What is Gravity?
What is the force that holds us down on the Earth and stops us from flying up in space? If you know the answer to this question then you know what is gravity. Gravity is one of the universe’s fundamental forces that dominate every moment of our conscious existence. It helps us to walk on the ground, pulls the basketballs and baseballs back to the ground, and gravity is also something our muscles cannot struggle against. The story of gravity goes together with the story of physics and many famous physicists have found ways to figure out what causes gravity.
What is Gravitation?
Scientists have defined four types of forces in the universe that either attract or repel an object from another. These are:
Strong force
Electromagnetic force
Weak force
Gravity
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Of these 4 types of forces, strong and weak forces operate within the center of an object’s atoms. The EM force applies to objects that have an excess of charge (electrons, protons) like socks that shuffle over a fuzzy carpet.
Gravity is the force that acts upon objects having mass. It is an all-pervasive force that keeps our foot on the ground yet it is still a puzzle for many and the answer to the question what do you mean by gravity.
What do You Mean by Gravitational Force
Gravity can be defined as the force which a planet (or a body) exerts on other objects and draws them towards its center. It is this force of gravity that keeps all the planets in their orbits around the Sun.
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Newton explained what is meant by gravitational force in his law of universal gravitation as a force of attraction that exists between any two objects having mass. This force is directly proportional to the masses of the objects and inversely proportional to the distance between them. It is mathematically expressed as:
F = G * (m1 m2) / R2
Where F - Force of gravity.
G - Gravitational constant and its value is 6.67 x 10-11 N * m2/kg2.
R - The distance between the objects.
What is Gravity Made of
Earth’s gravity comes from its entire mass. Earth’s entire mass exerts a combined gravitational pull on your body mass. It is this gravitational pull of the earth which gives you your weight. So if you are on a planet whose mass is less than the earth, then you would weigh less there. You and earth both exert gravitational force on each other, but since earth’s mass is way more than your weight, your force has no effect on the planet.
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A team of scientists based in the South Pole discovered the celestial fingerprint using a telescope, BICEP2. This fingerprint explains the beginning of time and reveals microscopic details of gravity. Quantum particles in an infant universe gave rise to “inflation,” which is the reason behind why the universe exploded outwards.
It also revealed that gravity is made up of quantum particles called gravitons. A single graviton is too small and massless, but these gravitons' quantum fluctuations in the infant universe bend pockets of this tiny space-time.
What is the Gravitational Force of Earth
The gravity of the earth is denoted by g and is defined as the net acceleration that is induced in objects owing to the gravitation (mass distribution within the earth) and centrifugal force due to the earth’s rotation.
Earth’s acceleration due to gravity is 9.8 m/s2 close to its surface (also called 1 g). Gravity decreases as the distance from the Earth increases.
Gravity on Earth is Not the Same Everywhere
The Sun’s gravity keeps the Earth in such a position that we can enjoy the sunlight without being burnt. Gravity holds our entire atmosphere and the air we breathe in, so living without gravity is impossible for us.
But the force of gravity is not the same at all parts of our planet Earth. There is more gravity in the places where there is more mass underground compared to places with less underground mass. The force of gravity is also dependent on where you are standing, near the equator, or further from it. This is due to Earth rotating on its axis; hence, the force of gravity at the equator is less (9.789 m/s2) than the gravity at the poles (9.832 m/s2). This means you would weigh slightly more at the poles than at the equator due to the centripetal force.
NASA has deployed 2 spacecrafts that measure these variations in Earth’s gravity. These spacecrafts are part of GRACE (Gravity recovery and climate experiment).
GRACE can detect small changes in gravity over time and has revealed crucial details about Earth. As an example, GRACE monitors sea level, and changes in it can make it determine changes in Earth’s crust brought upon by earthquakes.
Examples of Gravity
Some interesting examples of gravity are:
Gravity is the weakest force amongst all the fundamental forces known. A bar magnet can pull a paperclip upwards due to electromagnetic force, which is stronger than the gravitational pull of the whole earth on that paper clip.
It is gravity due to which a car coasts downhill even when you are not stepping on the gas.
Gravity holds the earth and all the planets around the Sun in their proper positions and orbits.
The force of gravity holds gases inside the Sun.
Tides in the oceans are led by the force of gravity of the moon.
The stars and planets have been created by gravity, which has pulled together the materials they are composed of.
Gravity not just pulls masses, but it affects light as well. Einstein proved this fact. If you send a flashlight upwards, it turns red with time due to gravitational pull. This color change is not perceptible by us, but scientists have been able to measure it.
