Earth --> Sun: Kaboum!!
- Thread starter c'est moi
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cest moi
Okay, but then the centripetal force and the gravitational force point BOTH towards the sun in this case. They can't be cancelling each other out like you suggested.
You're getting a little confused. Newtons first law states that an object will remain in motion unless acted upon by an unbalanced force. In other words, a body in motion will continue along a straight line unless acted upon by a force. The gravitational attraction is an unbalanced force that is accelerating the orbiting object towards the center of curvature.
Hence we have centripetal forces AND gravitational attraction, and they need to balance to create a stable orbit.
I hope this helps.
Okay, but then the centripetal force and the gravitational force point BOTH towards the sun in this case. They can't be cancelling each other out like you suggested.
You're getting a little confused. Newtons first law states that an object will remain in motion unless acted upon by an unbalanced force. In other words, a body in motion will continue along a straight line unless acted upon by a force. The gravitational attraction is an unbalanced force that is accelerating the orbiting object towards the center of curvature.
Hence we have centripetal forces AND gravitational attraction, and they need to balance to create a stable orbit.
I hope this helps.
I'm really not getting it 
okay, i know the law of Inertia
"In other words, a body in motion will continue along a straight line unless acted upon by a force. The gravitational attraction is an unbalanced force that is accelerating the orbiting object towards the center of curvature."
towards the centre -----> the earth is attracted towards the sun
again: what is keeping it from not going completely towards the sun? okay, let us say gravity is too weak cause we're too far, then why aren't we drifting of --> cause of the huge speed? (centrifugal force) well, gravity is just strong enough to counter that. Then again, for mercurius, as it so so much more close to the sun it must have a much greater orbital speed otherwise cause of gravity which is much stronger so close, it would smash against the sun ...... that's my reasoning so far
okay, i know the law of Inertia
"In other words, a body in motion will continue along a straight line unless acted upon by a force. The gravitational attraction is an unbalanced force that is accelerating the orbiting object towards the center of curvature."
towards the centre -----> the earth is attracted towards the sun
again: what is keeping it from not going completely towards the sun? okay, let us say gravity is too weak cause we're too far, then why aren't we drifting of --> cause of the huge speed? (centrifugal force) well, gravity is just strong enough to counter that. Then again, for mercurius, as it so so much more close to the sun it must have a much greater orbital speed otherwise cause of gravity which is much stronger so close, it would smash against the sun ...... that's my reasoning so far
If your to look at the full picture, it's not just the Earth spinning, while moving around the sun. There's all the other planets, the moon, Astroids, debris, meteors etc.
All of those things have a Gravity, and in space the larger the body or denser the mass, the greater the gravity.
Different bodies are drawn to and push away from one another, doing a kind of dance.
I mean I could mention here that you have Astrologer's that mention of planetry alignments and how it effects your mood. Well I might not be a great believer in it, but I do believe that they have a point that an alignment of planets can make an orbit change in shape, from circular to eliptical, and possible enducing velocity changes.
The Eliptical orbit, is made from such velocity changes, where the earth could be brought near the sun, and the near it get's it increase in veolcity enough to break free from the suns gravity and slingshot itself around.
In fact the whole slingshot usage of bodies is renound as a lifesaver in space. Especially for a nearly doomed spaceflight.
All of those things have a Gravity, and in space the larger the body or denser the mass, the greater the gravity.
Different bodies are drawn to and push away from one another, doing a kind of dance.
I mean I could mention here that you have Astrologer's that mention of planetry alignments and how it effects your mood. Well I might not be a great believer in it, but I do believe that they have a point that an alignment of planets can make an orbit change in shape, from circular to eliptical, and possible enducing velocity changes.
The Eliptical orbit, is made from such velocity changes, where the earth could be brought near the sun, and the near it get's it increase in veolcity enough to break free from the suns gravity and slingshot itself around.
In fact the whole slingshot usage of bodies is renound as a lifesaver in space. Especially for a nearly doomed spaceflight.
There is only one force to speak of on a planet in orbit: gravity.
Gravity <b>is</b> a centripetal force. It is a force which pulls the planet towards the sun (at the "centre" of the orbit).
There is no force balance happening in an orbit. If all the forces on a planet balanced, there would be no net force and so the planet would travel in a straight line with constant speed, according to Newton's first law. What happens instead is that gravity accelerates the planet, continually changing the direction of its velocity, so that it follows an elliptical path around the sun. The net force on the planet is the gravity force.
