NEET Study Notes on Gravitation: The force of attraction between two objects is defined as Gravitation. Gravitation is the force that governs all the objects in the universe. However, distance plays a factor in determining the gravitation force.

• In NEET Physics Syllabus, Gravitation is an important topic that serves as the fundamental for Keplers Law of planetary motion, the universal law of gravitation, acceleration due to gravity, and other similar topics.
• As per the previous year's analysis, Gravitation has a weightage of around 2%, With a focus on essential topics, candidates can score the marks from this unit in NEET 2022

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Important Topics in NEET Gravitation

NEET Study Notes on Universal Law of Gravitation

Newton formulated the study of forces that act on different particles. This is called Newton’s Universal Law of Gravitation or the Universal Law of Gravitation.

According to the law the force of attraction between two particles M1 and M2 is directly proportional to the masses product and inversely proportional to the square of the distnace between them.

The force of attraction between them is

F = G

where, m1 and m2 are the two masses at a distance r

G is the universal gravitational constant

Value of G-

• Gravitation force is the central force that follows the inverse square law.
• The work done by gravitational force is independent of the path followed, it is a conservative force.

Acceleration due to Gravity

The acceleration due to gravity is rate at which velocity increases when a body falls freely towards the earth. Shape, size and mass of the object do not affect acceleration due to gravity.

So, ……(i)

Me = mass of the earth

r= distance of the body from center of earth

Force of gravity can be written as F = mg…(ii)

where g= acceleration due to gravity

From (i) and (ii) we get,

From the expression (i) and (ii), we get

Also Read:

• NEET Study Notes on Law of Motion

• NEET Study Notes on Current Electricity

Variation of Acceleration due to Gravity

Acceleration due to gravity, g differs due to factors like depth and heighth. The variation in acceleration due to gravity are:

Due to height

..

where, g’ = acceleration at height h

h= height above earth surface

R= radius of the earth

When the height (h) increases, acceleration due to gravity (g) decreases.

Due to depth

where d= depth below earth surface

As the depth below the surface of the earth increases, acceleration due to gravity increases.

Due to latitude of earth

Earth rotates on its axis with angular velocity of ω. When latitude is θ, the acceleration at a place is

g’ =θ

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NEET Study Notes on Kepler's Law of Planetary Motion

Law of Orbits

As per Kepler’s First Law “ All planets revolve around the sun in elliptical orbits with sun at one of the foci’. Perihelion is the point at which the planet is close to the sun while aphelion is the point from where planet is far from the sun.

The sum of distance of any planet between two foci is constant,

Law of Equal Areas

As per Kepler’s Second Law of planetary motion, a line drawn from planet to the sun sweeps out equal areas in equal intervals of time.

I.e rate dA/dt, at which area is swept out A is constant

If , then A1= A2

Then

And L= mvr

Therefore, .

But if we apply conservation of angular momentum, , L is constant

Thus dA/dt is constant

Law of Periods

As per Kepler’s Tthird law of periods, “ The square of period of revolution of any planet is proportional is directly proportional to the cube of its semi-major axis of the orbit.

T2 ∝ r3

Read NEET Study Notes for Planar Motion

NEET Study Notes on Gravitational Field

Gravitation Field or Gravitational Field Intensity is the region or space around a body over which it can exert gravitational force on other masses.

At any point, gravitational field strength in the gravitational field is the force experienced by unit mass placed at a point. It is directed toward the field in which particles are produced.

Gravitation field intensity that is produced by mass, M at point P is force that is exerted on unit mass on point P, Gravitation Field intensity is represented by (Eg )

where m= mass

R is the distance between point M and m

Direction of (Eg ) is always pointed towards mass that produces it

S.I unit of (Eg ) is newton/kg with dimensions [M0LT–2].

What is Gravitational Potential?

In a Gravitation Field at any point, gravitational potential V is defined as the negative of work done per unit mass in carrying a rest mass from one point of refernce to another.

