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What happens when there is not enough gravitational pull?

For example, if I throw the ball very fast, and mv^2/r is bigger than gravitational force, does the ball gain radius or angular velocity or what? What if it's smaller that the gravitational force?

Does the same thing apply to satellites and alike? Hows their angular velocity controlled?
(edited 10 years ago)
Reply 1
Original post by Dog4444
For example, if I throw the ball very fast, and mv^2/r is bigger than gravitational force, does the ball gain radius or angular velocity or what? What if it's smaller that the gravitational force?

Does the same thing apply to satellites and alike? Hows their angular velocity controlled?


Your example of a ball won't work unless it is in circular motion.

Let us take a satellite in circular orbit around earth. Now the gravitational force has to equal mv2/r. The gravitational force is what causes the centripetal force, so they are always equal.

Remember this is just F=ma but in circular motion a=v2/r thus F=mv2/r.

Imagine a box being pushed along a smooth (no friction) floor for example. Lets adapt your question to this. If you push it with a force F, and it has mass m and acceleration a, we can say F=ma from Newton's second law. Now you were essentially saying, what if ma is bigger than F... Well ma has to equal F, because F=ma. I suppose it is like asking, 2=2 but what if the 2 on the right is bigger than 2? It can't happen.
Reply 2
Original post by fayled
Your example of a ball won't work unless it is in circular motion.

Let us take a satellite in circular orbit around earth. Now the gravitational force has to equal mv2/r. The gravitational force is what causes the centripetal force, so they are always equal.

Remember this is just F=ma but in circular motion a=v2/r thus F=mv2/r.

Imagine a box being pushed along a smooth (no friction) floor for example. Lets adapt your question to this. If you push it with a force F, and it has mass m and acceleration a, we can say F=ma from Newton's second law. Now you were essentially saying, what if ma is bigger than F... Well ma has to equal F, because F=ma. I suppose it is like asking, 2=2 but what if the 2 on the right is bigger than 2? It can't happen.


I meant sometimes theres not enought force from gravity what happens then and how it affects radius and angular of motion of the object (like ball or satellite). And what happens if there's too much gravity force (the opposite of first case).
Original post by fayled
Your example of a ball won't work unless it is in circular motion.

Let us take a satellite in circular orbit around earth. Now the gravitational force has to equal mv2/r. The gravitational force is what causes the centripetal force, so they are always equal.

Remember this is just F=ma but in circular motion a=v2/r thus F=mv2/r.

Imagine a box being pushed along a smooth (no friction) floor for example. Lets adapt your question to this. If you push it with a force F, and it has mass m and acceleration a, we can say F=ma from Newton's second law. Now you were essentially saying, what if ma is bigger than F... Well ma has to equal F, because F=ma. I suppose it is like asking, 2=2 but what if the 2 on the right is bigger than 2? It can't happen.

Wouldn't it just be a case of having unbalanced forces and so by Newton's first law, you'll cause an acceleration due to the resultant force.

Therefore in the case of the box, that will accelerate along the smooth surface if you give it that constant force.

And in the case of the OP, the ball would spiral outwards.

Am I right here? Be sure to check.

Stonebridge will better explain :smile:

Stonebridge
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Reply 4
Original post by Dog4444
I meant sometimes theres not enought force from gravity what happens then and how it affects radius and angular of motion of the object (like ball or satellite). And what happens if there's too much gravity force (the opposite of first case).


Smaller gravitational force -> smaller centripetal force -> for constant mass and speed, satellite would need to orbit with a larger radius -> angular velocity decreases

Larger gravitational force -> larger centripetal force -> for constant mass and speed, satellite would need to orbit with a smaller radius -> angular velocity increases

these consequences are derived from simple equations.
Reply 5
Original post by TheGrinningSkull
Wouldn't it just be a case of having unbalanced forces and so by Newton's first law, you'll cause an acceleration due to the resultant force.

Therefore in the case of the box, that will accelerate along the smooth surface if you give it that constant force.

And in the case of the OP, the ball would spiral outwards.

Am I right here? Be sure to check.

