## Class 9 - Physics

**Question:**

What is free fall?

**Answer:**

The falling of a body from height towards the earth under the influence of gravitation force of the earth alone only is called free fall.

## Questions and Answers

**Gravitation**- Questions

1. Name the scientist who identified the relation about force of gravitation. Name the law.

Answer

2. Fill in the blanks :

Assuming negligible air drag, if a steel ball and a table tennis ball are dropped from a height of 100 metres, both will reach the ground at ________ time(s). (same/different)

Assuming negligible air drag, if a steel ball and a table tennis ball are dropped from a height of 100 metres, both will reach the ground at ________ time(s). (same/different)

Answer

3. Fill in the blanks :

The force of gravity _________ with altitude. It also varies on the surface of the earth, _________ from poles to the equator.

The force of gravity _________ with altitude. It also varies on the surface of the earth, _________ from poles to the equator.

Answer

4. What are the factors the buoyant force depends on?

Answer

5. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Answer

6. A cuboid room has dimensions 50m x 15m x 3.5m. What is the mass of the air enclosed in the room if the density of air = 1.30 kg/m

^{3}. Answer

7. Fill in the blanks :

The force which keeps the object moving in a circular path is called _____ force. It always acts towards the _____.

The force which keeps the object moving in a circular path is called _____ force. It always acts towards the _____.

Answer

8. Fill in the blanks :

The value of Universal Gravitation constant (G) is ________ Nm

The value of Universal Gravitation constant (G) is ________ Nm

^{2}kgs^{-2}. Answer

9. What is the importance of Universal Law of Gravitation?

OR

Q: State any one phenomena related to the Universal Law of Gravitation.

OR

Q: State any one phenomena related to the Universal Law of Gravitation.

Answer

10. (a) What is the relationship between g and G?

(b) Show mathematically whether g (acceleration due to gravity on the earth) depends on the mass of the object itself or not.

(c) Show that for a given location on the surface of the earth g is almost constant.

(b) Show mathematically whether g (acceleration due to gravity on the earth) depends on the mass of the object itself or not.

(c) Show that for a given location on the surface of the earth g is almost constant.

Answer

11. How does value of 'g' vary on the earth? What are the factors responsible for this?

Answer

12. At what height above the surface of the earth, the value of 'g' becomes 64% of its value at the surface of the earth. Take radius of the earth = 6400 km.

Answer

13. What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (Mass of the earth is 6 × 10

^{24}kg and radius of the earth is 6.4 × 10^{6}m). Answer

15. Amit buys few grams of gold at the poles as per the instruction of one of his friends. He hands over the same when he meets him at the equator. Will the friend agree with the weight of gold bought? If not, why? [Hint: The value of g is greater at the poles than at the equator].

Answer

16. Write the standard equations in the form when an object is dropped from a height?

Answer

17. A ball is thrown vertically upwards with a velocity of 49 m/s. Calculate(i) the maximum height to which it rises.(ii)the total time it takes to return to the surface of the earth.

Answer

18. Define thrust.

Answer

19. Is weight a scalar quantity?

Answer

20. The tangential velocity of a planet or moon moving in a circle is at a right angle to ____________. (fill in the blank)

Answer

24. Why does an object float or sink when placed on the surface of the water?

Answer

25. What are Plimsoll Lines?

Answer

26. In what direction does the buoyant force on an object immersed in a liquid act?

Answer

27. Why does a block of plastic released under water come up to the surface of the water?

Answer

28. The volume of a 500 g sealed packet is 350 cm

^{3}. Will the packet float or sink in water if the density of water is 1 g cm^{-3}? What will be the mass of the water displaced by this packet? Answer

29. Fill in the blanks :

Geocentric model i.e. the Earth is the orbital centre of all celestial bodies was proposed by _____.

Geocentric model i.e. the Earth is the orbital centre of all celestial bodies was proposed by _____.

Answer

30. Fill in the blanks :

According to the Heliocentric theory, the _____ is at centre and _______ move around it.

According to the Heliocentric theory, the _____ is at centre and _______ move around it.

Answer

31. What was heliocentric theory? Who proposed this?

Answer

32. What are the Kepler Laws that govern the motion of the planets?

Answer

33. Who measured the value of G first time, experimentally?

Answer

34. How gravitational force is different from or similar to magnetic force (between two magnets) or coulomb force (between two charged object)?

Answer

35. Calculate the force of gravitation between the earth and the Sun, given that the mass of the earth = 6 × 10

^{24}kg and of the Sun = 2 × 10^{30}kg. The average distance between the two is 1.5 × 10^{11}m. Answer

36. Does the force of attraction between two objects depend on the properties of intervening medium?

Answer

37. Define mass.

Answer

38. Define weight? How it is related to mass?

Answer

39. What are the differences between the mass of an object and its weight?

Answer

40. A stone is thrown vertically upward with an initial velocity of 40 m/s. Taking g = 10 m/s

^{2}, find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone? Answer

41. A ball is thrown upwards from the ground of a tower with a speed of 20 m/s. There is a window in the tower at the height of 15m from ground. How many times and when will the ball pass the window? (g = 10 m/s

^{2}) Answer

43. Fill in the blanks :

All other variables remaining constant, the force of gravitation is ________ proportional to the square of distance between the two objects.

All other variables remaining constant, the force of gravitation is ________ proportional to the square of distance between the two objects.

Answer

44. Fill in the blanks :

The average acceleration of free fall on the surface of earth is ______ .

The average acceleration of free fall on the surface of earth is ______ .

Answer

45. The weight of a man is 690N which contains 5.3 x 10

^{-3}m^{3}of blood. Find (a) blood's weight. Take density of blood is 1060 kg/m^{3}(b) express it as a percentage of body weight. Answer

46. The following figure shows four containers containing a liquid at the same height. Rank them according to the pressure exerted by the liquid on the bottom of the containers in ascending order. a. 1, 2, 3, 4 b. 4. 3, 1, 2 c. 3, 4, 1, 2 d. same pressure in each case.

Answer

47. Define 1 Pascal.

Answer

48. The volume of 50 g of a substance is 20 cm

^{3}. If the density of water is 1 g cm^{-3}, will the substance float or sink? Answer

49. Why is it difficult to hold a school bag having a strap made of a thin and strong string?

Answer

50. A force of 200 N acts on a surface of area 10 cm

^{2}. Calculate thrust and pressure. How much the pressure changes if the area of contact increases to 50 cm^{2}. Answer

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