## Gravitation for Class 9

Earth attracts all things towards it through an unseen force of attraction. This force of attraction is called as gravitation or gravitational pull. You must have noticed that every time you throw an object upwards, it reaches a certain height and then falls down on the earth’s surface. So, we can conclude that these objects move under the gravitational forces or gravitational pull of the Earth which is nothing but a force of attraction. Gravitational force or gravity of earth is the reason why you remain on earth and it pulls you down. In this article, let us take a look at gravitation for class 9.

## Universal Law of Gravitation For Class 9

• The Universal Law of gravitation was coined by Sir Issac Newton.
• As per the law:

Everybody in the universe attracts every other body with a force, which is directly proportional to the product of their masses and inversely proportional to the square of distance between them. The force acts along the line joining the two bodies

• This law is fundamental to understanding gravitation for class 9. For understanding this concept better, let us consider two objects of masses M and m that are at a distance r from each other as shown in the figure below:

• Now, let’s assume that F is the force of attraction between two objects P and Q.
• So, if we take the universal law of gravitation into account, then the force F is proportional to the product of masses of objects P and Q that is
F∝M×m (1)
and the force is inversely proportional to the square of distance between these two objects that is
F∝1/r^2 (2)
• When we combine both equations (1) and (2), we derive
F∝Mm/r^2 (3)
or
F=GMm/r^2 (4)
• Note that in this case, G is the constant known as Universal Gravitational constant.
• Nm2Kg−2 is the SI unit of gravitational constant G
• The value of G was first established by Henry Cavendish (1731-1810) and it is
G=6.673×10−11Nm2Kg−2

## Main Characteristics of Gravitational Forces

• The gravitational force is basically a central force, which works along the line joining the centers of two bodies.
• It is always a long range force. The gravitational force occurs even when we’re talking about large distances.
• It is a conservative force. This implies that the work which is done by the gravitational force in displacing a body from one point to another is only dependent on the initial and final positions of the body and is independent of the path followed.
• It is crucial to know that the gravitational force is always attractive, unlike electrostatic and magnetic forces.

## The Importance of Universal Law of Gravitation

This is an important part of the theory of gravitation for class 9.

• Gravitational force is responsible for binding us to Earth.
• It is the reason behind the motion of moon around the earth.
• Planets revolve around the sun because of it.
• Even the tides that get formed by rising and falling of water level in the ocean happen because of the gravitational force exerted by both sun and moon on the earth.

## Understanding Free Fall

• Because of the gravitational force, the Earth attracts every object towards it.
• So, when an object falls towards earth, it happens so because of the influence of gravitational force. When only the gravitational force acts on the object during its free fall, then this motion of object can be called free fall motion. So, we can conclude that when a body (or object) falls from a height towards the earth only under the influence of the gravitational force of earth (with on other force acting on it), it is called free fall.
• Note that when an object falls freely towards the earth, there’s never a change in the direction of motion of the object. But because of the attraction of earth, there is a change in the magnitude of velocity.
• When we talk about change in velocity, then it involves acceleration. So, every time an object falls towards earth, the factor of acceleration is there due to earth’s gravitational force.

## Difference between Gravitation Constant (G) and Gravitational Acceleration (g)

 S. No. Gravitation Constant (G) Gravitational acceleration (g) 1. Its value is 6.67×10-11Nm2/kg2. Its value is 9.8 m/s2. 2. It is a scalar quantity. It is a vector quantity. 3. Its value remains constant always and everywhere. Its value varies at various places. 4. Its unit is Nm2/kg2. Its unit is m/s2.

## Motion of Objects because of the Gravitational Force of the Earth

Motion of objects is Let’s assume that an object is falling towards earth with an initial velocity u. And that its velocity, under the effect of gravitational acceleration g, changes to v after covering the height h in time t.

Then we can derive the three equations of motion as:

Velocity (v) after t seconds, v = u + ght

Height covered in t seconds, h = ut + & frac12; gt2

Relation between v and u excluding t, v2 = u2 + 2gh

Note that the value of g is considered as positive in the case of an object moving towards earth and considered as negative when the object is thrown in the opposite direction of earth.

Let’s understand gravitation better with a few questions and solutions.

### Question 1:

Why do we find it difficult to hold a school bag that has a long string and thin strap?

A thin and long string exerts a force that is equally spread over a small area, so the force applied due to the bag is more. The pressure is always inversely proportional to its area. So, the pressure that is created on the body by thin straps of the school bag is more and it becomes more uncomfortable to carry the bag. When we reduce the area of the bag, the pressure increases.

### Question 2:

What is buoyancy?

When an object or body is submerged in a fluid, the fluid applies an upward force on the sunken body. This upward force is the same as the weight of the fluid replaced by the submerged body and this is called the buoyancy. In simple words, it is the force put by the fluid when an object is submerged in it.

### Question 3:

Why does a block of wood come up to the surface when it is immersed in a fluid?

The density of water is more than the block of wood. So, the buoyant force in water is greater than that of the force exerted by the wood block. Therefore, whenever a block of wood is immersed, it comes back to the surface.

## Questions for Practice

Q1. What is the universal law of gravitation?

Q2. If we drop two stones of 150 gm and 500 gm from a height, which one will first reach the surface of the earth and explain why.

Q3. Why does any object float or sink when it is put on the surface of the water?

Q4. What is the direction in which the buoyant force acts on a given submerged body?

Q5. Explain why camels are able to walk easily on desert sand but we aren’t able to do the same?

Hope this article has helped you understand the topic of gravitation for class 9. For more such articles, keep following Toppr.

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