Stonebridge will better explain :smile:


ma or mv2/r is not a force, it is an expression for the resultant force of an object.

The sum of the forces=mass x acceleration

The OP seems to be asking what would happen if the RHS of the equation was larger than the LHS. This is not possible.

A resultant force causes the LHS of the equation to yield a non zero number thus yes there is an acceleration.
Reply 6
F=(mv^2)/r isn't really force, it is a description of a force for the specific case of circular motion. It's analogous to F=ma. Meaningless unless you equate it to something.

If your question was instead to consider, say, a satellite which was already in a circular orbit, which then fired it's boosters to increase its speed - then yes, the orbital radius would increase.

But your example of throwing a ball doesn't work because the circular motion condition is not fulfilled, so we can't use F=mv^2/r
Reply 7
Original post by OMGWTFBBQ
F=(mv^2)/r isn't really force, it is a description of a force for the specific case of circular motion. It's analogous to F=ma. Meaningless unless you equate it to something.

If your question was instead to consider, say, a satellite which was already in a circular orbit, which then fired it's boosters to increase its speed - then yes, the orbital radius would increase.

But your example of throwing a ball doesn't work because the circular motion condition is not fulfilled, so we can't use F=mv^2/r


Exactly.
Reply 8
Original post by fayled
ma or mv2/r is not a force, it is an expression for the resultant force of an object.

The sum of the forces=mass x acceleration

The OP seems to be asking what would happen if the RHS of the equation was larger than the LHS. This is not possible.

A resultant force causes the LHS of the equation to yield a non zero number thus yes there is an acceleration.


What I was asking is, if there's an object like a new satellite, and mv^2/r is more than gravity pull, what happens to satellites orbital radius, angular velocity and linear speed(radius time ang.vel). Especially, I'm interested in linear speed, would it remain constant or would it go up due acceleration?

Also, I'm not sure about the opposite situation where object is launched too slow so that mv^2/r is less than gravity pull.


Original post by OMGWTFBBQ

But your example of throwing a ball doesn't work because the circular motion condition is not fulfilled, so we can't use F=mv^2/r


What's wrong with the ball? If thrown with enough speed, it would remain in circular motion.
Reply 9
Original post by Dog4444
What's wrong with the ball? If thrown with enough speed, it would remain in circular motion.


If the ball is thrown fast enough such that it enters circular motion then the gravitational force = mv^2/r by definition because it is in circular motion

If you throw it too fast then it never meets the circular motion criteria, so talking of mv^2/r is nonsensical.
Reply 10
Original post by OMGWTFBBQ
If the ball is thrown fast enough such that it enters circular motion then the gravitational force = mv^2/r by definition because it is in circular motion

If you throw it too fast then it never meets the circular motion criteria, so talking of mv^2/r is nonsensical.


Taking mv^2/r and comparing it with gravity will show u that there's not enough gravity force to cause the ball with very high speed to move in circular motion, and I'm interested what happens with the ball's distance towards centre of mass of the earth, it's linear speed and ang.vel (if it will reach circular motion state) (I dunno, if it's me not explaining my question properly or theres other problem)
Reply 11
May be it's better to change the question: if there's a satellite orbiting around the earth with changing orbital radius (and still can be modelled by circular motion models due to small differences) (happens when it's not orbiting over equator plane). What happens with ang.vel and linear speed as radius changes?
Original post by fayled
ma or mv2/r is not a force, it is an expression for the resultant force of an object.

The sum of the forces=mass x acceleration

The OP seems to be asking what would happen if the RHS of the equation was larger than the LHS. This is not possible.

A resultant force causes the LHS of the equation to yield a non zero number thus yes there is an acceleration.


But of course :P

Had to get my head around that but I think I recall my understanding of it :P

Thanks!
Original post by Dog4444
May be it's better to change the question: if there's a satellite orbiting around the earth with changing orbital radius (and still can be modelled by circular motion models due to small differences) (happens when it's not orbiting over equator plane). What happens with ang.vel and linear speed as radius changes?


Please see attached file as my explanation contains a lot of equations and I don't know how to use TSR Latex.

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