Gravity of Earth

Earth pulls on everything.

This pull is called gravity. It keeps us on the ground.

Gravity is not the same everywhere. It is a little weaker at the equator. It is stronger at the poles. This happens because Earth is not a perfect ball. It bulges in the middle. Gravity also gets weaker as you go up. You are further from the center of the Earth.

This is why you feel lighter on a mountain. Gravity pulls us down toward the center.

Gravity is the pull that keeps us on the ground.

It comes from the mass of the Earth and its rotation. The strength of this pull is not the same everywhere.

One reason is the shape of our planet. The Earth is not a perfect sphere. It is an oblate spheroid. This means it is a bit flat at the poles. It also bulges out at the equator. Because of this bulge, you are further from the center at the equator. This makes gravity weaker there. At the poles, you are closer to the center. This makes gravity stronger.

Gravity also changes with your height. As you go up, you get further from Earth's center. This makes the pull weaker.

If you go up to 9,000 meters, you feel a little lighter. Even astronauts in orbit feel much gravity. It is still nearly 90% as strong as on the ground.

Finally, the ground under you matters. Denser rocks can make gravity stronger in one spot. This creates gravity anomalies, or local changes in pull.

Scientists use these changes to find oil or minerals.

Gravity is the force that pulls objects toward the Earth. It is the reason things fall down instead of floating away. This pull comes from two main things. First, it comes from the mass of the Earth itself. Second, it comes from the centrifugal force of the Earth's rotation. This rotation is the way the Earth spins on its axis. When you combine these two effects, you get the total gravity at any spot.

Gravity is a vector quantity, which means it has a specific direction. You can see this direction by using a plumb bob. A plumb bob is a weight on a string that hangs straight down.

Gravity is not the same strength everywhere on our planet. One reason is that the Earth is an oblate spheroid. This means the Earth is not a perfect, round ball. It is slightly flatter at the poles and bulges at the equator. Because of this bulge, people at the equator are farther from the center. This distance makes the pull weaker there. At the poles, you are closer to the center of the Earth. This makes the pull stronger at the poles.

The pull can change by about 0.7% across the surface. For example, gravity is 9.7639 m/s² on Nevado Huascarán in Peru. It is 9.8337 m/s² at the surface of the Arctic Ocean.

How high you are also changes how gravity feels. As you rise higher, you move farther from the Earth's center. This distance makes the gravitational pull get weaker. If you go from sea level to an altitude of 9,000 meters, your weight decreases by about 0.29%.

Some people think astronauts in orbit are weightless because gravity is gone. This is not true. At the altitude of the International Space Station, gravity is still nearly 90% as strong as on the ground. Astronauts feel weightless because they are actually in a state of free-fall.

In 1901, experts at the third General Conference on Weights and Measures set a standard. They chose a value of 9.80665 m/s² for Earth's gravity. This number was based on measurements taken in 1888. Those measurements were made at the Pavillon de Breteuil near Paris. This standard value is used even if it is not the exact local value. It helps scientists have a shared number to use for calculations.

This standard value is also used to define units like the kilogram force.

The ground beneath your feet can also change the local pull. These small changes are called gravity anomalies. Denser rocks, like those containing mineral ores, make gravity stronger in that spot. Less dense rocks, like sedimentary rocks, make it weaker.

Scientists use sensitive tools called gravimeters to measure these tiny differences. By studying these changes, people can find things like oil or mineral deposits. These local pulls can even be large enough to change the shape of the sea level. This is how we learn about what is hidden deep inside the Earth.

Gravity on Earth is the net acceleration experienced by objects due to two combined effects. These effects are gravitation, which comes from the distribution of mass within the planet, and centrifugal force, which results from the Earth's rotation. Gravity is a vector quantity, meaning it has both a specific magnitude and a direction. The direction of gravity coincides with the path of a plumb bob, which is a weight hanging from a string. In the International System of Units, this acceleration is expressed in meters per second squared (m/s²). Near the Earth's surface, the acceleration is approximately 9.8 m/s². This means a falling object's vertical velocity increases by about 9.8 meters per second every second, if air resistance is ignored.

While we often think of gravity as a constant, its precise strength varies depending on your location. A perfect, non-rotating sphere with uniform mass density would produce a uniform gravitational field. However, the Earth is an oblate spheroid, meaning it is not a perfect sphere. It is slightly flatter at the poles and bulges at the equator due to its rotation. This shape, combined with the rotation itself, causes deviations in gravity across the surface. The total variation in magnitude across the Earth is around 0.7%. For example, gravity is 9.7639 m/s² on Nevado Huascarán in Peru, but it is 9.8337 m/s² at the surface of the Arctic Ocean.

Latitude plays a major role in these variations. At latitudes closer to the equator, the outward centrifugal force from Earth's rotation is larger. This force counteracts gravity by up to a maximum of 0.3% at the equator, reducing the apparent downward acceleration. Additionally, the equatorial bulge means objects at the equator are further from the planet's center. Because gravitational attraction varies inversely with the square of the distance between two masses, this distance reduces the pull. Consequently, sea-level gravity increases from about 9.780 m/s² at the equator to about 9.832 m/s² at the poles. This means an object weighs approximately 0.5% more at the poles than at the equator.

Altitude also changes the strength of the gravitational pull. As you rise above the surface, you increase your distance from the Earth's center, which weakens the force. An increase in altitude from sea level to 9,000 meters results in a weight decrease of about 0.29%. It is a common misconception that astronauts in orbit are weightless because they have escaped Earth's gravity. In reality, at the altitude of the International Space Station, gravity is still nearly 90% as strong as it is on the ground. Astronauts feel weightless because they are in a state of free-fall, not because gravity has disappeared.

In 1901, the third General Conference on Weights and Measures established a standard gravitational acceleration. This conventional value is defined as 9.80665 m/s². It was based on measurements taken at the Pavillon de Breteuil near Paris in 1888. To create this standard, scientists applied a theoretical correction to convert the measurements to a latitude of 45° at sea level. This value is not an average of all locations, but a formal agreement to use when a specific local value is not required. This standard is also used to define the units known as kilogram force and pound force.

Local geology and topography create what are known as gravity anomalies. These are regional differences in the gravitational field caused by the density of the ground. Denser rocks, such as those containing mineral ores, create higher than normal local gravitational fields. Conversely, less dense sedimentary rocks cause a weaker field. Scientists use highly sensitive instruments called gravimeters to measure these fluctuations. By subtracting the effects of known factors like topography, researchers can use these anomalies to find oil and mineral deposits. These anomalies can even be large enough to cause bulges in sea level.

Understanding gravity also requires looking deep beneath the surface. According to the Shell theorem, proven by Isaac Newton, the gravitational force at a certain radius depends only on the mass inside that sphere. All the mass outside that radius cancels out. This means that if you were inside the Earth, the gravity you feel would depend on the density and mass of the layers beneath you. The Earth's density is not uniform; it changes as you move from the crust through the mantle to the core. Models like the Preliminary Reference Earth Model (PREM) help scientists map how this density distribution affects the overall gravitational field.