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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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