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Revision notes for Class 9 Science CBSE Chapter 10 Gravitation can be your best study guide while preparing the chapter for your exams. These revision notes can also be used in solving NCERT exercises or previous year questions related to the chapter, making it easier for you to understand every concept of the chapter. You have already learnt about the force and motion of objects. You might have also observed that when an object is dropped from a height, it falls towards the ground. The gravitation chapter teaches you about the universal law of gravitation and how the motion of objects on the earth is affected by gravitational force.
The online revision notes for Class 9 CBSE Chapter 10 Gravitation include topics like gravitation, the universal law of gravitation, the importance of the law of gravitation, free fall, the value of g, motion of objects under the gravitational pull of the earth, mass, weight, weight of an object on the moon, thrust and pressure, buoyancy, Archimed’s principle, and relative density. The revision notes prepared by askIITians help you understand every concept in detail. You can use these notes at any time and revise the chapter at your own pace. But, we guarantee that it would not take more than 20 minutes to complete the revision of this chapter.
Figure 1 Centripetal Force of Earth on Moon
Figure 2 Gravitational Force of Earth
From the above equation we can rewrite them as the following:
If we remove the proportionality we get proportionality constant G as the following:
The above equation is the mathematical representation of Newton’s Universal Law of gravitation
Hence, G = Fr2/ m1 m2
SI Unit: Nm2 kg-2
Value of G = 6.673 × 10-11 Nm2 kg-2 (was found out by Henry Cavendish (1731- 1810))
It explains many important phenomena of the universe –
Free Fall
The force (F) of gravitational attraction on a body of mass m due to earth of mass M and radius R is given by
We know from Newton’s second law of motion that the force is the product of mass and acceleration.
∴ F = ma
But the acceleration due to gravity is represented by the symbol g. Therefore, we can write
F = mg ….. (2)
From the equation (1) and (2), we get
When the body is at a distance ‘r’ from the centre of the earth then
the Value of ‘g' may vary at different parts of the earth –
We can find the value of acceleration due to gravity by the following –
When an object falls towards the earth due to the earth’s gravity and no other force is acting upon it, the object is said to be in a free-fall state. Free-falling objects are not even resisted by the air.
g = 9.8 m/s2 is also called the Free-fall Acceleration.
Value of ‘g' is the same on the earth, so the equations of motion for an object with uniform motion are valid where acceleration ‘a' is replaced by ‘g', as given under:
v = u + gt
s = ut + (1/2) gt2
2 g s = v2 – u2
Consider the equations of motion given in different scenarios:
When an object at rest falls towards earth – its initial velocity is zero
v = gt
s = t + (1/2) gt2
2 g s = v2
When an object with some initial velocity (u) falls towards earth –
When an object is thrown upwards from the earth – the gravitational force acts in opposite direction, hence g is negative
v = u - gt
s = ut - (1/2) gt2
-2 g s = v2 – u2
Mass
Weight
Mass is defined as the quantity of matter in an object.
The weight of an object is the force by which the gravitational pull of the earth attracts the object.
Mass is a scalar quantity
Weight is a vector quantity
The mass of an object is always constant as it depends upon the inertia of the object
The weight of an object can vary at different locations because of change in gravitational force of the earth
Mass can never be zero
Weight can be zero at places there is no gravitational force
Denoted as: m
Denoted as W
F = mg
where m = mass of object
a = acceleration due to gravity
Similarly, W is force, so
W = mg
SI Unit: kg
SI unit: N
Just like the Earth, the Moon also exerts a force upon objects. Hence, objects on the moon also have some weight. The weight will not be the same as on the earth. So, weight on the Moon can be calculated as -
Thrust
Pressure
Figure 4 Pressure
We know that pressure is inversely proportional to area. As area increases, pressure decreases and vice versa. So, nails' sharp edges make it easier for them to get into the wall because more pressure is exerted on the wall from a single point.
Buoyancy
Figure 5 Buoyancy
According to the Archimedes principle, whenever an object is immersed in a liquid (fully or partially), the liquid exerts an upward force upon the object. The amount of that force is equivalent to the weight of the liquid displaced by the object.
This means that if the weight of an object is greater than the amount of liquid it displaces, the object will sink into the liquid. However, if the weight of an object is less than the amount of water it displaces, the object will sink.
When density can be expressed in comparison with water's density it is called Relative Density. It has no unit because it is a ratio of two similar quantities.
Water is present everywhere on earth so it becomes easier to evaluate the density of a substance in relation to water.
How relative density can be used as a measure to determine if an object will sink or float in water?
Relative Density of an object
Float / Sink
Greater than 1
Sink in water
Less than 1
Float in water
Revision notes help you understand the basic concepts of the chapter easily. They provide you with easy to understand explanations, diagrams, comparison tables and notes that can help you master every concept for the exam.
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The main topics covered in online revision notes for the Class 9 Science Gravitation Chapter are gravitation, the universal law of gravitation, the importance of the law of gravitation, free fall, the value of g, motion of objects under the gravitational pull of the earth, mass, weight, weight of an object on the moon, thrust and pressure, buoyancy, Archimed’s principle, and relative density.
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