There are cases when what goes up doesn’t always come down. When an object reaches escape velocity, it defies gravity and doesn’t fall back to Earth.
The atmosphere in the playground was tense. The players from the two teams stood facing each other, holding hands to form unbreakable chains. A name was called, and a player from Team A stepped forward. Team B prepared for the attack. If this player could break through their linked arms, Team A would win.
Frowning in concentration and filled with determination, the player sprinted forward…
Reaching escape velocity on the playground was just as challenging as it is in space travel!
a quick game to understand escape velocity (Photo Credit : ANTSTUDIO/Shutterstock)
Our understanding of gravity is usually limited to its connection with the ground beneath us. Gravity pulls us towards the Earth (which we feel as our weight) and also pulls the Earth towards us. However, because the Earth is so massive, its movement towards us is imperceptible. Even though we pull the Earth slightly towards us, it goes unnoticed due to its much greater mass. The larger the mass of an object, the more difficult it is to move it – a concept explained by Newton’s first law of motion.
A math book thrown into the air from Earth will fall back down because it lacks the energy to break free from Earth’s gravitational pull. If you throw the book higher, it will stay in the air for a longer time before eventually returning to Earth. So, what goes up must come down, right? But if that were always true, how do satellites and rockets stay in orbit without falling back to Earth?
Escaping Gravity
Let’s imagine Newton sitting under a tree, peacefully contemplating the mysteries of the universe, when an apple suddenly hits him on the head. In a moment of brilliance, Newton throws the apple upwards, slightly annoyed after being hit by an unwanted fruit. However, he has an epiphany! If it weren’t for gravity, which causes the apple to fall, it would continue to travel indefinitely, slowed down only by atmospheric friction.
Newton wondered if gravity, which made the apple fall to the ground, could extend its reach beyond Earth and affect the moon and other planets. He proposed that both objects falling to the ground and the movement of planets around the sun could be explained by the same principle.
Newton’s discovery led to the creation of the universal law of gravitation. Newton realized that if an object were launched from the Earth’s surface with enough velocity, it could escape Earth and travel to infinity without falling back. This minimum velocity, known as escape velocity, is the precise amount of energy needed to break free from the gravitational pull of another body. The escape velocity from Earth, also referred to as the ‘second cosmic velocity’, is approximately 11.2 km/s. Russian scientists coined the term ‘Cosmic velocities’ to describe the important velocities associated with space travel. The ‘first cosmic velocity’, called orbital velocity, causes a projectile to orbit around a celestial body. A slower projectile will fall back to Earth. The International Space Station (ISS), for example, orbits the Earth at a speed of around 7.9 km/s, allowing it to remain in perpetual free fall without falling. This is similar to the spinning action of a yo-yo, where the string keeps the yo-yo from traveling off in a straight line. In his theory of general relativity, Einstein proposed that gravity is not a force between masses, but rather a distortion in the fabric of space-time caused by the presence of mass. This distortion bends the straight paths of objects, causing them to fall towards each other. The strength of a gravitational field is determined by the escape velocity associated with it. Strong fields have escape velocities approaching the speed of light, while weak fields have lower escape velocities. For low speeds and weak gravitational fields, the predictions of Newton’s theory and the general theory of relativity are approximately the same.
Is it Possible to Escape from a Black Hole?
The ability to escape from the gravitational pull of an object depends on two factors: the mass of the object and the distance to its center of mass. In order to break free from Earth’s gravity, a rocket must reach a velocity of 11.2 km/s. However, if we consider a planet with the same mass as Earth but half its diameter, the escape velocity would be different. This is because the distance from the planet’s surface to its center of mass is reduced, resulting in an increased escape velocity.
There is a specific radius called the Schwarzschild radius, at which the escape velocity of a gravitational mass is equal to the speed of light. In the 18th century, John Michell proposed that light would not be able to escape from the surface of a massive star when it reaches this critical radius, based on Newton’s law of gravitation.
Although Michell’s calculations were not accurate, Karl Schwarzschild later demonstrated that when a body collapses beyond the Schwarzschild radius, it becomes a black hole. The Schwarzschild radius represents the point at which gravity becomes the dominant force. Therefore, in order to escape the intense gravitational pull of a black hole, an escape velocity greater than the speed of light is required.
Even light cannot escape from a black hole (Photo Credit: Nasky/Shutterstock)
We All Require Assistance Sometimes
One may wonder, does the concept of escape velocity also apply to birds and airplanes? What about helium balloons?
Here’s the answer: escape velocity is only necessary when an object is attempting to break free from the gravitational pull of another body. To put it simply, an airplane does not need to reach escape velocity in order to fly, as it is not trying to escape from Earth. Nor does it need to be in orbit around Earth.
However, this means that in order to stay airborne, it must constantly overcome the downward force of gravity, which it accomplishes by generating lift with its wings and engines.
Birds also sustain flight using the same principle, with their wings creating areas of low and high pressure above and below the wings, enabling lift to be achieved. The crucial element here is the presence of air, without which it would not be possible to create changes in air pressure and, consequently, lift.
A seagull in flight (Photo Credit : twenty20)
Helium balloons are filled with helium, which is lighter than air. When a heavy gas (more dense) interacts with a light gas (less dense), the lighter gas floats on top. A helium balloon ascends in the air until it reaches an altitude where the density of the air inside the balloon equals the density of the air outside it. This creates the necessary lift.
In theory, at least, an object can achieve liftoff from a planet’s gravitational field at velocities other than the escape velocity. The escape velocity is only defined for non-propelled objects, meaning an object traveling at escape velocity requires no additional force to escape.
To leave Earth at a velocity lower than the escape velocity, one would need to continuously provide a boost to overcome gravity for the entire duration of the flight. For a spacecraft, this would require external assistance in the form of multiple booster rockets to maintain a continuous thrust and counteract Earth’s gravity – a thrust that is generated by burning an unimaginably large amount of fuel. Not only would this be extremely wasteful, but attempting to escape Earth or reach orbit in this manner, while carrying such massive amounts of fuel, would be highly impractical.
Gravity remains universally influential, even as its pull weakens with increasing distance. Ascending higher only diminishes its sphere of influence, but one can never truly escape it. However, with a bit of luck and the right velocity, it may be possible to evade its enticing allure!