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Class nine science Chapter 10 NCERT Solution
Chapter 10: Gravitation
Intext Questions (PAGE 134)
1. State the 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 ad inversely proportional to the square of the distance between them.
2. Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.
Where ‘G’ is the gravitational constant, ‘M’ the mass of the earth, ‘m’ the mass of the object and ‘d’ the distance between the centre of gravity of the object and the centre of the earth.
Intext Questions (PAGE 135)
1. What do you mean by free fall?
Ans. When an object falls with a constant acceleration, under the influence of the force of gravitation of the earth, an object is said to have free fall.
2. What do you mean by the acceleration due to gravity?
Ans. The constant acceleration experienced by a freely falling object towards the earth is called acceleration due to gravity. Its average value on the surface of the earth is 9.81 ms–2.
Intext Questions (PAGE 138)
1. What are the differences between the mass of an object and its weight?
| Mass | Weight |
| (a) It is the amount of matter in a body. | (a) It is the force of gravity acting on a body. |
| (b) It is a constant quantity at all the places in-universe. | (b) It is the variable quantity and changes with the change in acceleration due to gravity. |
| (c) It is measured by a physical balance. | (c) It is measured by a spring balance. |
| (d) Its SI unit is kilogram. | (d) Its SI unit is Newton. |
2. Why is the weight of an object on the moon 1/6 of the weight on the earth?
Mass of the Moon (M) = 7.4 × 1022 kg.
Radius of the Moon (R) = 1.74 × 106 m
Gravitational constant (G) = 6.7 × 10–11 Nm2/kg2
∴ Acceleration due to gravity on the Moon (gMoon)
⇒ gMoon = 1.63 ms–2.
Intext Questions (PAGE 141)
1. Why is it difficult to hold a school bag having a strap made of thin and strong string?
Ans. For the constant force, the pressure experienced is inversely proportional to the area of cross-section. Now, when the string is thin, it has less area of cross-section, and hence, exerts greater pressure on the hand for the given weight of the school bag. Thus, it becomes difficult to hold the school bag.
2. What do you mean by buoyancy?
Ans. The phenomenon due to which a body on being immersed in fluid experiences an upward force is called buoyancy.
3. Why does an object float or sink, when placed on the surface of the water?
Ans. When the object has density less than 1 g cm–3, it floats on the surface of the water, because, it always displaces more weight of water than its own weight. As buoyant force is more than its own weight, therefore, it floats. When the object has density more than 1 g cm–3, then it sinks in water, because, it always displaces less weight of water than its own weight. As buoyant force is less than its own weight, therefore, it sinks.
Intext Questions (PAGE 142)
1. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
Ans. Mass is always a constant quantity. So, it cannot be more or less than 42 kg.
2. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?
Ans. Iron is heavier.
Reason: Heaviness of an object is determined by its density. As the iron has far more density than cotton, therefore, in reality, iron is heavier than cotton.
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