Gravity
33 questions tagged Gravity — spanning every one of the seven Ws.
Why does gravity make objects accelerate at the same rate in a vacuum?
In a vacuum, all objects fall with the same acceleration because gravity pulls on mass, while heavier objects also have proportionally more inertia. Near Earth’s surface that acceleration is about 9.8 m/s².
Why does gravity get weaker with distance according to the inverse-square law?
Gravity spreads out over the surface of a sphere, so as distance doubles, the same pull is distributed over four times the area. That makes the force drop with the square of distance.
Why do astronauts appear weightless in orbit even though gravity is still strong there?
They are weightless because they are constantly falling around Earth instead of standing on a surface. At the International Space Station’s altitude, gravity is still about 90% as strong as at the ground.
Why does the Earth have tides on both sides of the planet at the same time?
Tides are caused by differences in the Moon’s gravity across Earth, not just its overall pull. The side facing the Moon and the opposite side both end up with tidal bulges.
When was gravitational lensing first observed?
In 1979, astronomers reported the first clear gravitational lensing of a distant quasar by a foreground galaxy.
When was the first confirmed detection of gravitational waves?
On 14 September 2015, LIGO made the first confirmed detection of gravitational waves, announced in 2016.
When was the first directly measured mass of the Earth’s moon obtained through gravity?
Newton’s laws were first used to estimate the Moon’s mass in the late 17th century, soon after the Principia appeared in 1687.
Which force is much weaker than electromagnetism but still dominates the universe on large scales?
Gravity is far weaker than electromagnetism, but it dominates large-scale structure because it is always attractive and acts over unlimited distance.
Whose law states that the force between two masses decreases with the square of the distance between them?
Newton’s law of universal gravitation. It says gravity acts between all masses and follows an inverse-square relationship with distance.
Whose theory first described gravity as the curvature of spacetime?
Albert Einstein’s general relativity. Published in 1915, it treats gravity as geometry rather than a force in the Newtonian sense.
How does gravity keep planets in orbit?
A planet stays in orbit because its forward motion constantly falls around the Sun while gravity pulls it inward. The balance between inertia and gravity creates a stable path.
How does gravity become weight?
Weight is the force gravity exerts on an object’s mass. On Earth, your weight depends on both your mass and Earth’s gravitational pull.
How do tidal forces stretch objects near a planet or moon?
Tidal forces happen because gravity is stronger on the near side of an object than on the far side. That difference can stretch, squeeze, or even break the object apart.
Why do tides form on opposite sides of Earth?
Tides form on both sides because the Moon’s gravity pulls more strongly on the near side than the far side, while Earth and Moon orbit a common center of mass. That creates a bulge toward the Moon and another on the opposite side.
Why can tidal forces tear objects apart near a massive body?
Tidal forces pull more strongly on the near side of an object than on the far side, stretching it. If the difference is strong enough, the object can break apart.
Why do objects fall at the same rate in a vacuum?
In a vacuum, all objects fall at the same rate because there is no air resistance to slow lighter or less streamlined items. Gravity gives them the same acceleration near Earth, about 9.8 m/s².
When was gravity first explained by a scientific law of universal attraction?
In 1687, Isaac Newton published the law of universal gravitation in Principia Mathematica. It was the first widely accepted mathematical theory of gravity.
When was general relativity first published?
In 1915, Albert Einstein presented general relativity. It describes gravity as the curvature of spacetime rather than a force in the Newtonian sense.
When was gravity first measured in a laboratory with a torsion balance?
In 1798, Henry Cavendish used a torsion balance to measure the gravitational attraction between masses. His experiment is often described as ‘weighing the Earth.’
Where was the Cavendish experiment conducted that first measured Earth’s density?
It was carried out in Henry Cavendish’s private laboratory in Clapham, London, in 1797–1798. The setup used a torsion balance to measure the tiny gravitational attraction between lead spheres.
Where is the gravitational constant measured in the Cavendish experiment usually identified?
It is identified with the torsion-balance laboratory in Clapham, London, where Cavendish’s 1797–1798 measurements were made. The result is often summarized as the first accurate weigh-in of Earth’s mass.
Which fundamental force explains why objects have weight?
Gravity explains why objects have weight. On Earth, weight is the gravitational force acting on mass.
How does the equivalence principle connect gravity and acceleration?
It says being in a gravitational field feels locally like being in an accelerating rocket. Einstein used this idea as a foundation for general relativity.
How do gravitational waves travel across the universe?
They are ripples in spacetime itself, produced by accelerating massive objects like merging black holes. They travel at the speed of light.
How do black holes fit into gravity without being a fundamental force?
Black holes are not a separate force; they are extreme solutions of general relativity where gravity becomes so strong that not even light escapes. Their effects are measured by the spacetime they warp around them.
What is the gravitational constant?
The gravitational constant, usually written as G, is the number used in Newton’s law of gravitation to calculate the strength of gravity. Its value is about 6.674 × 10^-11 in SI units.
When was Newton's law of universal gravitation first published?
It was first published in 1687, in Isaac Newton's Principia.
When was the Cavendish experiment first performed?
Henry Cavendish performed it in 1797–1798, making one of the first measurements of Earth's density.
When was the first experimental confirmation of the equivalence principle made?
A major early confirmation came in 1831 with the work of François Arago, later refined by Eötvös in the 1890s.
Where is the world’s most famous test of the equivalence principle performed?
Tests of the equivalence principle are performed in laboratories around the world, including precision facilities in Europe and the United States. They are not tied to one single famous site.
How does gravity keep the Moon in orbit around Earth?
Earth’s gravity constantly pulls the Moon inward while the Moon’s forward motion keeps it from falling straight down. Together, those effects create a stable orbit about 384,400 km away on average.
How does gravity make things fall at the same rate in a vacuum?
In a vacuum, gravity accelerates all objects equally, regardless of mass, if air resistance is removed. That means a feather and a hammer can hit the ground together.
How do forces create the tides on Earth?
Tides are mainly caused by the Moon’s gravity pulling more strongly on the side of Earth facing it than on the far side. The Sun also contributes, but less than the Moon.