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I was searching about 'why do not moon crash to earth due to its gravity?'Then by reading physics stake exchange existing questions I came to know that its the sideways motion of the moon that keeps moon in its orbit.

So my question is:- Moon is in circular or ellipthitical orbit and during its orbit moon is always pulled towards the centre of the earth but moon is changing its direction at every point because it is in circular or ellipthitical orbit, so the direction of force pulling moon towards the centre of the earth will also change at every point on the other hand the sideways motion of moon is always in a particular direction then there must be a point in the moon's orbit when sideways motion and gravity will act in same direction and moon should crash to earth.Then why it does not crash?

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    $\begingroup$ Imagine you had a giant mountain on earth, say 200 kilometres high, and you launched a rocket directly sideways from the top, so it went right around the earth . Then before it came back to the mountain, you chopped off enough of the mountain so the rocket could avoid the mountain top, it would circle the earth forever. Does that imaginary idea make any sense to you as to what is happening the moon, it is falling towards the earth but also falling around the earth. $\endgroup$
    – user81619
    Commented Aug 12, 2015 at 22:19
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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/9049/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Commented Aug 12, 2015 at 22:32
  • $\begingroup$ 1.Does Acid Jazz means to say that the sideways motion of moon or rocket will change at every point due to gravity? 2.Then every object having sideways motion,passing through the earth will start revolving around earth and will keep revolving forever.Isn't it? If yes then there must be a lot of stones,rocks,meteors etc.etc. revolving around earth? 3.What if the rocket is launched straight forward away from the earth?Please answer. $\endgroup$
    – Ujjval Narang
    Commented Aug 12, 2015 at 22:56
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    $\begingroup$ Possible duplicate of Why doesn't the Moon fall onto the Earth? $\endgroup$
    – luchonacho
    Commented Sep 27, 2018 at 7:35

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If someone (like superman) could stop the moon from its orbital motion then yes it would fall towards the Earth. Only then the direction of motion would be parallel to gravity.

Same with the I.S.S or the satellites orbiting Earth. They could also spiral in and crash because the atmosphere is taking away their kinetic energy.

Just like the comment says imagine that you throw a stone from a high point horizontally (see picture). The more initial (horizontal) speed you give to the stone the further it will land. But the earth isn't flat ! If you give it too much speed , it will run out of land to crash! It will keep flying under the perpendicular force of gravity. Just like a bull running around in circles while the cowboy pulls him with a rope. So it will fly a full circle and hit you at the back!

In fact you can calculate that speed because then Gravity serves as the centripetal force: $G\frac{M_{earth}M_{moon}}{R^2}=M_{moon}\frac{v^2}{R}$, If you know the mass of the Earth and the distance between them. Although the mass of the moon is large and this serves only as an approximation (see two body problem).

enter image description here

Check also the xkcd blog for some ideas about orbits.

https://what-if.xkcd.com/58/

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  • $\begingroup$ Isn't your second paragraph a contradiction of newton's theory "When we apply a force on an object in the universe to bring it in motion,it will always remain in motion unless an equal and opposite force is applied" ##Lefteris $\endgroup$
    – Ujjval Narang
    Commented Aug 12, 2015 at 23:19
  • $\begingroup$ Which part? The only force acting is gravity which is towards the centre and thus perpendicular the motion. So the tangential speed is unaffected and a satellite can orbit endlessly. If there were a tangential force (for example atmospheric friction) , the tangential speed would gradually decrease in accordance to Newton’s law and spiral in. Remember velocity is a vector analysed into a radial and a tangential component $\endgroup$
    – Lefteris
    Commented Aug 12, 2015 at 23:27

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