V= –W//m

S.I unit of V is joule/kg

Gravitational Potential due a uniform solid sphere

Case 1: at any external point ‘P’ (r>R)

Case 2: at an internal point ‘P’ (r<R)

Case 3: at the center of the solid sphere, the potential is

Gravitation Potential Energy

If we assume that potential energy at infinity is zero,

What is the gravitational potential energy at a two-particle system?

Gravitation potential energy of particles m1 and m2 that are separated by a distance r is

U=-Gm1m2/r

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NEET Study Notes on Satelites

Satelite may be defined as any body that continuously moves around a large body, due to the gravitational force of the other body. Satellites may be natural or artificial.

• Natural Satelites- Any celestial body that revolves around the the planet, is a natural satellite. Example- moon
• Artificial Satelite- Artificial satellites are that ones that are made and developed by humans and placed in orbits around the earth.

Geostationary Satelite

• Geostationary Satellite revolve around the in the same direction of the earth, around its axis and also the same velocity.
• Due to this, they appear to be stationary. Geostationary satellites are also known as parking satellite or synchronous satellite.
• Such satelites are used for the purpose of communication. Orbit of geostationary satellites is aslo called Parking Orbit.
• With respect to earth, relative velocity of geostationaryvelocity is zero.

Time Period of Satelite

Time Period of Satelite (T) of an artificial satellite at a distance (h) above earth surface is:

where, Re= radius of earth

g= accelerate due to gravity

If Re>h ,

Then

here , Re = 6.4 × 106 m ; g = 9.8 ms–2

T= 5075 seconds or 84.58 minutes

Also Read:

• NEET Study Notes for Kinematics

• NEET Study Notes for Thermodynamics

Orbital Velocity

Or

[∵ and r = (R + h)]

Where the satellite is at height (h) above surface of earth

• Orbital velocity does not depend on the mass of the satellite
• Orbital velocity around the earth is 7.92 km/s.
• As the height of the satellite increases, orbital velocity decreases.

Escape Velocity

Escape velocity is the minimum velocity of a body that is projected so that it escapes the gravitational attraction of the earth.

• Escape velocity deos not depend upon the mass of the body that is being projected but the mass as as well as the radius from the body from where it is being projected.
• Escape velocity from surface of earth is 11.2km/s.
• For escape, the minimum energy needed is GMm/r.
• When from an infinite distance, a body falls freely it will reach the earth’s surface with velocity of 11.2 km/s.

Relationship between Orbital Velocity and Escape Velocity (V₀ and V_e)

Check NEET Study Notes on Wave Optics

NEET Sample Questions on Gravitation

Question: What is the force of attraction between two objects M1 and M2 that lie at distance d , directly proportional to

1. Product of the mass M1 and M2
2. Sum of mass of M1 and M2
3. Difference between mass of object M1 and M2
4. Sum of the square of the mass of object M1 and M2

Answer: Product of the mass of object M1 and M2

The universal law of gravitation states that two objects in the universe are attracted by a force that acts on a straight line between center of mass of both the objects. Force of attraction is directly proportional to the product of mass of two objects and inversely proportional to square of the distance.

Question: When a satellite is launched into orbit, the force that governs motion is—?

1. Elasticity
2. Friction
3. Gravitation
4. Fuel driven

Answer: Gravity

Satelite is an object that revolves around a larger body due to the force of gravity.

Question: Where is the maximum intensity of Gravitational field of the earth experienced?

1. Poles
2. Surface of the earth
3. Equator
4. Center of the eath

Answer: Poles

The maximum intensity of gravitational field of the earth is maximum at the poles and minimum at the equator.

Question: What is the weight of a body at center of the earth?

1. Same as on the surface of the earth
2. Zero
3. Less than weight on surface of earth
4. None of the above

Answer: Zero

Question: What does the orbital velocity of a satellite that orbits around earth’s surface depend upon?

1. Orbital Radius
2. Radius of Earth
3. Mass of the satellite
4. Mass of the earth

Answer: Radius of the earth

Question: What does Kepler’s second law of constancy show the conservation of?

1. Linear Momentum
2. Angular Momentum
3. Energy
4. None of the above

Answer: Angular Momentum

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