Gravity is one of the fundamental forces of nature, governing the motion of celestial bodies, the behavior of objects on Earth, and even the structure of the universe. From keeping planets in orbit to pulling apples to the ground, gravity is a force that affects everything with mass. But what is gravity, and how does it work?
Key Takeaways: What Is Gravity?
- Gravity is a fundamental force of nature that causes attraction between objects with mass.
- The strength of gravity varies on different celestial bodies, depending on mass and radius.
- Newton’s law of universal gravitation describes gravity as a force between two masses, decreasing with distance.
- Einstein’s general theory of relativity redefines gravity as the curvature of spacetime caused by mass.
- Gravity is responsible for keeping planets in orbit, controlling tides, and shaping galaxies.
- Gravity is weakest of the four fundamental forces, yet it has an infinite range and governs large-scale cosmic structures.
Definition of Gravity
Gravity is a force of attraction that acts between all objects with mass. It is responsible for keeping us on Earth, determining the orbits of planets, and shaping the large-scale structure of the universe. The force of gravity depends on the mass of objects and the distance between them.
Gravity Formulas
Several key equation describe gravity:
Gravity can be described using several key equations:
- Newton’s Law of Universal Gravitation
F = G · (m1m2)/r2- Used to calculate the force of gravity between two masses.
- Weight Formula
W= mg- Used to find the force of gravity acting on an object at the surface of a planet.
- Acceleration Due to Gravity on a Planet
g = GM/R2- Used to calculate gravity on a planet’s surface, where:
- M is the planet’s mass,
- R is its radius.
- Used to calculate gravity on a planet’s surface, where:
- Escape Velocity
ve = √ (2GM/R)- The minimum velocity required to leave a planet’s gravitational influence.
The Importance and Effects of Gravity
Gravity is essential for many physical and astronomical phenomena:
- Keeps Planets and Moons in Orbit – The Sun’s gravity holds planets in the solar system, and Earth’s gravity keeps the Moon in orbit.
- Shapes Galaxies and the Universe – Gravity binds stars into galaxies and structures the cosmic web.
- Controls Tides – The Moon’s gravitational pull causes ocean tides on Earth.
- Influences Weight – Your weight is the force of gravity acting on your body.
- Affects Motion of Projectiles – Objects fall to the ground due to gravity, following parabolic trajectories.
- Determines Escape Velocity – The speed required to overcome a celestial body’s gravity and enter space.
Gravity on Different Celestial Bodies
A planet’s mass and radius determine its gravity. The following table shows the acceleration due to gravity (g) on different celestial bodies compared to Earth’s gravity (gE = 9.8 m/s²). Note that the values for surface gravity on other planets varies for the Sun and gas giants because defining their surface is challenging.
| Celestial Body | Surface Gravity (g, in m/s²) | Gravity Relative to Earth |
|---|---|---|
| Sun | 274.0 | 28.0 × Earth |
| Mercury | 3.7 | 0.38 × Earth |
| Venus | 8.87 | 0.90 × Earth |
| Earth | 9.81 | 1.00 × Earth |
| Moon | 1.62 | 0.165 × Earth |
| Mars | 3.71 | 0.38 × Earth |
| Jupiter | 24.79 | 2.53 × Earth |
| Saturn | 10.44 | 1.07 × Earth |
| Uranus | 9.1 | 0.91 × Earth |
| Neptune | 11.15 | 1.14 × Earth |
Using Gravity for Calculating Weight on Other Planets
You can calculate your weight on different planets using the formula:
Weight on planet = Weight on Earth × gplanet / gEarth
Alternatively, just enter in your weight in this handy tool:
Weight on Other Planets Calculator
Enter your weight:
Historical Understanding of Gravity
While people had an intuitive understanding of gravity (objects fall when dropped), Galileo furthered our understanding by showing that it is (more or less) a constant. Newton’s classical formula allowed for calculations involving gravity, while Einstein’s general relativity defined gravity in terms of spacetime and mass.
Early Ideas About Gravity
- Ancient Greeks: Aristotle believed heavier objects fell faster than lighter ones.
- Galileo Galilei (1590s): Showed that all objects fall at the same rate regardless of mass.