FAQs on Gravity
1. What is gravity in simple terms?
Gravity is the fundamental force of attraction that exists between any two objects that have mass. It is the force that pulls objects towards each other. For instance, the Earth's gravity is what keeps you on the ground, holds the atmosphere in place, and causes objects like an apple to fall downwards when dropped.
2. What is the main difference between 'gravity' and 'gravitation'?
While often used interchangeably, there is a subtle distinction between the terms:
- Gravitation refers to the universal force of attraction acting between any two objects in the universe. It is a general term described by Newton's Law of Universal Gravitation.
- Gravity typically refers to the specific gravitational force exerted by a very large celestial body, like a planet or a star, on objects near its surface. For example, we talk about Earth's gravity pulling on a person.
3. What is Newton's Law of Universal Gravitation and its formula?
Newton's Law of Universal Gravitation states that every particle in the universe attracts every other particle 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. The formula is:
F = G (m₁m₂ / r²)
- F is the gravitational force.
- G is the universal gravitational constant (approximately 6.674 × 10⁻¹¹ N⋅m²/kg²).
- m₁ and m₂ are the masses of the two objects.
- r is the distance between the centres of the two masses.
4. What is the significance of the value g = 9.8 m/s² for Earth's gravity?
The value g = 9.8 m/s² represents the acceleration due to gravity near the Earth's surface. It means that, ignoring air resistance, any object in free-fall will increase its downward velocity by 9.8 metres per second for every second it falls. This value is an average and can slightly vary based on location (e.g., at the poles vs. the equator) and altitude.
5. How are mass and weight different, and what is gravity's role in connecting them?
Mass and weight are fundamentally different concepts, connected by gravity:
- Mass is the amount of matter an object contains. It is a scalar quantity, measured in kilograms (kg), and remains constant regardless of location.
- Weight is the force of gravity acting on an object's mass. It is a vector quantity, measured in Newtons (N), and changes depending on the strength of the local gravitational field.
Gravity connects them through the formula Weight (W) = Mass (m) × Acceleration due to gravity (g). Your mass is the same on Earth and the Moon, but your weight is much less on the Moon because its gravity is weaker.
6. Why is gravity on the Moon so much weaker than on Earth?
The strength of a celestial body's gravitational pull depends directly on its mass. The Moon has significantly less mass than the Earth—about 1.2% of Earth's mass. Because of this smaller mass, the force of gravity on the Moon's surface is only about one-sixth of the gravity on Earth's surface. This is why astronauts can jump much higher on the Moon than on Earth.
7. How does the force of gravity change as you move away from or deeper into the Earth?
The force of gravity is not constant and changes with position:
- With Altitude (moving away): As you move away from the Earth's surface, the force of gravity decreases. This follows the inverse square law from Newton's formula—doubling the distance reduces the gravity to one-quarter of its original strength.
- With Depth (moving deeper): As you go deeper into the Earth, the force of gravity also decreases. This is because the mass of the Earth's shell above you no longer contributes to the downward pull. The effective mass pulling you towards the centre becomes smaller. At the exact centre of the Earth, the gravitational force would be zero.
8. If gravity is the weakest fundamental force, why does it seem so powerful and dominate the universe?
Although gravity is the weakest of the four fundamental forces at a particle level, it dominates on a large scale for two key reasons:
- It is always attractive and cumulative: Unlike electromagnetism, which has positive and negative charges that can cancel each other out, gravity is only attractive. Every bit of mass adds to the total gravitational pull, allowing it to build up to immense forces over large objects like planets and stars.
- It has an infinite range: The strong and weak nuclear forces only act over subatomic distances. Gravity, however, extends infinitely, allowing it to influence objects across vast cosmic distances and shape the structure of galaxies and the universe itself.
9. How can astronauts be 'weightless' on the International Space Station (ISS) if Earth's gravity is still strong there?
Astronauts on the ISS are not truly weightless; they are in a constant state of free-fall. The ISS is orbiting Earth at such a high speed that as it falls towards the planet due to gravity, it also moves horizontally, constantly 'missing' the ground. This continuous falling motion creates the sensation of weightlessness for the astronauts and everything inside the station, even though Earth's gravity is still about 90% as strong as it is on the surface.
10. What are some important real-world examples of gravity in action?
Beyond making objects fall, gravity has many crucial applications and effects:
- It keeps the planets of our solar system in stable orbits around the Sun.
- It holds Earth's atmosphere and the air we breathe from drifting into space.
- The Moon's gravity is the primary cause of ocean tides on Earth.
- It is the force responsible for the formation and structure of stars, planets, and entire galaxies by pulling together vast clouds of gas and dust.