Gravity <b>is</b> a centripetal force. It is a force which pulls the planet towards the sun (at the "centre" of the orbit).
There is no force balance happening in an orbit. If all the forces on a planet balanced, there would be no net force and so the planet would travel in a straight line with constant speed, according to Newton's first law. What happens instead is that gravity accelerates the planet, continually changing the direction of its velocity, so that it follows an elliptical path around the sun. The net force on the planet is the gravity force.
"What happens instead is that gravity accelerates the planet, continually changing the direction of its velocity, so that it follows an elliptical path around the sun."
so the velocity is due to gravity
BUT, as I said before, isn't it because this speed that the earth stays where it is??
and aren't you wrong when you say "acceleration", if that would be true, we would have a very unpleasant stay here on earth ...
and how does it come gravity "accelerates" (i really don't think that's correct, would be falling of my chair if that happens) the earth and making it *change from direction* --> can't we see the force of gravity as a perpendicular force on the sun towards the earth, like an arm reaching out for the earth, or does that "arm" turn along with the spin of the sun.....
"The net force on the planet is the gravity force."
ok.
me is mucho confused
so the velocity is due to gravity
BUT, as I said before, isn't it because this speed that the earth stays where it is??
and aren't you wrong when you say "acceleration", if that would be true, we would have a very unpleasant stay here on earth ...
and how does it come gravity "accelerates" (i really don't think that's correct, would be falling of my chair if that happens) the earth and making it *change from direction* --> can't we see the force of gravity as a perpendicular force on the sun towards the earth, like an arm reaching out for the earth, or does that "arm" turn along with the spin of the sun.....
"The net force on the planet is the gravity force."
ok.
me is mucho confused
James R
There is no force balance happening in an orbit. If all the forces on a planet balanced, there would be no net force and so the planet would travel in a straight line with constant speed, according to Newton's first law.
Although I agree with you're explanation in that gravity is the only net force acting as the centripetal force on the orbiting body, I don't agree there is no balance of forces. If we take the Earth-Moon system for example, the force of gravity of the orbiting body, the Moon, must also be taken into account as another force acting on the system. This force needs to balance out the centripetal force of gravity from the orbited body for the orbiting body to remain stable.
Aside from that, your explanation is spot on.
cest moi
When no work is done upon an object by external forces, the potential and kinetic energies of the object remain constant. In other words, if the body is moving at a constant speed, the centripetal force doesn't alter the total mechanical energy, in this case the kinetic energy. The centripetal force can accelerate the object by changing its direction, but it can't change its speed. There would need to be a force acting in the direction or opposite direction of the motion of the object in order to change its speed and direction.
Remember, speed refers to how fast an object is moving (scalar) and velocity refers to the rate at which an object changes its position (vector).
There is no force balance happening in an orbit. If all the forces on a planet balanced, there would be no net force and so the planet would travel in a straight line with constant speed, according to Newton's first law.
Although I agree with you're explanation in that gravity is the only net force acting as the centripetal force on the orbiting body, I don't agree there is no balance of forces. If we take the Earth-Moon system for example, the force of gravity of the orbiting body, the Moon, must also be taken into account as another force acting on the system. This force needs to balance out the centripetal force of gravity from the orbited body for the orbiting body to remain stable.
Aside from that, your explanation is spot on.
cest moi
When no work is done upon an object by external forces, the potential and kinetic energies of the object remain constant. In other words, if the body is moving at a constant speed, the centripetal force doesn't alter the total mechanical energy, in this case the kinetic energy. The centripetal force can accelerate the object by changing its direction, but it can't change its speed. There would need to be a force acting in the direction or opposite direction of the motion of the object in order to change its speed and direction.
Remember, speed refers to how fast an object is moving (scalar) and velocity refers to the rate at which an object changes its position (vector).
I have an analogy which you can try yourself which demonstartes very clearly why we maintain our current orbit.
Get a foot or two of string, and tie a metal nut to one end. Dangle it from your hand, and start it swinging around in a circle, maybe a foot in diametre. Keep it at that speed/diametre. Notice that it's path is a balance between gravity (pulling it toward the centre, or toward hanging straight down), and the force of motion which tends to make it want to go up and out.
