If I drop a tennis ball, does it fall towards the earth? Or does the earth fall towards the tennis ball?
If I drop a tennis ball...
| Neilos wrote: |
| If I drop a tennis ball, does it fall towards the earth? Or does the earth fall towards the tennis ball? |
Both thing will happen simultaniously. This is a case of Conservation of momentum
Before dropping, both ball and earth have Zero momentum, so after dropping total momentum
should be Zero.
MV = mv
M=mass of earth
m=mass of ball
V=Velocity with which earth move towards tennis ball
v=velocity with which ball moves towards earth
Thus we see that as M is greater than m, means V is very smaller than v
Conclusion: earth will move towards tennis ball with very very small velocity, than
ball move towards earth
thank you
Very nice answer. I was hoping for more discussion on the matter before we hit the solution but very nice!
Neilos
ps. I'll have to come up with some harder questions
Neilos
ps. I'll have to come up with some harder questions
| Neilos wrote: |
| Very nice answer. I was hoping for more discussion on the matter before we hit the solution but very nice!
Neilos ps. I'll have to come up with some harder questions |
that was the physicists answer, but practically (i.e. from an engineering standpoint) speaking it doesn't matter.
Actually, when you drop it at the side of the earth that is away from the big bang, the ball will move away from the earth, slower and slower, while the earth will move to the ball a lot faster, at more or less constant speed.
| Stubru Freak wrote: |
| Actually, when you drop it at the side of the earth that is away from the big bang, the ball will move away from the earth, slower and slower, while the earth will move to the ball a lot faster, at more or less constant speed. |
Remember that he's talking a closed system here, Stubru Freak. It's also being pulled by the gravitational pull of the moon, but that isn't mentioned in the question.
| Hogwarts wrote: |
| Remember that he's talking a closed system here, Stubru Freak. It's also being pulled by the gravitational pull of the moon, but that isn't mentioned in the question. |
The notion that there is a side of the earth that faces 'away' from the Big Bang is a nonsense, which relies on a misunderstanding of the physics - the BB didn't occur at a 'place' in spacetime.
As far as I understand the universe is expanding from the place where the big bang was. So the whole galaxy is moving away from there at high speed, including the ball and the earth. Anyway, of course chatrack gave the obvious answer, but Neilos wanted a discussion, so I gave another (unusual) reference point.
| Stubru Freak wrote: |
| As far as I understand the universe is expanding from the place where the big bang was. So the whole galaxy is moving away from there at high speed, including the ball and the earth. Anyway, of course chatrack gave the obvious answer, but Neilos wanted a discussion, so I gave another (unusual) reference point. |
Yes - a lot of people share this faulty picture. It really isn't like that. There was no 'place' where the Big Bang was. Spacetime itself is expanding - in a very real sense we are IN the Big Bang right now. There is no area in the sky you can point to and say 'that is where the BB happened'.
Let's try an analogy and see if it helps.
You take a blob of dough (spacetime) with some currents (or raisins) in it (galaxies). You stick it in the oven and the dough expands. The currents appear to move away from each other, but they are not actually moving through the dough (spacetime) - it is the dough (spacetime) that is expanding.
It makes no sense for one current to say - 'the expansion started over there' - because EVERY current will see every other current moving away from it.
Does that help?
This faulty picture is because people imagine an explosion at a point in space - the media do little to correct this, so people think they understand.
| Bikerman wrote: | ||
Yes - a lot of people share this faulty picture. It really isn't like that. There was no 'place' where the Big Bang was. Spacetime itself is expanding - in a very real sense we are IN the Big Bang right now. There is no area in the sky you can point to and say 'that is where the BB happened'. Let's try an analogy and see if it helps. You take a blob of dough (spacetime) with some currents (or raisins) in it (galaxies). You stick it in the oven and the dough expands. The currents appear to move away from each other, but they are not actually moving through the dough (spacetime) - it is the dough (spacetime) that is expanding. It makes no sense for one current to say - 'the expansion started over there' - because EVERY current will see every other current moving away from it. Does that help? This faulty picture is because people imagine an explosion at a point in space - the media do little to correct this, so people think they understand. |
I understand now.
