Why Do planets Move ? Which force keep them moving in thier orbits?

Why Do planets Move ? Which force keep them moving in thier orbits? google says..... because of the gravitational pull of the sun....... but isn't it the force which only keeps the planets in their  orbits......... ? is it also the same gravitational pull of the sun which keeps them moving ? if so , then how ? 

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-max-1 year ago

I think you have a lot of physics to learn. I'll try to give a detailed answer. Starting with the simple stuff, things in motion will stay in motion unless acted upon by a net force. If you push a bowling ball, it will just keep moving forward and slow down as the friction forces and air resistance pushes against the motion, stopping it.

Only the acceleration of an object and forces acting upon it are related and proportional. This is Force = Mass*Acceleration. So as long is there is no force on an object, it will keep moving in a straight line or stay still.

Acceleration is the rate at which velocity changes over time. In calculus terms, this is dV/dT. Just think of it as change in velocity over change in time (from the start time to the end time.) Velocity, in a similar way, the the rate at which the position of an object changes over time. Or dP/dT. Just think of it as change in position over change in time (from the start time to the end time.) Understanding a bit of calculus can go a long ways here in understanding these concepts fully. (They are actually really simple concepts once you get them.)

With gravity, things can get complicated with the concept of warped spacetime, but to our observations, everything that has mass is attracted to each other. There is no such thing place with no gravity. That would imply that you are infinitely far away from all matter, and that you have no mass. The force of gravity is directly proportional to the mass of both objects, and inversely to the square of the distance between them. That means that if you double the distance between 2 objects, the force of gravity is 4 times weaker!!!

So when it comes to orbits, it is all about forces and vectors. Vectors are just imaginary arrows, that are super useful because they can be used to describe ANYTHING that has magnitude and direction. The length of the imaginary arrows, err vectors represents magnitude. How strong the force it, how speedy something is, how fast is it accelerating, etc. Vectors can be used to describe forces, velocities, accelerations, electric fields, all that good stuff. The vector for gravity between 2 objects will be always be inward facing arrows between the 2 objects. The length of the arrows between, say, earth and sun will depend on the product of their masses and distance. As you get closer to the sun, the pull of gravity grows strong pretty fast.

So, imagine you try to launch a cannon ball around the moon. If you do not give it enough velocity horizontal velocity (----->) the force of gravity cause the cannonball to always accelerate towards the center of the moon. However, as it falls, because it had all that initial velocity, the pull of gravity causes it to wrap around the moon and it just endlessly falls around and around and around. To really understand why and how these force, acceleration, and velocity vectors all interacts over the entire timespan, you really need to take university level physics and a semester or two of calcus.

_Boltz_ (author) 1 year ago

ummmmmm................. a new question arises ......... let there be two magnetic balls......... one to be fixed at a place another to be thrown towards it........ let the situation be that the ball passes about 5 cm away from the first ball..... the second ball gets deflected due to the magnetic attraction........ but why it doesn't get into an imaginary orbit and without colliding with the first ball .... it moves in circular orbit and then stops after some time due to friction.... and then get's attracted to the first ball......... why it just get deflected and carry onn with thier straight path ? instead of orbiting the first ball ???? are you able to visualise ? then why this happens ? and dude....... i googled it but didn't got the expected answer..... and bout the textbook...... the author just can't predict each and every questions arising in the generation's mind...... and maybe the situation is that i don't want to search it in a book of 870 pages with more derivations than the theory .

Setup right, it WILL orbit the ball, if you launch the second ball at just the right speed into the field around the first.

All of orbital dynamics arises from the fact that gravity is a conservative field, that is energy isn't lost as you interact with the field.

Your problem is you are still fixed in your thinking about the world you move it, and can't conceive of a system where energy is essentially perfectly conserved.

Don't expect there to be a pat answer that can be expressed in a couple of lines.

If the situation is that you don't want to read a book, you ain't cut out for science stuff. Try liberal arts.

_Boltz_ (author)  steveastrouk1 year ago

so, a lot of emotions..... running inside me.... but....... #Steve....... you made my day, now I have quite a good reason to read a book........... thanks brother......... :)

Newton's own method of visualising orbital motion was to imagine a gun firing a projectile. The fast it leaves the gun, the further from the gun it lands. At some point, the curvature of the earth falls off before the projectile lands, and the projectile is in free-fall around the earth

_Boltz_ (author)  steveastrouk1 year ago

you mean to say , like angry birds space, we just need the right orientation and speed..... to get anything into an orbit........

Essentially, yes. You have to get to going sideways at around 18,000 MPH/ 30,000 KMH to get into earth orbit. That's the difference between orbital, and sub-orbital flight. You can get on a fairly fast thing and get to 100km above the earth and be "in space", but you will fall down again.


iceng1 year ago

Regarding your magnetic balls scenario, which I tried

While an unknown NIB material, maybe the passing velocity and
magnetic orientation of the balls is very important to the ballistic
path... At 5 cm the inverse square attraction of the magnets will inexorably pull them together !

-max-1 year ago

A good physics game demonstrating all that nonsense below works out, might be Angry birds Space. You can launch the birds around planets, and see if you can get one stuck in orbit!!! The physics are not totally realistic, but it is probably close enough to give you an intuitive understanding. In short, Things that orbit are just freefalling things that keep missing.

