physics - gravitation

Class 9 - Physics


Why do sewing needles and all pins have a sharp pointed tip?

Pressure = Force/Area
⇒ Pressure is inversely proportional to the surface area on which the force acts.When we pin up the
papers or sew the clothes, we apply force over a very small area which leads to a larger pressure. That's why sewing needles and all pins have pointed tips.

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Questions and Answers

Gravitation - Questions
1. Name the scientist who identified the relation about force of gravitation. Name the law.
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)
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.
4. What are the factors the buoyant force depends on?
5. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
6. Fill in the blanks :
The value of Universal Gravitation constant (G) is ________ Nm2kgs-2.
7. What is the importance of Universal Law of Gravitation?
Q: State any one phenomena related to the Universal Law of Gravitation.
8. (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.
9. How does value of 'g' vary on the earth? What are the factors responsible for this?
10. 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.
11. 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 × 1024 kg and radius of the earth is 6.4 × 106 m).
12. Mass of an object is 10 kg. What is its weight on the Earth? (g = 9.8 m/s2.)
13. 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].
14. Write the standard equations in the form when an object is dropped from a height?
15. 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.
16. Is weight a scalar quantity?
17. The tangential velocity of a planet or moon moving in a circle is at a right angle to ____________. (fill in the blank)
18. Fill in the blanks :
A body is said to be in _______ when only gravity acts on it.
19. Fill in the blanks :
The SI unit of weight is _______.
20. Why does an object float or sink when placed on the surface of the water?
21. What are Plimsoll Lines?
22. In what direction does the buoyant force on an object immersed in a liquid act?
23. Why does a block of plastic released under water come up to the surface of the water?
24. The volume of a 500 g sealed packet is 350 cm3. 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?
25. 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/m3.
26. Fill in the blanks :
Geocentric model i.e. the Earth is the orbital centre of all celestial bodies was proposed by _____.
27. Fill in the blanks :
The force which keeps the object moving in a circular path is called _____ force. It always acts towards the _____.
28. What was heliocentric theory? Who proposed this?
29. Who measured the value of G first time, experimentally?
30. How gravitational force is different from or similar to magnetic force (between two magnets) or coulomb force (between two charged object)?
31. Calculate the force of gravitation between the earth and the Sun, given that the mass of the earth = 6 × 1024 kg and of the Sun = 2 × 1030 kg. The average distance between the two is 1.5 × 1011 m.
32. Does the force of attraction between two objects depend on the properties of intervening medium?
33. Define mass.
34. Define weight? How it is related to mass?
35. What are the differences between the mass of an object and its weight?
36. A stone is thrown vertically upward with an initial velocity of 40 m/s. Taking g = 10 m/s2, find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone?
37. 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/s2)
38. Define thrust.
39. Fill in the blanks :
________ of an object is the force of gravity acting on it.
40. Fill in the blanks :
During free fall condition, the body is in state of __________.
41. Fill in the blanks :
All other variables remaining constant, the force of gravitation is ________ proportional to the square of distance between the two objects.
42. Fill in the blanks :
The average acceleration of free fall on the surface of earth is ______ .
43. 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.
44. Define 1 Pascal.
45. The volume of 50 g of a substance is 20 cm3. If the density of water is 1 g cm-3, will the substance float or sink?
46. Why is it difficult to hold a school bag having a strap made of a thin and strong string?
47. A force of 200 N acts on a surface of area 10 cm2. Calculate thrust and pressure. How much the pressure changes if the area of contact increases to 50 cm2.
48. Fill in the blanks :
Heliocentric model was proposed by ______.
49. Fill in the blanks :
According to the Heliocentric theory, the _____ is at centre and _______ move around it.
50. Fill in the blanks :
Kepler's Law of ______ states that a line that connects a planet to the Sun sweeps out equal ______ in the plane of the planet's orbit in _____time intervals.
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