Why Satellites don't fall down? Well, technically they fall down soon or later... let me explain...

Newton confirms that they must to fall back to Earth as the apple did on his head.

**Newton's Law of Universal Gravitation **

where:** F **is the force between the masses;

Then... If every mass attracts every other mass with a force along the line intersecting both masses, why Satellites still stay up? Easy: it's because they are flying fast.

Then... If they are flying fast... Why they are not going far away and instead they are turning around the Earth?

Luckily there is another Force.

Let me try to explain what is the centripetal force. When you run normally you are moving straight foreward.

Now imagine that you run with a hand caught to a lamppost. What happens? you are running as before but a Force (your arm) does not let you get away from the pole. Your arm is the centripetal force. A Satellite is always falling to the Earth. But velocity tries to push it away. The sum of forces is a circular motion.

** F** is the Centripetal force

Surely you see that: Centripetal Force and Gravity are the same force... Yes

* Fc *is Centripetal Force,

That means:

At the moment we can see that Satellite mass is not critical. We continue..

Great! we have a formula to know the velocity of our Satellite. We can see the most important things are the mass of the planet to orbit and the distance between the centre of the planet and the satellite.

Now we are going to concentrate on Earth numbers, thanks again to Wikipedia. Remember to use the standard units (meters and kilograms, not Km or Tons, etc...)

Now we are going to use the ISS as an example. The ISS orbits normally arround 400 km high. That means R = RT + R iss = 6370 Km + 400 Km = 6770 Km

Knowing that Newtons(N) are mass(Kg) x Gravity(m/s2) we can finish the calculations:

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