## Gravitation Notes Class 9 Pdf

Contents

To understand this chapter you must have the basic knowledge of Motion and force, which causes motion.

So, before going through this chapter you must read Chapter 9: Force and Laws of Motion Class 9

Let us firstly take a look at the topics we are going to read in this ‘Gravitation Notes of Class 9′.

### Gravitation

What is Gravitation or Gravitational Force?

Answer: It is the force of attraction between two or more objects/bodies. It is weak force unless very large mass bodies are involved.

• The force also acts between you and things around you but since the mass of bodies involved are very small, you don’t feel it.
• It is defined or governed by a law called “Universal Law of Gravitation“.

### Universal Law of Gravitation

The law of gravitation states that-

• the force of attraction between any two objects is proportional to the product of their masses
• it is also inversely proportional to the square of the distance between two objects.
• The force acts along the line connecting the centres of the masses of the objects.

Question: Why this law of Gravitation is called “Universal Law”?

Answer: Since this law applies to objects anywhere in the universe. So this law is called universal law.

Question: What is the Formula to find the Gravitational Force?

Answer: if you want to find the Gravitational Force between two bodies then use the formulas given below.

As stated above, according to the law of Gravitation-

Force ∝ product of masses (let say M and m)——- (i)

Force ∝ 1/d2 (inversely proportional to distance square)— (ii)

From Equation (i) and (ii) we get

$\mathbf{F∝\frac{Mm}{d^{2}}}$    or, $\mathbf{F=G\frac{M\times m}{d^{2}}}$

Here is Constant of Proportionality and it is called Universal Gravitation Constant.

Question: What is the value of Universal Gravitation Constant (G) and the SI unit of G?

The accepted value of G is The accepted value of G is 6.673 × 10–11 N m2 kg-2.

The value of G was founded by Henry Cavendish. He used a sensitive balance to find its value.

The SI Unit of G can be found by substituting the units of Force, masses, and distance in the above formula. It is N m2 kg-2.

#### Importance of the Universal Law of Gravitation

the Universal Law of Gravitation is very important because it helps us to understand various phenomena such as-

• the force that is binds us to the earth
• motion of the planets around the sun and the moon around the earth
• the rise and fall of tides due to the moon and the sun

### Define Free Fall

When an object falls only under the influence of gravity, it is said to be in free fall. In other words, when an object falls, and the only force that is acting on it is force of gravity, it is said to be in free fall.

Question: What is ‘g’ or acceleration due to gravity?

Answer: Whenever objects fall on the earth or it is attracted by the gravitational force of the earth, it undergoes acceleration (because of change of speed). This acceleration is called acceleration due to gravity.

Question: What is the symbol of acceleration due to gravity and the value of ‘g’.

Answer: It is denoted by the letter ‘g’. The value of acceleration due to gravity is 9.8 m s-2.

### Calculating of the value of ‘g’

Let us calculate the value of acceleration due to gravity (g).

As we know that for objects on or near the surface of the earth, ‘g’ is given by-

$\mathbf{g=G\frac{M}{R^{2}}}$

Here G (Universal Gravitational Constant) = 6.673 × 10–11 N m2 kg-2

M (Mass of earth) = 6 × 1024 kg

R (Radius of Earth) = 6.4 × 106 m

Putting these values in the above equation and then solving the equation-

g = [(6.673 × 10–11 N m2 kg-2 )× (6 × 1024 kg)]/(6.4 × 106 m)

g = 9.8 m/s2

### Variation of ‘g’

As we know that the earth is not a perfect or a complete sphere, it’s radius increases from poles to the equator-

• It also varies on the surface of the earth. It decreases from poles to the equator, as you can relate from the below formula.
• The force of gravity also decreases with altitude.

### Mass and Weight

#### Define Mass and Weight

Mass can be defined as the amount or quantity of matter present in a body. Weight is a force, a force exerted by gravity on a body.

#### What is the difference between Mass and Weight

In general students often get confused with the term mass and weight. This is because people in day to day life use these two terms as same thing whereas Mass and Weight are  two different quantities.

Let’s look at the difference between mass and weight from the class 9 point of view.

 Mass Weight Amount of matter present in a body. It is a force exerted by gravity on a body. It is scalar quantity. It is vector quantity. SI Unit of mass is Kg SI unit of Weight is newton or kg ms-2 Mass is always constant or doesn’t change with location It changes with location Mass of any object can never be zero Wight of an object can be zero, when no gravity acts

### Weight of an object on the Moon

Weight of an object on the moon can be defined as the force with which the moon attracts an object.

Weight of the object on the moon = one-sixth of the Weight of the object on the earth

Why is the weight of an object on the moon 1/6th its weight on the earth?

Since the mass of the moon is less than that of the earth. So, the moon exerts lesser force of attraction.

Mass of earth is = 5.98 × 1024

Mass of Moon is = 7.36 × 1022

### Centripetal Force

What is Centripetal Force?

Centripetal Force is the force that makes a body move in a circular path with uniform speed. The force always acts towards the center.

### Kepler’s Law of Planetary Motion

Johannes Kepler derived three laws, which govern the motion of planets and these laws are called the Kepler’s Law of Planetary Motion.

Kepler’s 1st Law

The plants orbit in an elliptical path with the Sun remaining at one of the foci.

Kepler’s 2nd Law

The line joining the planet and the Sun sweep equal areas in equal intervals of time.

Kepler’s 3rd Law

The cube of the mean distance of a planet from the Sun is proportional to the square of its orbital period T.

r3/T2 = constant

### Thrust and Pressure

What is thrust?

Thrust can be defined as the net force acting in a particular direction.

What is Pressure?

It is the force or the thrust acting per unit area.

### Pressure in Fluids

Fluids are the substances that has the tendency to flow. Liquids and Gases are called fluids.

Like solids exert pressure due to its weight. Similarly Fluids also exert pressure the base and walls of the container in which they are kept.

Pressure exerted by fluids at a depth is given by- p = ρgh

Here, p = pressure, ρ = density of the fluid, g = acceleration due to gravity, h = given depth

### Buoyancy

When a body is immersed in a fluid, an upward force is exerted by the fluid on the body which is equal to the weight of the fluid displaced by the body. This upward force is called buoyant force and the phenomenon is called buoyancy.

### Archimedes’ Principle

When a body is submerged either fully or partially then it is acted upon by a Force of Buoyancy in the vertical direction which is equal to the weight of the liquid displaced by the body.

### Relative Density

Define Relative Density.

Relative Density of a substance is defined as the density of the substance to that of the density of water.

R.D = density of the substance/density of water

As we know, Density = mass/Volume

or, Relative Density =(mass of the substance/Volume of the substance) × (Volume of Water/Mass of Water)

Now, if we consider equal volume of water and the substance, then

Relative Density = Mass of the Substance/Mass of Water of equal volume

SO, the other definition of Relative Density is the ratio of the mass of the substance of any volume to the mass of the water of the equal volume.

Relative Density is a relative quantity or the ratio of similar quantity, it has no unit.

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