Key Takeaways
- Everything in the universe possesses gravity, including you.
- We use gravitational energy in many ways, from roller coasters to space exploration.
- Isaac Newton described gravity in 1666 after an apple fell on his head.
The most simple definition of gravitational energy is an object’s stored energy based on its height above the Earth.
The further away from the Earth’s gravity something is, the more gravitational energy it has. Likewise, the closer an object is to the surface of the Earth, the less gravitational energy it possesses.
It sounds simple. In essence, gravitational energy is straightforward. But, like many physics topics, its beauty is in the detail. So hop aboard this literal, not metaphorical, roller coaster and whizz through the ups and downs, highs and lows, of gravitational energy.
Gravitational Energy: Harnessing the Earth’s Forces
Everything in the universe possesses gravity. You, the screen you are reading, planets, and even the fluffiest muffins in the world have gravitational energy. And gravity starts with Newton.
Most people associate Isaac Newton with his discovery of gravity when an apple fell on his head in England in 1666. From that fruit came Newton’s law of gravitation. His theorem explained that every particle of matter in the universe attracts any other object. The amount of this force, or gravity, relies on the mass of the objects and the distance between them.
The Earth is the largest object close to us all, right under our feet. All earthly objects are attracted to its force of attraction, which we call its gravitational field or pull. The Earth’s pull is more significant than any other force attraction.
Wherever we or other objects are situated on the surface of the Earth defines their gravitational energy. A basketball resting on the floor has little gravitational energy. Try to shoot a hoop, and that falling basketball might hurt you a bit if it were to land on your head. Drop that basketball on someone from the top of a high-rise building, and you could do severe damage.
By changing the basketball’s positioning in relation to the Earth, we alter its gravitational energy. The distance between our objects alters the force of gravity between Earth and the basketball. This positioning is called the gravitational potential energy of an object.
How Does Gravitational Potential Energy Relate to an Object’s Position in a Gravitational Field?
First, we need to take a quick step back and understand the different types of energy. There are two primary energy forms:
Potential energy: This is stored energy, the potential for an object to do what physicists call “work.”
Kinetic energy: This describes the energy of movement an object has while it moves around, performing work.
These two principal energy definitions have different energy subsections, like heat energy or sound energy.
First, let’s get back to our basketball. Dormant on the floor, it has a form of potential energy. Let’s lift it, ready to pass it to a friend. We have given the basketball some gravitational potential energy. Drop it, and the basketball’s gravitational potential energy converts into kinetic energy. Once the ball reaches a standstill on the floor, it reverts to having (very little) potential energy.
Newton also explained two critical rules related to gravity:
- The larger an object’s mass, the greater gravity it has.
- The gravitational pull between objects is greatest when they are closer.
An elephant has more gravitational potential energy than our basketball because it has more mass. And for proximity, we can sense the Earth’s gravitational pull when we jump to shoot a hoop. We return to the Earth’s surface and don’t float into space.
There are other types of potential energy. One is elastic potential energy. We can create this, for example, by pulling back the string of a bow or stretching a coil. Finally, there is chemical potential energy relating to energy storage in the chemical bonds of substances. This includes food, which our bodies convert into mechanical energy and movement, or burning gasoline in a combustion engine.
What Is the Connection Between Mass, Height, and Gravitational Energy?
We can calculate an object’s gravitational potential energy thanks to Newton’s Laws and his apple. We measure gravitational energy in joules.
How much is a joule? Well, an apple weighs around one Newton, around 3.6 ounces, or 102 grams. Lift that apple one meter, and you’ve performed one joule of energy, or work, on it. Because of its new, higher position, its gravitational potential energy has increased, too. Release the apple, and the force of gravity will perform one joule of work as it drops that one meter.
Here’s what we need to calculate gravitational potential energy:
- E is energy (joules)
- Mass (m) is measured in kilograms
- G (g) is gravitational acceleration, which is always 9.81 m/s2 on Earth, making it a gravitational constant
- Height (h) is measured in meters and represents the object’s height when dropped
That gives us this equation:
Gravitational Potential Energy = mgh
Let’s compare our basketball and elephant and lift both to 328 feet (100 meters) above the surface of the Earth. A Spalding basketball weighs around 1.37 lbs or 625 grams, and our elephant weighs around 3.3 U.S. tons or about 3,000 kilograms. Which takes more work to lift, giving it more potential energy?
Our basketball has 613 joules of energy at 328 feet, while our elephant has almost 3,000,000 joules. Unsurprisingly, our heavy elephant has much more gravitational potential energy than the lighter basketball at a higher distance from Earth. The amount of work to get the elephant in place was more.
Helpful tip: This online calculator is a good reference point if you want to find something’s gravitational potential energy.
What Role Does Gravitational Energy Play in Celestial Bodies Like Planets and Stars?
We’ve seen that all particles in the universe, including all the moons, planets, stars, and galaxies, contain gravity. Gravity is one of the four fundamental forces in the universe, alongside electromagnetic, strong nuclear, and weak nuclear forces.
The center of our solar system is the sun, with its eight orbiting planets (sorry, Pluto!). These vast bodies have incredible gravitational forces, with the sun having the greatest of them all. However, the planets aren’t pulled into the sun. Their own gravitational fields and spinning help them resist the sun’s draw. This tug-of-war created the planets’ circular orbits around the sun.
