Gravitation, often referred to as gravity, is a fundamental force of nature that attracts all objects with mass towards each other. It is the force that keeps planets in orbit around stars, moons in orbit around planets, and holds us firmly to the ground.
The concept of gravitation has been studied and understood for centuries. One of the earliest known attempts to explain gravity was by the ancient Greek philosopher Aristotle, who believed that objects fell to the ground because they were seeking their natural place. However, this explanation was later proven to be incorrect.
A significant breakthrough in understanding gravitation came in the 17th century with the work of Sir Isaac Newton. Newton developed the Law of Universal Gravitation, which states that every particle in the universe attracts every other particle with a force that is directly proportional
Newton's law of gravitation was a major accomplishment, but it did not fully explain the nature of gravity. It was not until the early 20th century that Albert Einstein developed the Theory of General Relativity, which provides a more comprehensive understanding of gravitation. Einstein's theory describes gravity as a curvature of spacetime caused by the presence of mass and energy.
Newton's Law of Universal Gravitation
Mathematical Expression
Newton's Law of Universal Gravitation states that the force of attraction between two objects is directly proportional to the product of their masses and inversely proportional
F = G * (m1 * m2) / r^2
where:
- F is the gravitational force between the two objects
- G is the 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 centers of the two objects
Explanation
This equation means that the stronger the gravitational force between two objects, the:
- Larger their masses are
- Closer they are to each other
Examples of Gravitational Force in Action
- Earth's gravity: The gravitational force between the Earth and objects on its surface is responsible for the weight of those objects. This force is what keeps us grounded.
- The moon's orbit: The gravitational force between the Earth and the moon is what keeps the moon in orbit around our planet.
- Planetary orbits: The gravitational force between planets and the sun is what keeps them in their respective orbits.
- Tidal forces: The difference in gravitational force between the moon's near side and far side of the Earth is responsible for tidal forces in the oceans.
- Black holes: Black holes have such a strong gravitational force that nothing, not even light, can escape their pull.
Gravitational Field Intensity
Definition
A gravitational field is a region of space around a massive object where another object experiences a gravitational force. The gravitational field intensity at a point in this field is defined as the gravitational force per unit mass acting on a test mass placed at that point.
Calculation
The gravitational field intensity (g) at a distance (r) from a point mass (M) is given by:
g = GM / r^2
where:
- G is the gravitational constant
- M is the mass of the point mass
- r is the distance between the point mass and the point
where the field intensity is being calculated
Relationship Between Gravitational Force and Field Intensity
The gravitational force (F) acting on a test mass (m) placed in a gravitational field of intensity (g) is given by:
F = mg
This equation shows that the gravitational force acting on a test mass is directly proportional to the gravitational field intensity at its location. In other words, the stronger the gravitational field intensity, the greater the gravitational force acting on a test mass.
Gravitational Potential
Concept of Gravitational Potential Energy
Gravitational potential energy is the energy possessed by an object due to its position in a gravitational field.
Calculation of Gravitational Potential Difference
The gravitational potential difference between two points in a gravitational field is
For a point mass (M), the gravitational potential (V) at a distance (r) from it is given by:
V = -GM / r
where:
- G is the gravitational constant
- M is the mass of the point mass
- r is the distance between the point mass and the point
where the gravitational potential is being calculated
The gravitational potential difference (ΔV) between two points A and B is then given by:
ΔV = VB - VA
where VA and VB are the gravitational potentials at points A and B, respectively.
Note: The negative sign in the formula for gravitational potential indicates that gravitational force is attractive. As an object moves closer to the source of the gravitational field, its gravitational potential energy decreases.
Acceleration Due to Gravity
Definition
Acceleration due to gravity (g) is the acceleration experienced by an object falling freely under the influence of gravity. It is the rate at which the velocity of a falling object increases due to the gravitational force acting on it.
Calculation
The acceleration due to gravity at the surface of a spherical planet is given by:
g = GM / R^2
where:
- G is the gravitational constant
- M is the mass of the planet
- R is the radius of the planet
Variation of Acceleration Due to Gravity
1. Variation with Altitude
- As an object moves away from the Earth's surface, its distance from the center of the Earth increases. This means that the gravitational force acting on it decreases, and consequently, the acceleration due to gravity also decreases.
- The variation of acceleration due to gravity with altitude is approximately linear for small altitudes.
2. Variation with Depth
- As an object moves deeper into the Earth, the mass of the Earth below it decreases. This means that the gravitational force acting on it decreases, and consequently, the acceleration due to gravity also decreases.
- The variation of acceleration due to gravity with depth is more complex than with altitude, as it depends on the distribution of mass within the Earth.
3. Variation with Rotation of Earth
- Due to the Earth's rotation, objects at the equator experience a centrifugal force that acts outwards. This force reduces the effective gravitational force acting on them.
- As a result, the acceleration due to gravity is slightly less at the equator than at the poles.
- The variation of acceleration due to gravity with latitude is approximately sinusoidal.
Kepler's Laws of Planetary Motion
Johannes Kepler, a German astronomer, formulated three laws that describe the motion of planets around the Sun. These laws are based
Kepler's First Law: The Law of Elliptical Orbits
- Statement: Every planet revolves around the Sun in an elliptical orbit, with the Sun at one focus.
- Explanation: This law deviates from the earlier belief that planets move in circular orbits. The elliptical orbit means that the distance between a planet and the Sun varies throughout its orbit.
Kepler's Second Law: The Law of Areas
- Statement: A planet sweeps out equal areas in equal intervals of time.
- Explanation: This law implies that a planet moves faster when it is closer to the Sun and slower when it is farther away. This is because the gravitational force
is stronger when the planet is closer, causing it to accelerate.
Kepler's Third Law: The Law of Harmonies
- Statement: The square of the orbital period of a planet is proportional to the cube of its average distance from the Sun.
- Explanation: This law relates the orbital period of a planet to its distance from the Sun. Planets farther from the Sun have longer orbital periods.
Applications of Kepler's Laws in Astronomy
- Predicting planetary positions: Kepler's laws can be used to predict the positions of planets in the sky at any given time.
- Calculating orbital periods: Knowing the average distance of a planet from the Sun, Kepler's third law can be used to calculate its orbital period.
- Understanding the motion of other celestial bodies: Kepler's laws are not limited to planetary motion but can also be applied to other celestial bodies, such as comets and asteroids.
- Space exploration: These laws are essential for planning and executing space missions, as they help in determining the trajectories of spacecraft.
- Studying the formation and evolution of the solar system: Kepler's laws provide insights into the formation and evolution of the solar system, as they reveal the relationships between the planets and the Sun.
Escape Velocity
Definition
Escape velocity is the minimum speed that an object needs to acquire to escape the gravitational pull of a celestial body and travel infinitely far away from it.
Calculation
The escape velocity (Ve) from a celestial body of mass (M) and radius (R) is given by:
Ve = √(2GM / R)
where:
- G is the gravitational constant
Factors Affecting Escape Velocity
- Mass of the celestial body: A more massive celestial body has a stronger gravitational pull, so the escape velocity is higher.
- Radius of the celestial body: A smaller celestial body has a weaker gravitational pull at its surface, so the escape velocity is lower.
- Distance from the center of the celestial body: The escape velocity decreases with increasing distance from the center of the celestial body.
Example: The escape velocity from Earth is approximately 11.2 km/s. This means that an object launched from Earth's surface with a speed of at least 11.2 km/s can overcome Earth's gravity and escape into space.