A famous Apollo 15 experiment demonstrates how, in the near vacuum of the Moon, a hammer and a feather fall at the same rate—just as Galileo predicted. Without air resistance, gravity acts equally on all objects, regardless of mass.:
Newton’s Law of Universal Gravitation (1687)
Sir Isaac Newton formulated the first mathematical model of gravity:
F = G · (m1m2)/r2
where:
- F = gravitational force,
- G = gravitational constant (6.674 × 10−11 N·m²/kg²),
- m1, m2 = masses of objects,
- r = distance between their centers.
Einstein’s General Relativity (1915)
Albert Einstein proposed that gravity is not a force but the curvature of spacetime caused by mass. This theory explains phenomena such as:
- The bending of light around massive objects (gravitational lensing).
- Time dilation near strong gravitational fields.
Worked Example Problems
1. Weight on Another Planet
Problem: A person weighs 70 kg on Earth. What is their weight on Mars?
Formula: W = mg
Weight on Mars = 70 × 3.719.81 = 2 6.5 kg
Answer: The person weighs 26.5 kg on Mars.
2. Gravitational Force Between Two Masses
Problem: Find the gravitational force between Earth (5.97 × 1024 kg) and the Moon (7.35 ×1022 kg), separated by 3.84 × 108 m.
Formula: F = G · (m1m2)/r2
F = (6.674 × 10−11) · (5.97 × 1024)(7.35 × 1022) / (3.84 × 108)2 = 1.98 × 1020 N
Answer: The gravitational force between Earth and the Moon is 1.98×1020 N.
Common Misconceptions and FAQs About Gravity
Gravity Is the Same Everywhere on Earth
Misconception: Gravity is uniform across the planet.
Reality: Gravity varies slightly with altitude, latitude, and local geology. It’s weaker at higher elevations and stronger near the poles due to Earth’s rotation and shape.
Gravity Requires Mass, So Why Does It Affect Light?
Misconception: Only objects with mass experience gravity, so light should not be affected.
Reality: Einstein’s general relativity shows that gravity warps spacetime, and light follows these curves. This effect, called gravitational lensing, explains why photons bend around massive objects like black holes.
Astronauts in Space Experience No Gravity
Misconception: There is no gravity in space.
Reality: Gravity exists everywhere, but astronauts appear weightless because they are in continuous free fall around Earth, not because gravity is absent.
Heavier Objects Fall Faster Than Lighter Ones
Misconception: More massive objects fall faster under gravity.
Reality: In the absence of air resistance, all objects accelerate at the same rate under gravity. Galileo and the Apollo 15 astronauts demonstrated this principle.
Gravity Acts Instantly
Misconception: If the Sun disappeared, Earth would immediately fly off into space.
Reality: Changes in gravity propagate at the speed of light, so it would take about 8 minutes for Earth to be affected.
Gravity Can Be Blocked or Shielded
Misconception: Like electromagnetism, gravity can be blocked by certain materials.
Reality: Gravity cannot be shielded or canceled—all mass is affected by it, regardless of barriers.
Gravity Only Pulls, Never Pushes
Misconception: Gravity can attract or repel like electric charges.
Reality: Gravity only attracts. However, dark energy appears to act as a repulsive force on cosmic scales, accelerating the universe’s expansion.
The Moon’s Gravity Does Not Affect Humans
Misconception: The Moon’s gravity only affects oceans, not people.
Reality: The Moon’s gravity affects all objects on Earth, but its pull is much weaker than Earth’s own gravity, so we do not feel it directly.
Artificial Gravity Is Impossible
Misconception: Gravity cannot be simulated in space.
Reality: Artificial gravity can be simulated using centrifugal force, such as in a rotating space station.
References
- Dyson, F.W.; Eddington, A.S.; Davidson, C.R. (1920). “A Determination of the Deflection of Light by the Sun’s Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919”. Phil. Trans. Roy. Soc. A. 220 (571–581): 291–333. doi:10.1098/rsta.1920.0009
- Ehlers, Jurgen (1997). “Examples of Newtonian limits of relativistic spacetimes”. Classical Quantum Gravity. 14 (1A): 122–123. doi:10.1088/0264-9381/14/1A/010
- Halliday, David; Robert Resnick; Kenneth S. Krane (2001). Physics. New York: John Wiley & Sons. ISBN 978-0-471-32057-9.
- Pickover, Clifford (2008). Archimedes to Hawking: Laws of Science and the Great Minds Behind Them. Oxford University Press. ISBN 9780199792689.
- Thorne, Kip S.; Misner, Charles W.; Wheeler, John Archibald (1973). Gravitation. W.H. Freeman. ISBN 978-0-7167-0344-0.