to the ground.

Q4. A man’s weight when taken at the poles is 600 N. Will his weight remain the same when measured at the equator? Will there be an increase or decrease is his weight? Explain .

Q1. (i) A bar of metal has a mass 200 g and a certain weight. Mass remains the same when weighed at equator but weight decreases. Why?

(ii) Differentiate between mass and weight. Write any three differences.

(ii) Difference between mass and weight.

Q2. Write three points of differences between mass and weight. How much would a 70 kg astronaut weigh on moon? What would be his mass on the earth and on the moon?

Differences between mass and weight:

^{2}Kg^{-2}**Short Answer Questions-I (SAQs-I) : 2 Marks**

Q1. Give reason-stone falls towards the earth but earth do not rise towards the stone.

**Answer**

Stone falls due to gravitational force exerted by the earth. Earth do not move towards the stone as the mass of earth is much more than that of the stone.

Q2. Account for the following:

(i) On moon, man feels lighter than earth.

(ii) Mass is scalar while weight is a vector quantity.

Q2. Account for the following:

(i) On moon, man feels lighter than earth.

(ii) Mass is scalar while weight is a vector quantity.

**Answer**
(i) Due to less gravitational force exerted by moon on man.

(ii) Mass do not have direction while weight has direction.

Q3. Name the positions on earth where the value of ‘g’ is

(i) Maximum

(ii) Minimum? Justify your answer.

(ii) Mass do not have direction while weight has direction.

Q3. Name the positions on earth where the value of ‘g’ is

(i) Maximum

(ii) Minimum? Justify your answer.

**Answer**
On earth value of g is maximum at poles and minimum at the equator. At poles radius of earth is less so value of g is more, at equator radius of earth is more so value of g is less. g =

Q4. (i) What keeps the moon in uniform circular motion around the earth?

(ii) Why do astronauts in space feel weightless?

Q4. (i) What keeps the moon in uniform circular motion around the earth?

(ii) Why do astronauts in space feel weightless?

**Answer**
(i) Gravitational force between the moon and the earth keeps moon in uniform circular motion around the earth.

(ii) They do not exert any force/weight on their spaceship due to the absence of gravity in space.

Q5. State any four natural phenomena explained by universal law of Gravitation.

(ii) They do not exert any force/weight on their spaceship due to the absence of gravity in space.

Q5. State any four natural phenomena explained by universal law of Gravitation.

**Answer**

(i) The force that binds us to earth.

(ii) The motion of moon around earth.

(iii) The motion of planets around sun.

(iv) Formation of tides.

Q6. Explain what happens to the force between two objects if:

(i) The mass of one object is doubled ?

(ii) The distance between the object is tripled.

(ii) The motion of moon around earth.

(iii) The motion of planets around sun.

(iv) Formation of tides.

Q6. Explain what happens to the force between two objects if:

(i) The mass of one object is doubled ?

(ii) The distance between the object is tripled.

**Answer**

We know the force between two objects of mass m

(i) F = Gm

Let mass of first object m

F'=(G(2m

So, the force is doubled.

(ii) If the distance is trippled, the new distance will be d/3

The graviational force now will be F'' = (m

So, the force will be 9 times of initial force.

Q7. State two factors on which value of ‘g’ depends.

_{1}and m_{2}is given(i) F = Gm

_{1}m_{2}/d^{2}--- (i)Let mass of first object m

_{1}is doubled, then the gravitational force F' will beF'=(G(2m

_{1})m_{2})/d^{2}= 2(Gm_{1}m_{2})/d^{2}= 2F --- (from i)So, the force is doubled.

(ii) If the distance is trippled, the new distance will be d/3

The graviational force now will be F'' = (m

_{1}m_{2})/(d/3)^{2}= 9m_{1}m_{2}/d^{2}= 9FSo, the force will be 9 times of initial force.

Q7. State two factors on which value of ‘g’ depends.

**Answer**
(i) Latitude of the place.

(ii) Mass of the earth. Value is maximum at poles and minimum at equator of the earth.

(ii) Mass of the earth. Value is maximum at poles and minimum at equator of the earth.

**Short Answer Questions-II (SAQs-II) : 3 Marks**

Q1. Explain :

(i) Universal gravitational constant

(ii) Free fall

**Answer**

(i) Universal gravitational constant is the constant ‘G’ appearing in Newton’s law of gravitation.

F=GMm/r

where, F is the force between two masses m and M at a distance r apart. The numerical value of G is equal to 6.673×10

F=GMm/r

^{2}where, F is the force between two masses m and M at a distance r apart. The numerical value of G is equal to 6.673×10

^{-11}Nm^{2 }kg^{-2}. The value of G was found out by Henry Covendish by using a sensitive balance.
(ii) Free fall: Whenever objects fall towards the earth under the gravitational force alone, we can say that the objects are in free fall. While falling there is no change in the direction of motion of the objects. But due to earth’s attraction, there will be a change in the magnitude of the velocity.