For a larger circle (orbit), apply more speed, and watch the circle grow. Again, it will maintain a stable orbit in its new diametres by finding equilibrium between gravity pulling it inward and the force of motion pulling it outward.
For any given weight of the metal nut (planet), there are different balances of speed and diametre which will result in stable orbits, or equilibrium.
I hope this helps people picture easier how planetary orbits work.
Get a foot or two of string, and tie a metal nut to one end. Dangle it from your hand, and start it swinging around in a circle, maybe a foot in diametre. Keep it at that speed/diametre. Notice that it's path is a balance between gravity (pulling it toward the centre, or toward hanging straight down), and the force of motion which tends to make it want to go up and out.
For a larger circle (orbit), apply more speed, and watch the circle grow. Again, it will maintain a stable orbit in its new diametres by finding equilibrium between gravity pulling it inward and the force of motion pulling it outward.
For any given weight of the metal nut (planet), there are different balances of speed and diametre which will result in stable orbits, or equilibrium.
I hope this helps people picture easier how planetary orbits work.
okay (Q)! centripetal force changes only direction
after all these replies, I am still finding myself lost
Maybe someone could sum it up with an example (not a dummie like me):
Imagine I throw a globe the size of the earth in our solar system:
...oooo
..oooooo............ooo
.ooooooo..........oooo.. .^
..oooooo........... ooo... .|
...oooo....................... | (gets thrown from this direction)
...SUN..............GLOBE
what will happen and why?
I still don't get why gravity causes things to orbit instead of just pulling it completely towards the mass who produces the gravity. When I jump of my house I get smacked on the earth ... I don't orbit it ... actually it seems I don't even understand why satellites orbit the earth.
after all these replies, I am still finding myself lost
Maybe someone could sum it up with an example (not a dummie like me):
Imagine I throw a globe the size of the earth in our solar system:
...oooo
..oooooo............ooo
.ooooooo..........oooo.. .^
..oooooo........... ooo... .|
...oooo....................... | (gets thrown from this direction)
...SUN..............GLOBE
what will happen and why?
I still don't get why gravity causes things to orbit instead of just pulling it completely towards the mass who produces the gravity. When I jump of my house I get smacked on the earth ... I don't orbit it ... actually it seems I don't even understand why satellites orbit the earth.
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yeah, i was busy with that
BUT if you look up you'll see that I've exactly told already what you are telling now! (or is it on page 1)
I said exactly the same thing but nobody answered directly to it ... instead the centripetal force etc. was mentioned and I forgot all about what I had said myself
BUT if you look up you'll see that I've exactly told already what you are telling now! (or is it on page 1)
I said exactly the same thing but nobody answered directly to it ... instead the centripetal force etc. was mentioned and I forgot all about what I had said myself
that's how I said it:
"Then again, for mercurius, as it so so much more close to the sun it must have a much greater orbital speed otherwise cause of gravity which is much stronger so close, it would smash against the sun ...... that's my reasoning so far"
but you say it better
but I think there's something wrong with the reasoning ... it's far too simple!
"Then again, for mercurius, as it so so much more close to the sun it must have a much greater orbital speed otherwise cause of gravity which is much stronger so close, it would smash against the sun ...... that's my reasoning so far"
but you say it better
but I think there's something wrong with the reasoning ... it's far too simple!
Hi c'est moi,
Long time no see you.
Ok, I'm not the smart guy you wish for, but i like to post my thought here.
I think we should calculate all of forces in several position of planet orbits. Let's take a look at mass of two objects (sun and earth) as gravity source (gravity depends on it's mass isn't it?). Then we can calculate the vector forces between them, to check is it equilibrioum in certain positions (snapshots of dynamic movement). If it does, so it solved. No further question.
But if we ask WHAT cause the initial movement creates centifugal force which precisely balance to the gravity... I think no one really sure, since the big bang theory itself; why/how it's happen still in debates among several scientist.
Long time no see you.
Ok, I'm not the smart guy you wish for
, but i like to post my thought here.I think we should calculate all of forces in several position of planet orbits. Let's take a look at mass of two objects (sun and earth) as gravity source (gravity depends on it's mass isn't it?). Then we can calculate the vector forces between them, to check is it equilibrioum in certain positions (snapshots of dynamic movement). If it does, so it solved. No further question.
But if we ask WHAT cause the initial movement creates centifugal force which precisely balance to the gravity... I think no one really sure, since the big bang theory itself; why/how it's happen still in debates among several scientist.