Isaac newton (basicly the creator of calculus and physics) talked about the example of a cannon ball, and shooting it fast enough to make it circle the world. https://en.wikipedia.org/wiki/Newton%27s_cannonball

-max- -max-1 year ago

One last good example of a (2D) orbit might be those funnels that you drop coins in. The coins are attracted to the center because of the funnel shape, and *ahem* gravity, and because they have that initial velocity in a direction tangential to the force towards the center.

iceng1 year ago

Then YOU 'an ugly bag of water' quoting a star trek episode, who inhabit the surface of this planet would also resist motion.....

Think about it,....., what force is acting on the atmosphere in the vacuum of space..

GRAVITY and our moon is the worst offender it pulls the atmosphere and oceans making our tides...

No. The atmosphere is carried on the planet's gravity.

Yonatan241 year ago

Google it

Vyger1 year ago

I always thought it was the result of Superman playing planet pool in his spare time.

You need to read your textbook, your reading assignment. The answers will be there. Put down the phone and pick up a for real book.

It was in motion when the solar system was formed.

It fell into an orbit around the sun as the planet formed gathering debris that formed the solar system.

Gravity keeps it in its orbit the attraction of two objects.

What keeps it moving is inertia the resistance of any physical object to any change in its state of motion. Or simply put what is in motion wants to remain in motion and what is at rest wants to remain at rest.

The planet wants to go straight from Inertia but the gravitational pull of the sun keeps it in its orbit.

The two forces working together formed the planet in motion in its orbit.

Now if you ask why the earth just doesn't slow down and fall into the sun?

It is just not today or any day in the near future.

It turns out you don't actually need a force for an object to keep moving.

The motion of an object without any external forces acting on it, is a trajectory with constant speed, in a straight line.

The word "inertia" is a good word for this concept, the idea that some thing moving with a particular speed, in a particular direction, will tend to keep moving with that particular speed in a particular direction. The word "momentum" is a related concept.

Assuming you have a planet moving in a circular orbit, the actual instantaneous speed of the planet is tangential to the orbit, and at a right angle to the force of gravity, which acts in a direction along the straight line connecting the planet to its star.

As sort of a weird question, I ask: What would happen to the planet if you could somehow magically turn off its star's gravity?

I claim, the answer is the planet's motion would change from circular motion, to motion in a straight line. The planet would keep moving in a straight line, with the same speed it was moving when the gravity was switched off.

As a more realistic, down to Earth example, the same kind of motion happens when a rock is released from a sling.


When the rock is held in the sling, it is constrained to a circular path, because the rope of the sling is continually pulling the rock inwards, towards the hand holding the sling.

But the rock has huge inertia. It wants to fly off in a straight line, and it will do so, if it can slip free from the sling somehow.

Sort of the moral of this story is there are sort of two things necessary to keep a planet (or anything else) in an orbit. One is the continual pull of gravity. The other is momentum of the planet. As vectors, they don't point in the same direction.

Speaking vaguely, these two things (Don't call both them "forces" because one is a force, and the other is momentum, the thing force affects.) kind of struggle against each other. Gravity wants to pull the planet and the star into each other. But the planet's momentum makes it want to leave the star, and just go off travelling forever in a straight line with constant speed. The result of this "struggle", between gravity and momentum, is a circular, or more generally, an elliptical, orbit.

See also:



petercd1 year ago

Probably the same force that keeps the electrons 'orbiting' the nucleus.

I cant particularly imagine a pencil ceasing to be a pencil beyond the borders of the suns influence.

Nor for that matter do I get how the gravitational pull pushes the planet for the 2nd half of its orbit either.

...probably falls into that 3rd category of Unknowable.

Nothing to do with electrons, I'm afraid.

An orbiting celestial body tries to move in a straight line, but gravity constantly pulls (no push) at it, forcing the straight line into an elliptical curve.

iceng1 year ago

When any object with mass (weight) is moving in the near perfect vacuum of space it keeps on moving forever !! Unless acted upon by another force like another mass object like a gas in our atmosphere, another planet or a star.

You have to accept the above statement....

Have you seen water spinning around a bath tub drain ?.... So as leftover bits from an exploded star draw together by static attraction until they get big enough to use gravity to pull big chunks which may miss each other and turn pulling closer all the time... Eventually all the loose matter joins the gravity rotation combining gas and other matter combining unimaginable weight until there is enough internal pressure to cause a nuclear fire (joining-ubundant-hydrogen=into-helium) releasing energy and forming a stream of particles with light pushing away from the newly formed star... This solar wind as it is called helps form planets rather then absorbing them... all these clumps were spiraling into the sun.... Some clumps ie planets they may find a balance orbit not always round where the speed of rotation wanting to fling away is matched by gravity pulling back....

Try swinging a bucket on a rope then add 3 glasses of water and swing again what happens to the water ?

rickharris1 year ago


Gravity effects everything, planets included.

It is pulling them all towards the sun and towards each other.

What is stopping the planets falling into the sun is the fact they are moving, in orbit.

This causes them to try to move in a straight line (Newtons laws) but gravity pulling on them pulls then into orbit around the sun.

The web site above has some diagrams and links to other interesting places covering this sort of thing.