Q2. State the universal law of gravitation. Mention four phenomena which can be explained by this law.

Q2. State the universal law of gravitation. Mention four phenomena which can be explained by this law.

**Answer**

(i) The force that binds us to earth.

(ii) The motion of the moon around the earth.

(iii) The motion of planets around the sun.

(iv) The tides due to the moon and the sun.

Detailed Answer: Universal law of gravitation states that the force of attraction between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.

(ii) The motion of the moon around the earth.

(iii) The motion of planets around the sun.

(iv) The tides due to the moon and the sun.

Detailed Answer: Universal law of gravitation states that the force of attraction between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Four phenomena which can be explained by this law are:

(i) The force that binds us to earth.

(ii) The motion of the moon around the earth.

(iii) The motion of planets around the sun.

(iv) The tides due to the moon and the sun.

Q3. In which direction do the following forces act when an object is in motion? Explain with the help of an example. (i) Frictional force (ii) Gravitational force

(i) The force that binds us to earth.

(ii) The motion of the moon around the earth.

(iii) The motion of planets around the sun.

(iv) The tides due to the moon and the sun.

Q3. In which direction do the following forces act when an object is in motion? Explain with the help of an example. (i) Frictional force (ii) Gravitational force

**Answer**(i) Frictional force: Backwards

Example: If a book slides across the surface of a desk, then the desk exerts a frictional force in opposite (i.e. backwards) direction of its motion.

(ii) Gravitational force: Downwards

Example: When we throw a ball in the air, it returns to the ground.

Q4. A man’s weight when taken at the poles is 600 N. Will his weight remain the same when measured at the equator? Will there be an increase or decrease is his weight? Explain .

**Answer**

No, his weight will not remain same as that at the poles. There will be a decrease in his weight at the equator. As the radius of the earth increases from the poles to the equator, the value of ‘g’ becomes greater at poles decreasing towards equator. Also, the force of gravity decreases from poles to the equator.

Q5. Give reasons:

(i) A piece of paper takes much longer to fall than a stone through the same distance, when both are dropped simultaneously from roof.

(ii) The mass is constant everywhere the weight keeps changing.

(iii) The value of ‘g’ keeps changing as we move away from the earth whereas value of ‘G’ remains constant all over the universe.

Q5. Give reasons:

(i) A piece of paper takes much longer to fall than a stone through the same distance, when both are dropped simultaneously from roof.

(ii) The mass is constant everywhere the weight keeps changing.

(iii) The value of ‘g’ keeps changing as we move away from the earth whereas value of ‘G’ remains constant all over the universe.

**Answer**
(i) This is because a piece of paper has larger surface area and therefore experiences more friction due to air than a stone which has less surface are.

(ii) Because acceleration due to gravity varies from place to place.

(iii) The value of g depends on Latitude of the place and the mass of the earth while G is called universal constant as its value remains constant at all the places in the universe.

Q6. In which direction do the following forces act when an object is in motion:

(i) Frictional force

(ii) Gravitational force

(iii) Centripetal force

(ii) Because acceleration due to gravity varies from place to place.

(iii) The value of g depends on Latitude of the place and the mass of the earth while G is called universal constant as its value remains constant at all the places in the universe.

Q6. In which direction do the following forces act when an object is in motion:

(i) Frictional force

(ii) Gravitational force

(iii) Centripetal force

**Answer**
(i) Backward

(ii) Downward

(iii) Towards the centre

(ii) Downward

(iii) Towards the centre

**Long Answer Questions (LAQs) : 5 Marks**

Q1. (i) A bar of metal has a mass 200 g and a certain weight. Mass remains the same when weighed at equator but weight decreases. Why?

(ii) Differentiate between mass and weight. Write any three differences.

**Answer**

(i) Weight is dependent on gravitational force. Since, on equator, gravitational force is less, so the weight of the bar of metal decreases.

(ii) Difference between mass and weight.

Mass | Weight |

Mass is the quantity of matter contained in the body.
| Weight is the force of gravity acting on the body. |

It is the measure of inertia of the body. | It is the measure of gravity. |

Mass is a constant quantity. | Weight is not a constant quantity. It is different at different places. |

Q2. Write three points of differences between mass and weight. How much would a 70 kg astronaut weigh on moon? What would be his mass on the earth and on the moon?

**Answer**

Differences between mass and weight:

Mass | Weight |

Mass is the quantity of matter contained in the body.
| Weight is the force of gravity acting on the body. |

It is the measure of inertia of the body. | It is the measure of gravity. |

Mass is a constant quantity. | Weight is not a constant quantity. It is different at different places. |

Mass of astronaut on moon = 70kg. | Value of g on moon = (value of g on moon on earth)/6 |