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C'est moi:
<i>so the velocity [of an orbiting planet] is due to gravity</i>
No. Not at all. The velocity is just something the planet happens to have had from the time of the formation of the solar system. Gravity acts at right angles to the velocity (for a circular orbit). Therefore, the gravitational force does not change the <b>speed</b> (i.e. the magnitude of the velocity), but it does change the direction of motion. A change in the direction of motion is a change of velocity, and a change of velocity is an acceleration. So gravity is correctly said to accelerate the Earth in its orbit around the sun.
We do not feel the acceleration because we are accelerated by the sun's gravity at the same rate that the rest of the Earth is accelerated. Both the Earth and the people on it are in free fall towards the sun at all times. We don't move closer to the sun, though, because our pre-existing velocity makes us go round. We continually fall towards the sun, but "miss".
Think about throwing a ball horizontally off a high tower. If you drop it (zero horizontal velocity), it simply falls to the ground. If you throw it outwards (some horizontal velocity), it hits the ground some distance away. If you could throw the ball very fast, as it fell towards the ground, the ground would curve away from it (due to the Earth's curvature). The ball would never hit the ground, although it would be continually pulled downwards by gravity. In other words, it would be in orbit, for the same reasons that the Earth orbits the sun.
(Q):
<i>If we take the Earth-Moon system for example, the force of gravity of the orbiting body, the Moon, must also be taken into account as another force acting on the system.</i>
The force of gravity due to the Moon acting on the Earth is equal and opposite to the force of gravity due to the Earth acting on the Moon (see Newton's third law). In fact, the Earth and Moon orbit their common centre of mass. However, the centre of mass is a point inside the Earth, so it is more or less correct to say that the Moon orbits the Earth.
My point was that there is no "outward" force on the Moon. The only force on the Moon, from an inertial point of view, is the centripetal gravitational force from the Earth. Thus, there is no
"balance of forces" if you look only at the Moon, for example.
<i>so the velocity [of an orbiting planet] is due to gravity</i>
No. Not at all. The velocity is just something the planet happens to have had from the time of the formation of the solar system. Gravity acts at right angles to the velocity (for a circular orbit). Therefore, the gravitational force does not change the <b>speed</b> (i.e. the magnitude of the velocity), but it does change the direction of motion. A change in the direction of motion is a change of velocity, and a change of velocity is an acceleration. So gravity is correctly said to accelerate the Earth in its orbit around the sun.
We do not feel the acceleration because we are accelerated by the sun's gravity at the same rate that the rest of the Earth is accelerated. Both the Earth and the people on it are in free fall towards the sun at all times. We don't move closer to the sun, though, because our pre-existing velocity makes us go round. We continually fall towards the sun, but "miss".
Think about throwing a ball horizontally off a high tower. If you drop it (zero horizontal velocity), it simply falls to the ground. If you throw it outwards (some horizontal velocity), it hits the ground some distance away. If you could throw the ball very fast, as it fell towards the ground, the ground would curve away from it (due to the Earth's curvature). The ball would never hit the ground, although it would be continually pulled downwards by gravity. In other words, it would be in orbit, for the same reasons that the Earth orbits the sun.
(Q):
<i>If we take the Earth-Moon system for example, the force of gravity of the orbiting body, the Moon, must also be taken into account as another force acting on the system.</i>
The force of gravity due to the Moon acting on the Earth is equal and opposite to the force of gravity due to the Earth acting on the Moon (see Newton's third law). In fact, the Earth and Moon orbit their common centre of mass. However, the centre of mass is a point inside the Earth, so it is more or less correct to say that the Moon orbits the Earth.
My point was that there is no "outward" force on the Moon. The only force on the Moon, from an inertial point of view, is the centripetal gravitational force from the Earth. Thus, there is no
"balance of forces" if you look only at the Moon, for example.
Okay James R, so what I and then Adam said was thus correct (except that I thought the speed was due to gravity)
so mercurius has a much higher speed than the earth and pluto must be the slowest ...
and any object that enters the solar system and that has no balance between its initial speed and the force of gravity will or drift away from our solar system or be attracted towards the sun and burned
so mercurius has a much higher speed than the earth and pluto must be the slowest ...
and any object that enters the solar system and that has no balance between its initial speed and the force of gravity will or drift away from our solar system or be attracted towards the sun and burned