What is Gravitation?
Gravitation is the force of attraction between any two objects in the universe.
Every object that has mass attracts every other object. This attraction is called gravitational force.
The force is very small between ordinary objects, but it becomes very important when the objects are very massive, such as planets, stars, and moons.
For example, Earth attracts all objects toward its centre. This is why a stone thrown upward comes back down. This is also why we stay on the ground instead of floating away.
Gravitation is responsible for many important phenomena in nature:
- Objects falling toward Earth
- The Moon revolving around Earth
- The planets revolving around the Sun
- The formation of tides
- The motion of artificial satellites
- The structure of galaxies
Universal Law of Gravitation
Sir Isaac Newton stated the universal law of gravitation.
According to this law:
Every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.
Mathematically,
where:F = gravitational force between the two objects
m₁ and m₂ = masses of the two objects
r = distance between the centres of the two objects
G = universal gravitational constant
The value of G is:
Key Points
- The gravitational force increases when the masses of the objects increase.
- The gravitational force decreases when the distance between the objects increases.
- If the distance between two objects is doubled, the gravitational force becomes one-fourth of the original force.
- This is because gravitational force follows the inverse square law.
Gravitational Force
Gravitational force is always attractive. It never repels.
This means that two masses always pull each other toward themselves. Earth pulls an apple downward, and the apple also pulls Earth upward. However, Earth is extremely massive compared to the apple, so the motion of Earth is not noticeable.
Acceleration Due to Gravity
When an object falls freely toward Earth, its velocity increases continuously. This happens because Earth’s gravitational force produces acceleration.
This acceleration is called acceleration due to gravity.
It is denoted by g.
Near the surface of Earth,
g=9.8m/s2
This means that the velocity of a freely falling object increases by about 9.8 m/s every second, if air resistance is ignored.
Mass and Weight
Mass and weight are related, but they are not the same.
Mass is the amount of matter present in an object. It remains the same everywhere.
Weight is the gravitational force acting on an object. It depends on the value of gravity.
W=mg
where:
W = weight
m = mass
g = acceleration due to gravity
An object has the same mass on Earth and on the Moon, but its weight is less on the Moon because the Moon’s gravity is weaker than Earth’s gravity.
Free Fall
When an object falls under the influence of gravity alone, it is said to be in free fall.
During free fall, all objects fall with the same acceleration, provided air resistance is neglected.
This means that a heavy object and a light object fall at the same rate in vacuum.
Why Gravitation is Important
Gravitation helps us understand both everyday events and large-scale motions in space.
It explains why objects fall, why planets move around the Sun, why satellites stay in orbit, and why stars and galaxies hold together.
Without gravitation, the universe would not have the structure we observe today.
Motion of Planets and Satellites
Gravitation plays a major role in the motion of planets and satellites.
The Sun attracts the planets by gravitational force. At the same time, each planet has its own motion. Because of this combination, the planets do not fall directly into the Sun. Instead, they move around the Sun in fixed paths called orbits.
Similarly, Earth attracts the Moon by gravitational force. This force keeps the Moon moving around Earth. Artificial satellites also move around Earth because of Earth’s gravitational pull.
A satellite stays in orbit because of two effects:
- It moves forward with a certain speed.
- Gravity pulls it toward Earth.
Because of this, the satellite continuously “falls” toward Earth, but it also keeps moving forward. As a result, it keeps going around Earth instead of crashing directly down.
This is why gravitation is responsible for the motion of:
- Planets around the Sun
- The Moon around Earth
- Artificial satellites around Earth
- Comets and other celestial bodies
In simple words, gravity provides the force needed to keep planets and satellites in orbit.
Escape Velocity
When we throw an object upward, it rises for some time and then falls back to Earth. This happens because Earth’s gravity pulls it downward.
But if an object is thrown upward with a very high speed, it may escape Earth’s gravitational pull and never return. The minimum speed required for this is called escape velocity.
Escape velocity is the minimum velocity required for an object to escape from the gravitational field of a planet or other celestial body.
For Earth, the escape velocity is about:
11.2 km/s
This means that an object must have a speed of about 11.2 kilometres per second to escape Earth’s gravity, neglecting air resistance.
Escape velocity depends on:
- The mass of the planet
- The radius of the planet
A planet with greater mass has stronger gravity, so its escape velocity is higher.
A smaller planet with weaker gravity has a lower escape velocity.
For example, the Moon has much weaker gravity than Earth, so its escape velocity is much smaller.
Escape velocity is important in space travel. Rockets must reach very high speeds to move beyond Earth’s gravitational influence and enter space.
Applications of Gravitation
Gravitation is not just a theoretical concept. It has many important applications in daily life, astronomy, and space science.
The major applications are:
1. Falling Objects
2. Weight of Objects
3. Motion of the Moon
4. Motion of Planets
5. Artificial Satellites
6. Tides
7. Space Missions
8. Structure of the Universe
1. Falling Objects
Gravitation explains why objects fall toward Earth.
