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NCERT Class 9 Science Textbook Chapter 10 With Answer PDF Free Download
Chapter 10: Gravitation
We know that the moon goes around the earth. An object when thrown upwards, reaches a certain height and then falls downwards.
It is said that when Newton was sitting under a tree, an apple fell on him. The fall of the apple made Newton start thinking.
He thought that: if the earth can attract an apple, can it not attract the moon? Is the force the same in both cases?
He conjectured that the same type of force is responsible in both cases. He argued that at each point of its orbit, the moon falls towards the earth, instead of going off in a straight line.
So, it must be attracted by the earth. But we do not really see the moon falling towards the earth. Before the thread is released, the stone moves in a circular path at a certain speed and changes direction at every point.
The change in direction involves a change in velocity or acceleration.
The force that causes this acceleration and keeps the body moving along the circular path is acting towards the center. This force is called the centripetal (meaning ‘center-seeking’) force.
The motion of the moon around the earth is due to the centripetal force. The centripetal force is provided by the force of attraction of the earth.
If there were no such force, the moon would pursue a uniform straight-line motion.
It is seen that a falling apple is attracted to the earth. Does the apple attract the earth? If so, we do not see the earth moving towards an apple.
Why? According to the third law of motion, the apple does attract the earth. But according to the second law of motion, for a given force, acceleration is inversely proportional to the mass of an object [Eq. (9.4)].
The mass of an apple is negligibly small compared to that of the earth. So, we do not see the earth moving towards the apple. Extend the same argument for why the earth does not move towards the moon.
In our solar system, all the planets go around the Sun. By arguing the same way, we can say that there exists a force between the Sun and the planets.
From the above facts, Newton concluded that not only does the earth attract an apple and the moon, but all objects in the universe attract each other. This force of attraction between objects is called the gravitational force.
We have learned in the previous chapter that the mass of an object is the measure of its inertia (section 9.3). We have also learned that the greater the mass, the greater the inertia.
It remains the same whether the object is on the earth, the moon, or even in outer space. Thus, the mass of an object is constant and does not change from place to place.
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NCERT Solutions Class 9 Science Chapter 10 Gravitation
1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?
It is tough to carry a school bag having a skinny strap because of the pressure that is being applied on the shoulders.
The pressure is reciprocally proportional to the expanse on which the force acts. So, the smaller the surface area, the larger is going to be the pressure on the surface.
In the case of a skinny strap, the contact expanse is quite small. Hence, the pressure exerted on the shoulder is extremely huge.
2. What do you mean by buoyancy?
The upward force possessed by a liquid on an object that’s immersed in it is referred to as buoyancy.
3. Why does an object float or sink when placed on the surface of the water?
An object floats or sinks when placed on the surface of the water because of two reasons.
(i) If its density is greater than that of water, an object sinks in water.
(ii) If its density is less than that of water, an object floats in water.
4. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
A weighing machine measures the body weight and is calibrated to indicate the mass. If we stand on a weighing machine, the weight acts downwards while the upthrust due to air acts upwards.
So our apparent weight becomes less than the true weight. This apparent weight is measured by the weighing machine and therefore the mass indicated is less than the actual mass. So our actual mass will be more than 42 kg.
5. 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 the other. Can you say which one is heavier and why?
The correct answer is the cotton bag is heavier than an iron bar. The bag of cotton is heavier than the bar of iron. The cotton bag experiences a larger air thrust than the iron bar. Therefore, the weighing machine indicates less weight than its actual weight for the cotton bag. The reason is
True weight = (apparent weight + up thrust)
The cotton bag’s density is less than that of the iron bar, so the volume of the cotton bag is more compared to the iron bar. So the cotton bag experience more upthrust due to the presence of air.
Therefore, in the presence of air, the cotton bag’s true weight is more compared to the true weight of the iron bar.
Gravitation NCERT Textbook With Solutions PDF Free Download