Extra Questions for Class 9th: Ch 10 Gravitation Science

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?
Answer
Sir Issac Newton.

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)?
Answer
The unit of gravitational constant is Nm2Kg-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.


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.


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?


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.

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.

Answer

We know the force between two objects of mass m1 and m2 is given
(i) F = Gm1m2/d2 --- (i)
Let mass of first object m1 is doubled, then the gravitational force F' will be
F'=(G(2m1)m2)/d2 = 2(Gm1m2)/d2 = 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'' = (m1m2)/(d/3)2 = 9m1m2/d2 = 9F
So, 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.

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/r2

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 Nmkg-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.

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.
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

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.


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


Answer

(i) Backward
(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

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