When we drop a stone, a ball, or any object, Earth pulls it downward due to gravity.
2. Weight of Objects
The weight of an object is due to gravitational force.
W=mg
where W is weight, m is mass, and g is acceleration due to gravity.
An object weighs more on Earth than on the Moon because Earth’s gravity is stronger.
3. Motion of the Moon
The Moon revolves around Earth because Earth’s gravity pulls it toward itself.
Without Earth’s gravitational pull, the Moon would not stay in its orbit.
4. Motion of Planets
The planets revolve around the Sun because of the Sun’s gravitational force.
The Sun’s gravity keeps the solar system together.
5. Artificial Satellites
Artificial satellites are launched into orbit around Earth.
They are used for communication, weather forecasting, television broadcasting, GPS navigation, scientific research, and Earth observation.
6. Tides
Tides in oceans are mainly caused by the gravitational pull of the Moon.
The Sun also affects tides, but the Moon has a stronger effect because it is much closer to Earth.
7. Space Missions
Gravitation is very important in planning space missions.
Scientists use gravitational calculations to launch rockets, place satellites in orbit, send spacecraft to other planets, and bring astronauts safely back to Earth.
8. Structure of the Universe
Gravitation helps form and hold together large structures in the universe.
Stars, planets, solar systems, galaxies, and clusters of galaxies are all influenced by gravitational force.
Glossary of Key Terms
Recap of the Key Terms in Gravitation
- Acceleration Due to Gravity: Acceleration due to gravity is the acceleration produced in a freely falling object due to Earth’s gravitational pull.
- g=9.8m/s2
- Escape Velocity: Escape velocity is the minimum velocity required for an object to escape from the gravitational field of a planet or other celestial body.
- Free Fall: Free fall is the motion of an object under the influence of gravity alone.
- Gravitation: Gravitation is the force of attraction between any two objects that have mass.
- Gravitational Constant: The gravitational constant, denoted by G, is the constant used in Newton’s law of gravitation.
- Gravitational Force: Gravitational force is the attractive force between two masses.
- Mass: Mass is the amount of matter present in an object. It remains the same everywhere.
- Orbit: An orbit is the curved path followed by a planet, moon, or satellite around another body.
- Satellite: A satellite is an object that revolves around a planet. The Moon is a natural satellite of Earth. Artificial satellites are man-made satellites.
- Tides: Tides are the periodic rise and fall of sea levels mainly caused by the gravitational pull of the Moon and the Sun.
- Universal Law of Gravitation: Newton’s universal law of gravitation states that every object in the universe attracts every other object with a force that depends on their masses and the distance between them.
- Weight: Weight is the gravitational force acting on an object.
W = mg
Questions and Answers
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is Gravitation?
Gravitation is the force of attraction between any two objects that have mass.
Who proposed the universal law of gravitation?
Sir Isaac Newton proposed the universal law of gravitation.
State Newton’s universal law of gravitation.
Newton’s universal law of gravitation states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.
What is the formula for gravitational force?
The formula for gravitational force is:
where F is gravitational force, G is the gravitational constant, m₁ and m₂ are the masses of the two objects, and r is the distance between their centres.
What is the value of acceleration due to gravity near Earth’s surface?
The value of acceleration due to gravity near Earth’s surface is approximately:
g=9.8m/s2
What is the difference between mass and weight?
Mass is the amount of matter present in an object. It remains the same everywhere.
Weight is the gravitational force acting on an object. It changes from place to place depending on the value of gravity.
Why do objects fall toward Earth?
Objects fall toward Earth because Earth attracts them by gravitational force.
What is free fall?
Free fall is the motion of an object under the influence of gravity alone, without considering air resistance.
Why does the Moon revolve around Earth?
The Moon revolves around Earth because Earth’s gravitational force pulls the Moon toward itself and keeps it in orbit.
What is escape velocity?
Escape velocity is the minimum velocity required for an object to escape from the gravitational field of a planet or other celestial body.
For Earth, escape velocity is about:
11.2 km/s
Why are artificial satellites important?
Artificial satellites are important because they are used for communication, weather forecasting, television broadcasting, GPS navigation, scientific research, and Earth observation.
What causes tides?
Tides are mainly caused by the gravitational pull of the Moon. The Sun also affects tides, but the Moon has a stronger effect because it is closer to Earth.
Why do planets revolve around the Sun?
Planets revolve around the Sun because the Sun’s gravitational force pulls them toward itself and keeps them in orbit.
Is gravitational force attractive or repulsive?
Gravitational force is always attractive. It always pulls objects toward each other.
Why is gravitation important?
Gravitation is important because it explains falling objects, weight, planetary motion, satellite motion, tides, space travel, and the structure of the universe.