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  • More than six thousand light years from the surface of the earth,

  • a rapidly spinning neutron star

  • called the Black Widow pulsar, blasts its companion brown dwarf star with radiation

  • as the two orbit each other every 9 hours.

  • Standing on our own planet,

  • you might think you're just an observer of this violent ballet.

  • But in fact, both stars are pulling you towards them.

  • And you're pulling back,

  • connected across trillions of kilometers

  • by gravity.

  • Gravity is the attractive force between two objects with mass

  • any two objects with mass.

  • Which means that every object in the universe attracts every other object:

  • every star, black hole,

  • human being, smartphone, and atom

  • are all constantly pulling on each other.

  • So why don't we feel pulled in billions of different directions?

  • Two reasons: mass and distance.

  • The original equation describing the gravitational force between two objects

  • was written by Isaac Newton in 1687.

  • Scientists' understanding of gravity has evolved since then,

  • but Newton's Law of Universal Gravitation

  • is still a good approximation in most situations.

  • It goes like this:

  • the gravitational force between two objects

  • is equal to the mass of one

  • times the mass of the other,

  • multiplied by a very small number

  • called the gravitational constant,

  • and divided by the distance between them, squared.

  • If you doubled the mass of one of the objects,

  • the force between them would double, too.

  • If the distance between them doubled,

  • the force would be one-fourth as strong.

  • The gravitational force between you and the Earth pulls you towards its center,

  • a force you experience as your weight.

  • Let's say this force is about 800 Newtons

  • when you're standing at sea level.

  • If you traveled to the Dead Sea,

  • the force would increase by a tiny fraction of a percent.

  • And if you climbed to the top of Mount Everest, the force would decrease

  • but again, by a minuscule amount.

  • Traveling higher would make a bigger dent in gravity's influence,

  • but you won't escape it.

  • Gravity is generated by variations in the curvature of spacetime

  • the three dimensions of space plus time

  • which bend around any object that has mass.

  • Gravity from Earth reaches the International Space Station,

  • 400 kilometers above the earth,

  • with almost its original intensity.

  • If the space station was stationary on top of a giant column,

  • you'd still experience ninety percent

  • of the gravitational force there that you do on the ground.

  • Astronauts just experience weightlessness

  • because the space station is constantly falling towards earth.

  • Fortunately, it's orbiting the planet fast enough that it never hits the ground.

  • By the time you made it to the surface of the moon,

  • around 400,000 kilometers away,

  • Earth's gravitational pull would be

  • less than 0.03 percent of what you feel on earth.

  • The only gravity you'd be aware of would be the moon's,

  • which is about one sixth as strong as the earth's.

  • Travel farther still

  • and Earth's gravitational pull on you will continue to decrease,

  • but never drop to zero.

  • Even safely tethered to the Earth,

  • we're subject to the faint tug of distant celestial bodies and nearby earthly ones.

  • The Sun exerts a force of about half a Newton on you.

  • If you're a few meters away from a smartphone, you'll experience

  • a mutual force of a few piconewtons.

  • That's about the same as the gravitational pull

  • between you and the Andromeda Galaxy,

  • which is 2.5 million light years away

  • but about a trillion times as massive as the sun.

  • But when it comes to escaping gravity,

  • there's a loophole.

  • If all the mass around us is pulling on us all the time,

  • how would Earth's gravity change

  • if you tunneled deep below the surface,

  • assuming you could do so without being cooked or crushed?

  • If you hollowed out the center of a perfectly spherical Earth

  • which it isn't, but let's just say it were

  • you'd experience an identical pull from all sides.

  • And you'd be suspended, weightless,

  • only encountering the tiny pulls from other celestial bodies.

  • So you could escape the Earth's gravity in such a thought experiment

  • but only by heading straight into it.

More than six thousand light years from the surface of the earth,

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B1 US TED-Ed gravity gravitational earth gravitational force gravitational pull

How far would you have to go to escape gravity? - Rene Laufer

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    11101130 posted on 2019/11/26
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