Change Big Bang to any galaxy except ours and my post is still valid though.Not really. You would have to calculate the net gravitational influence due to ALL masses in the universe. That would be a huge task. The local group of galaxies clearly exert the greatest 'pull', but whether that would be specifically directional or not is something I've not really thought about - it certainly wouldn't be measurable by any technique I can think of...
| Bikerman wrote: |
| Not really. You would have to calculate the net gravitational influence due to ALL masses in the universe. That would be a huge task. The local group of galaxies clearly exert the greatest 'pull', but whether that would be specifically directional or not is something I've not really thought about - it certainly wouldn't be measurable by any technique I can think of... |
The direction doesn't really matter, both the ball and the earth will move away from the other galaxy at a high speed, and the earth will catch the ball as it slows down a little.
| Stubru Freak wrote: | ||
The direction doesn't really matter, both the ball and the earth will move away from the other galaxy at a high speed, and the earth will catch the ball as it slows down a little. |
Nope, you are still stuck with this false notion of galaxies flying apart. 'T'aint so.
We could go into relativity, but you need to get your Newtonian physics straight first...
| Bikerman wrote: | ||||
Nope, you are still stuck with this false notion of galaxies flying apart. 'T'aint so. We could go into relativity, but you need to get your Newtonian physics straight first... |
Why not? The universe is expanding, so the distance between the galaxies increases. Or not?
| Stubru Freak wrote: |
| Why not? The universe is expanding, so the distance between the galaxies increases. Or not? |
So even if the galaxies were moving (which they are not, since it is the space between them that is expanding - to a first approximation*) it wouldn't matter.
* The local group are actually gravity-bound anyway and are not moving apart - in fact Andromeda is moving towards the Milky-way and will, one day, 'collide'.
| Neilos wrote: |
| ps. I'll have to come up with some harder questions |
If you're falling at terminal velocity and you sneeze, would your sneeze break your skull in?
| Bikerman wrote: | ||
So even if the galaxies were moving (which they are not, since it is the space between them that is expanding - to a first approximation*) it wouldn't matter. * The local group are actually gravity-bound anyway and are not moving apart - in fact Andromeda is moving towards the Milky-way and will, one day, 'collide'. |
But since the space between the galaxies expands, our galaxy is moving, in a purely mathematical sense, in the frame of reference of another galaxy.
| Stubru Freak wrote: | ||||
But since the space between the galaxies expands, our galaxy is moving, in a purely mathematical sense, in the frame of reference of another galaxy. |
Yes indeed. That would not, however, have any influence on the tennis ball.
| Bikerman wrote: | ||
The notion that there is a side of the earth that faces 'away' from the Big Bang is a nonsense, which relies on a misunderstanding of the physics - the BB didn't occur at a 'place' in spacetime. |
Perhaps I implied support of his argument by saying 'also', when I was just trying to list something else outside the closed system -- sorry >.>
| Bikerman wrote: | ||||||
Yes indeed. That would not, however, have any influence on the tennis ball. |
Well, the tennis ball is part of our galaxy so it's also moving away from the other galaxy, at the same speed we are. The moment you drop it, that speed changes somewhat (in direction or in actual speed), so it collides with the earth.
| Stubru Freak wrote: |
| Well, the tennis ball is part of our galaxy so it's also moving away from the other galaxy, at the same speed we are. The moment you drop it, that speed changes somewhat (in direction or in actual speed), so it collides with the earth. |
Let's forget, for the moment, that the galaxy is not, in fact, moving. Let's consider a very simple example.
If I am travelling on a train at (say) 100mph, and I drop a tennis ball, does the ball change its downward path because of my apparent movement? (Note the word 'apparent').
Nope - it falls downward in my frame of reference, due to gravity. If I mark a spot on the train floor beneath my hand then the ball will hit it. Now, a 'stationary' observer will see the ball move in the diagonal - but that is HIS frame of reference, not mine.
This is called Galilean relativity and you really need to understand it before you can make any sense of more advanced physics.
| Bikerman wrote: | ||
Let's forget, for the moment, that the galaxy is not, in fact, moving. Let's consider a very simple example. If I am travelling on a train at (say) 100mph, and I drop a tennis ball, does the ball change its downward path because of my apparent movement? (Note the word 'apparent'). Nope - it falls downward in my frame of reference. This is called Galilean relativity and you really need to understand it before you can make any sense of more advanced physics. |
Yes I get that. But let's say there's no air resistance, and you drop the ball outside the window of the train. In your frame of reference, the ball goes straight to the ground. In the frame of reference of someone outside the train, the tennis ball will fall in a parabolic path.