This idea repeats at larger scales. For example, our sun orbits the center of the Milky Way.
Is Gravitational Energy Vital to Space Exploration?
Gravitational energy has helped humans explore our solar system. Satellites and rockets can only travel so far into space before running out of fuel. The sun’s massive gravitational pull would eventually stop them and draw them into its fiery center. Fortunately, they can hitch a ride to travel further thanks to the gravitational energy of nearby planets.
Voyager I is the most distant spacecraft from Earth, more than 15 billion miles away. It has relied on gravitational energy since its 1977 launch. How? Voyager I initially used engines to travel through space, which is empty. They eventually expired, but Voyager I kept moving through the void.
Eventually, it encountered planets like Saturn with gravitational energy, which started to draw the spacecraft into its orbit, called positive work. The closer Voyager I got to Saturn, the greater the pull. Voyager I sped up, falling towards the giant planet. However, because of the craft’s speed and angle of approach, a tug-of-war ensued between Voyager’s momentum and the planet’s gravitational energy.
A combination of Saturn’s movement in space, its gravitational field, and Voyager’s momentum allowed the craft to slingshot itself out of the planet’s pull with extra speed and a new direction.
The International Space Station (ISS), 125 miles above the surface, doesn’t crash to Earth despite being drawn to the ground by our planet’s gravitational field. However, precisely because of its orbit, kinetic energy (aka the energy of movement) counteracts the gravitational pull, allowing the ISS to maintain its position.
Theoretically, the Earth’s gravitational field doesn’t stop; it just weakens and dissipates the further away from the planet you travel. The escape velocity from Earth’s atmosphere is around 25,000 mph (40,000 km/h). Of course, in practice, other planets’ gravitational pulls will be more significant once you reach a certain distance from Earth, and their gravitational fields will pull you in.
Can You Explain the Concept of Gravitational Potential Energy Conversion in Practical Scenarios?
The law of conservation of energy states that energy may only be converted from one type to another and never lost.
So those 3,000,000 joules of gravitational potential energy in our elevated elephant can’t be lost; they will only be converted into other energies when the elephant falls, the conservation of energy within a system.
As the elephant drops, our potential energy becomes kinetic energy. When it hits the ground, it may also produce heat and sound energy as it splats. This is energy conversion. Only an external force can add energy to a system like this.
Of course, dropping elephants from a great height is cruel and serves no purpose. However, we do use gravitational potential energy conversion to good use in our daily lives.
Thrillseekers love roller coasters. These fun rides use the coaster’s gravitational potential energy at its highest point to propel it around the tracks. Conversion creates movement (kinetic energy), heat (thermal energy), and mightily clattering noises (sound energy) as the coaster zooms around.
Hydropower is a beautiful example of gravitational potential energy conversion. The water behind the dam sits as a store of potential energy. Releasing it downstream creates kinetic energy and movement, which drive turbines (mechanical energy) that produce electricity (electrical energy).
Are There Any Alternative Technologies Leveraging Gravitational Energy?
Pumped hydro storage technology allows us to capture excess renewable energy and store it for use later in a simple manner. There are two reservoirs, one higher than the other, with electricity-generating turbines between them.
Unused electricity can pump water from the lower reservoir to the higher one. This creates a store of potential energy. If the power grid requires more energy, the upper reservoir releases its water to the lower one, turning the turbines en route to create electricity. The water will be pumped back up to the upper reservoir once there is another electricity surplus.
What about gravitational energy batteries? The principle is similar to pumped hydro storage but uses weights instead of water. Electricity lifts a heavy weight or rocks in a tower or to the top of a deep shaft. When electricity is required, the weight drops slowly towards the ground via winches, turning turbines to produce electricity.
Both pumped hydro storage and gravitational batteries are very efficient. These systems seem to negate the need to build lithium-ion utility-scale batteries to store excess renewable energy. However, drawbacks include flooding areas for reservoirs and cost and space requirements for both.
Gravitational energy is not a panacea for all our energy needs. As Newton demonstrated, it requires energy to move objects away from the surface of the Earth and give them gravitational potential energy. Gravity is not energy itself; it is a force that shows us how objects interact. However, it does offer a way to store energy for later use as gravitational potential energy.
The Force of Gravitational Energy Can Be Harnessed for Good
Gravity is a fundamental force of nature. We can harness this force as gravitational energy or gravitational potential energy. We do this by moving objects with mass to different areas on Earth. These objects then interact with each other by falling or moving or in other ways to create kinetic or moving energy.
This conversion of energy allows us to create electricity, heat, sound, movement, and much more. We can store this energy for later use in reservoirs or gravity batteries, such as when there is excess renewable energy that would otherwise go to waste.
Gravitational energy has also allowed our brightest minds to send space stations, rockets, and satellites into orbit. Thanks to the speed boost given by the planets they pass, many are exploring the farthest reaches of our solar system. The force that keeps our feet on the ground may eventually propel us to an intergalactic future. Gravitational energy, this fundamental force of the universe, is truly helping us understand its fundamentals.
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