CBSE – 9th Standard Science

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Class 9 Question Bank Chapter 10 – Gravitation

Q1. What made Newton start thinking of gravity?

Ans. The fall of the apple on his head made Newton start thinking of gravity.

Q2. What is Centripetal force?

Ans. The force that causes this acceleration and keeps the body moving along the circular path is acting towards the centre. This force is called the centripetal force.

Q3. The motion of the Moon around Earth is due to which force?

Ans. The motion of the moon around the earth is due to the centripetal force.

Q4. What is gravitational force?

Ans. The force of attraction between objects is called the gravitational force.

Q5. What is Universal Law of Gravitation?

Ans. Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Q6. What do you mean by ‘G’?

Ans. G is the constant of proportionality and is called the universal gravitation constant.

Q7. What is the S.I unit of ‘G’?

Ans. The SI unit of G is N m2 kg-2.

Q8. What is the value of ‘G’?

Ans. The accepted value of G is 6.673 x 10-11 N m2 kg-2.

Q9. What is the importance of Universal law of Gravitation?

Ans. They are:-

(i) The force that binds us to the 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.

Q10. What do you mean by acceleration due to gravity?

Ans. Whenever an object falls towards the earth, an acceleration is involved. This acceleration is due to the earth’s gravitational force. This acceleration is called the acceleration due to the acceleration due to gravity.

Q11. What do you mean by ’g’?

Ans. Acceleration due to gravity is denoted by g.

Q12. What is the unit of ’g’?

Ans. The unit of g is the same as that of acceleration, that is, m s-2.

Q13. What is the magnitude of the Gravitational force?

Ans. The magnitude of the gravitational force Fwill be equal to the product of mass and acceleration due to the gravitational force.

Q14. As the radius of the earth increases from the poles to the equator, what happens to ‘g’?

Ans. As the radius of the earth increases from the poles to the equator, the value of gbecomes greater at the poles than at the equator.

Q15. What is the value of ‘g’?

Ans. The value of acceleration due to gravity of the earth, g= 9.8 m s-2.

Q16. What do you mean by weight of an object?

Ans. The force of attraction of the earth on an object is known as the weight of the object. It is denoted by W.

Q17. What is the weight of object on the moon?

Ans. Weight of the object on the moon = (1/6) x its weight on the earth.

Q18. What is Thrust?

Ans. The force acting on an object perpendicular to the surface is called thrust.

Q19. What is Pressure?

Ans. The thrust on unit area is called pressure.

Q20. What is the S.I. unit of Pressure?

Ans. SI unit of pressure is N/m2 or N m-2.

Q21. What is S.I unit of preesure called?

Ans. SI unit of pressure is called Pascal and is denoted by Pa.

Q22. What is Buoyant force?

Ans. The upward force exerted by the water onthe bottle is known as upthrust or buoyantforce.

Q23. What is Density?

Ans. The density of a substance is defined as the mass per unit volume.

Q24. What do you mean by Archimedes’ principle?

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

Q25. What are the applications of the Archimedes’ principle?

Ans. (i) It is used in designing ships and submarines.

(ii) Lactometers, which are used to determine the purity of a sample of milk

(iii) Hydrometers used for determining density of liquids.

Q26. What is Relative density?

Ans. The relative density of a substance is the ratio of its density to that of water.

Q27. Why objects sink when placed on the surface of water?

Ans. When the downward force acting on the object is greater than the upthrust of water on the object. So it sinks.

Q28. Why objects float when placed on the surface of water?

Ans.When the upthrust of water on an object is greater than the weight of the

object. So it floats.