So similarly, in the frame of reference of someone watching us from another galaxy, the ball and the earth are moving, and the ball just changes speed and direction.
| Stubru Freak wrote: |
| Yes I get that. But let's say there's no air resistance, and you drop the ball outside the window of the train. In your frame of reference, the ball goes straight to the ground. In the frame of reference of someone outside the train, the tennis ball will fall in a parabolic path.
So similarly, in the frame of reference of someone watching us from another galaxy, the ball and the earth are moving, and the ball just changes speed and direction. |
| Bikerman wrote: | ||
|
Yes that was my point. The question was if the earth moves to the ball or the ball to the earth. That question depends on your frame of reference. If your frame of reference moves at the maximum speed at which the ball falls, and you drop the ball from high enough, the ball will slow down and the earth will move to the ball instead. I think that's somewhat what he expected when he asked the question: everything depends on your frame of reference.
The easy answer is too obvious anyway: the ball moves towards the earth. The earth moves a little little bit in a closed system, but in practice, if someone drops a football at the other side of the earth, it's moving to the other side anyway.
| Stubru Freak wrote: |
| Yes that was my point. |
| Quote: |
| If your frame of reference moves at the maximum speed at which the ball falls, and you drop the ball from high enough, the ball will slow down and the earth will move to the ball instead. |
| Bikerman wrote: | ||||
|
The ball falls towards the earth. Because of friction, it will reach a maximum speed, if the ball falls from high enough. At that point, its speed is constant.
So if you take that speed as a reference frame, you will see this: the ball will slow down because of gravity. Instead of reversing towards the earth, it will stop, because of the friction of all the wind (just still air from the earth's reference frame, but wind from the ball's reference frame). The earth will move towards the ball. Not because of gravity of course, but because the earth is moving, and there's no force stopping it.
| Stubru Freak wrote: | ||||||
The ball falls towards the earth. Because of friction, it will reach a maximum speed, if the ball falls from high enough. At that point, its speed is constant. So if you take that speed as a reference frame, you will see this: the ball will slow down because of gravity. Instead of reversing towards the earth, it will stop, because of the friction of all the wind (just still air from the earth's reference frame, but wind from the ball's reference frame). The earth will move towards the ball. Not because of gravity of course, but because the earth is moving, and there's no force stopping it. |
LOL...you have some strange ideas.
If we are ignoring air resistance then no - there is no friction.
Now, let's say there is friction due to air resistance. The ball will reach a terminal velocity (probably about 20m/s). It won't 'slow down' or 'stop'. I think you really need to do a basic course in newtonian mechanics. I can help, but first you need to understand that this is bunk.
| Bikerman wrote: | ||||||||
LOL...you have some strange ideas. If we are ignoring air resistance then no - there is no friction. Now, let's say there is friction due to air resistance. The ball will reach a terminal velocity (probably about 20m/s). It won't 'slow down' or 'stop'. I think you really need to do a basic course in newtonian mechanics. I can help, but first you need to understand that this is bunk. |
We aren't ignoring air resistance. That was just to simplify the train example. Now it's important that it exists.
Say your reference frame has a speed of 20m/s, and the direction is from the ball to the earth. It's still inert, it's not accelerating. So it's perfectly possible to use normal physics in that reference frame. The moment you drop the ball, in that reference frame, it will slow down from a speed of 20m/s to 0, because of gravity (it's moving away from the earth, and it doesn't want to). But because of the friction of the wind, it won't fall further towards the earth. Gravity and the friction of the wind are opposite forces with the same strength, so the ball won't move. The earth will keep moving, because there's no force stopping it. So in that reference frame, the earth moves towards the ball. It was moving in the direction of the ball, and its gravity made the ball stop. (And it accelerated the earth a little little bit, but nothing important.)
nobody answered my physics question...| medesignz wrote: | ||
If you're falling at terminal velocity and you sneeze, would your sneeze break your skull in? |
you are reacted with sneeze force only
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