chapter 09 gravitation class 9th PSEB, NCERT, CBSE part 2

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Observation Activity 9.1
When stone is released, it takes a tangential movement and after some time drops on the ground.
Explanation
Before releasing
The stone moves in a circular path and changes direction at every point of the circle. The change in direction involves a change in velocity.
A force causes acceleration (change in velocity) and keeps the stone moving along the circular path is acting towards the center (here towards hands) is called centripetal force.
After releasing When the stone is released, it moves along a straight line that is tangent to the circular path. After some distance, it falls down to the ground.
Important questions on activity 01
What is the cause of the motion of the moon around the earth?
Ans. The motion of the moon around the earth is due to the centripetal force. This force is provided by the force of attraction of the earth.
What is a tangent to a circle?
Ans. A straight line that meets the circle at one point is called a ‘tangent’ of the circle.
Activity 9.2
Observation: The stone goes upward and at a certain height, it starts falling down towards the earth.
Explanation: When a stone is thrown upward, it goes away from the gravitational pull of the earth so there is uniform retardation in the upward direction. At this time the acceleration of stone is decreasing at a constant rate of 9.8m/s2. It means acceleration is due to gravity - 9.8 m/s2.
At a certain height, the velocity of the stone reduces to zero. Now the stone moves downward with an acceleration of 9.8m/s2.
Important question
Define acceleration due to gravity?
Ans.- The acceleration with which an object falls freely towards the earth is known as acceleration due to gravity.
Activity 9.3
Observation: Paper reaches the ground a little later than the stone.
Explanation: This happens due to the resistance offered by the air. Air resistance is directly related to the area of the body or object.
If the area is larger then the resistance applied increases and on the other hand, if the area is lesser then the resistance applied decreases.
In this case, the area of the paper is more than that of stone, so the air resistance is larger on the paper and it falls down slowly.
We know that force is the product of mass and acceleration
F=ma
Here acceleration is due to gravity so
F = mg
Force applied on the free-falling object can be obtained by the following formula :
F= GMm/r²
When the same activity is done in the jar in which air is absent, we find that paper and stone reach the bottom at the same time because there is no air resistance present in the jar.
Conclusion : we understood the effect of gravity on free-falling objects.
Activity 9.4
Observation: The bottle starts floating in the water but when we apply external force to immerse it, at the same time we feel an upward force acting upon the bottle.
Explanation: The gravitational attraction of the earth acts on the bottle in a vertically downward direction. When the bottle is released, it experiences an upward force which is called upthrust or buoyant force.The upward force exerted by a liquid on a body that is immersed in the liquid is known as the upthrust or buoyant force.
The bottle displaces water of volume equal to its own volume.The bottle starts floating because the weight of displaced water is equal to the upthrust.When the bottle is released, it doesn’t stay immersed in water because at this time the buoyant force exerted by the liquid is more than the weight of the object.
We can immerse the bottle in water by filling it with water, then its weight will be more than the upthrust and it will sink.
Activity 9.5
Observation: The iron nail sinks into the beaker.
Explanation: The gravitational force acts on the nail vertically downward. The weight of the nail is more than the upthrust force or buoyant force, that’s why the nail sinks in water.The density of an iron nail is more than the density of water so the upthrust of water is less than the weight of the nail so it sinks in water.
Conclusion :
This activity shows that the relation between the weight of the object and upthrust is responsible for the floatation of the object.
Activity 9.6
Observation:
The iron nail sinks into the beaker while the cork floats on water.
Explanation: The iron nail sinks in the water while the cork floats on the surface of the water because the density of the nail is greater than the upthrust and on the other hand the density of the cork is less than the upthrust
Conclusion:
Activity 9.6 shows the relation between the density of the object and the upthrust of liquid.
questions
1. What is density?
Ans.- The density of a substance is defined as the mass per unit volume.