# Why don't all massive objects collide?

1. May 27, 2006

### toddman

I am not a mathematician or a physicist so this may be a dumb question. If the force we call gravity is caused by an object's mass bending or curving space/time and creating a gravity well that causes other objects to be drawn into this well; it seems that all objects should eventually spiral into the object with the largest mass. Is the fact that this does not happen due to the gravity waves produced by a rotating or moving object cancelling out some of the gravitational effect and allowing smaller objects to orbit an object with larger mass without being drawn in? For example, our moon's orbit used to be much closer than it is now and it's orbit distance has been increasing over time, which is the opposite of what you would expect. Or does the increased acceleration as the two objects move closer together allows for sufficient momentum to overcome the effects of gravity, providing the objects trajectories are not on a collision course.

As I said at the beginning of this post I am not well versed in either mathematics or physics so if you reply try to keep it fairly simple and if I have overlooked some simple fundamental principle please don't be to rough on me.

Thanks,
Toddman

2. May 28, 2006

### Gokul43201

Staff Emeritus
Have you ever rolled a coin along a parabolic pit with a hole in the bottom (of the kind that is found in every other science museum) ? Depending on how fast you started the coin off, it would either fall into the bottom of the well (if it starts off slowly) or fly off the top edge (if it started off really fast). If there were no friction between the coin and the surface of the well, you could flick the coin at just the right speed and direction that it makes circles forever...

Much the same thing happens when bodies are gravitationally attracted to each other. but to make things more complicated, there are other forces acting between bodies (or once did) besides gravity.

3. May 28, 2006

### pervect

Staff Emeritus
It's almost the opposite, actually. The way to think about this to get the correct answers is to think about the energy of the system, and where it goes. To do this in detail takes a lot of math (as does a lot of physics), but it's not too hard to give a brief overview without the equations.

In the simplest case, when two objects orbit each other, it does not take any energy, no energy is dissipated, and the orbits would never decay.

Gravity waves are one reason this doesn't happen. When an object orbits another object, it emits a very tiny amount of gravitational radiation. This drains energy from the system, causing the orbit to decay.

Gravity waves are one example of an extermely tiny effect that slowly drains energy away from a system. There are other sorces of drag and drag forces as well. Even light can act as a drag force (see for instance the Poynting-Robertson drag in the link below) which is mostly important for very small particles, but would have an extremly tiny effect on larger objects as well).

http://en.wikipedia.org/wiki/Poynting-Robertson_effect

[/quote]
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For example, our moon's orbit used to be much closer than it is now and it's orbit distance has been increasing over time, which is the opposite of what you would expect. Or does the increased acceleration as the two objects move closer together allows for sufficient momentum to overcome the effects of gravity, providing the objects trajectories are not on a collision course.
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The moon's orbital increase is being powered by the spin of the Earth. This is another example of how it is convenient to "track the energy". The Earth is gradually losing its energy as it "tide-locks" with the moon. As the Earth's days get shorter, the moon's orbit gets higher. Some of the energy of the Earth's spin is being dissipated in moving the water around on the Earth, but the rest of the energy is going into raising the Moon's orbit.

Sometimes I tend to get too technical, but I hope I kept things really simple. If there is something in my response that wasn't very clear, please don't hesitate to ask.

Don't worry about asking questions, or not knowing the answer ahead of time. I'd much rather talk to someone who is curious and doesn't know the answer than (for a bad example) to someone who thinks he knows what the answer is and is totally wrong and won't listen...

4. May 28, 2006

### toddman

Thanks to all who replied, your answers helped me to conceptualize this and brought into play forces and energy I had overlooked in my simplified examples. Thanks again
Toddman