# Acceleration Due to Gravity in Physics

Gravity is a force we experience every moment of every day. At its heart, gravity is an unseen pull that objects with mass have for one another. On the surface of the Earth, the acceleration due to gravity has a value of around 9.81 m/s².

In this article, we will look at the concept of acceleration due to gravity in detail. We’ll explore what it is, the physics behind it, and how gravitational acceleration differs on other planets.

##### PHYSICS

Relevant for

Learning about acceleration due to gravity.

See explanation

##### PHYSICS

Relevant for

Learning about acceleration due to gravity.

See explanation

## Basic Concepts

Before we explore the details of acceleration due to gravity, let’s first look at the key concepts of acceleration, free fall, and the influence of gravity.

### Acceleration

In physics, acceleration refers to the rate of change of velocity of an object with respect to time. It is a vector quantity, meaning it has both a magnitude (speed) and a direction. An object is said to be accelerating if it is changing its velocity – this could mean that the object is speeding up, slowing down, or simply changing direction.

### Free Fall

A free-falling object is one that is falling under the sole influence of gravity. This means that the only force acting upon the object is gravity. In the absence of air resistance, all free-falling objects near the Earth’s surface accelerate at the same rate, regardless of their mass.

Something important to note here is that the object does not have to be falling directly downwards to be in free fall. An object thrown upwards or sideways is also in free-fall after it leaves the thrower’s hand, as the only force acting upon it is gravity.

### Force of Gravity

Gravity is the force that attracts two bodies towards each other. On Earth, we commonly experience it as the force that keeps us grounded. It’s why when we jump, we don’t continue drifting upwards into the sky, but instead come back down to the ground. The force of gravity is what gives weight to physical objects.

Gravity is described by Newton’s law of universal gravitation, which states that every particle of matter 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 centers.

## Acceleration Due to Gravity on Earth

To understand the acceleration due to gravity, let’s consider the following diagram, which shows an apple being dropped and falling to the ground:

The diagram is based on a multiflash photograph that shows the apple at equal intervals of time.

We can see that the spaces between the images of the apple increase steadily. This means that the apple’s velocity increases as it falls, that is, it’s accelerating.

Acceleration due to gravity is a form of acceleration experienced by objects in free fall under the influence of gravity. Given that gravity is acting as the sole force, all objects (regardless of their mass), fall to the ground at the same rate in a vacuum.

### What is ‘g’ on Earth?

The acceleration due to gravity is symbolized by the lowercase letter ‘g’. On Earth, it has an approximate value of 9.81 m/s². This is known as the acceleration of free fall.

Acceleration of free fall
$latex g=9.81~\frac{\text{m}}{\text{s}^2}$

In the absence of other forces (like air resistance), an object in free fall close to the Earth’s surface will increase its speed by 9.81 meters per second every second.

For example, if an object is dropped with an initial velocity of 0, after one second, its velocity will be 9.81 m/s; after two seconds, 19.62 m/s; and so forth.

### Variations in ‘g’

While we often use the average value for simplicity, it’s important to note that the exact value of ‘g’ can vary slightly in different parts of the world. This variation is due to factors such as altitude, latitude, and local geological formations.

At higher altitudes, farther from the Earth’s core, ‘g’ is slightly smaller. Additionally, because Earth isn’t a perfect sphere but rather an oblate spheroid (wider at the equator than at the poles), ‘g’ is also slightly smaller at the equator compared to the poles.

Despite these variations, for most everyday calculations and physics problems, the average value of 9.81 m/s² is used for the acceleration due to gravity. However, when precision is necessary, such as in space travel, these small differences become important.

## Acceleration Due to Gravity Beyond Earth

The concept of acceleration due to gravity is not limited to Earth alone. Each celestial body in the universe, from the smallest asteroid to the largest star, exerts its own gravitational pull.

The value of ‘g’ varies significantly across different celestial bodies due to differences in their mass and radius. We can look at a few examples in the following table:

For example, we see that the acceleration due to gravity is approximately 1.62 m/s² on the Moon—about one-sixth of its value on Earth—due to the Moon’s smaller size and mass. On Jupiter, the largest planet in our solar system, ‘g’ is a hefty 24.79 m/s² due to its massive size.

### Gravity in space travel

The differences in gravitational acceleration beyond Earth have important implications for the prospects of space travel and colonization. For instance, the lower gravity on the Moon and Mars affects everything from the physical health of astronauts to the construction of potential bases or habitats.

In addition, the concept of gravitational acceleration is crucial in planning space missions, particularly for gravity-assist maneuvers, or “slingshot” effects. This technique, used by many interplanetary spacecraft, involves using the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, saving propellant, and time.

## The Physics Behind the Phenomenon

Now that we have a basic understanding of acceleration due to gravity, let’s take a look at how Einstein’s General Theory of Relativity revolutionized our understanding of gravity.

### Einstein’s Theory of General Relativity

Prior to the 20th century, gravity was largely understood through Newton’s law of universal gravitation, which states that every pair of objects in the universe attracts each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

However, when Albert Einstein published his general theory of relativity in 1915, he offered a new way of looking at gravity not as a force, but as a consequence of the curvature of spacetime caused by mass and energy.

Central to the theory of general relativity is the equivalence principle, which states that the effect of gravity on an object is indistinguishable from the effect of acceleration. For example, if you’re in an elevator that’s accelerating upwards, you feel heavier, just as you would feel if you were in a stronger gravitational field.

### The Curvature of Spacetime

In general relativity, Einstein proposed that gravity is the result of the warping or curving of spacetime by mass and energy. Essentially, the presence of a massive object like Earth deforms the spacetime around it, creating a sort of “dip” that smaller objects move along. This motion along the curved path is what we perceive as gravitational attraction or acceleration due to gravity.

Einstein’s equations of general relativity accurately predict the behavior of gravity in a wide range of scenarios, from the motion of planets in our solar system to the bending of light from distant stars. This view of gravity has stood the test of time and numerous experimental verifications, cementing it as our current best description of gravity.

## Implications and Applications

The concept of acceleration due to gravity is not just a theoretical concept confined to the realm of physics textbooks. It has significant practical implications and applications in our daily lives, and it is relevant to various scientific and engineering disciplines.

Daily Life

Acceleration due to gravity shapes the world around us as it gives weight to physical objects, governs the motion of falling objects, and is the reason why we stay grounded on Earth.

The functioning of our vehicles, machines, and even our own bodies is impacted by gravity. For example, human locomotion and balance are inherently tied to our body’s response to gravity’s pull.

Engineering

Engineering applications, from the design of buildings and bridges to the flight of aircraft and rockets, are heavily influenced by the acceleration due to gravity. Civil engineers must account for the force of gravity when designing structures, and mechanical engineers must consider it when developing machines and vehicles.

For aerospace engineers, understanding and working with ‘g’ is critical, as the force of gravity must be overcome to launch satellites and spacecraft.

Geology and Geophysics

In geology and geophysics, variations in ‘g’ can provide information about the distribution of mass in the Earth’s interior. These variations are also useful in studying the Earth’s crust and mantle, the movement of tectonic plates, and even in predicting volcanic eruptions and earthquakes.

Astronomy and Cosmology

In astronomy, the concept of acceleration due to gravity plays a key role in understanding the motion of planets, stars, and galaxies. It helps in calculating the orbits of planets around the Sun, the lifecycle of stars, the dynamics of galaxies, and the expansion of the universe.

It is also crucial in the study of black holes, neutron stars, and other astrophysical phenomena.