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JohnnyCode

What will heppen in real world?

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I have a rigid body response dilema as to their real world behaviour.

Consider a cube (in space- no gravity) that moves its direction and hits a static (super stable) platform orthogonaly like this:

 

      

    ooo

    ooo

    ooo

oooo

 

I then wonder wheather the fact that the cube mass center being over the static platform in the direction of cube movement, will couse the cube to not to "try" hang over the edge at all, or, the fact that some part of mass is still over the edge to move freely, will couse the cube to emit some tendence of movement to the right over the egde. So will it slightly bend over the platform or not at all?

 

And second question is what would happen if there was attractive force in the direction of cube movement against the platform?

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In reality, neither object can be perfectly stationary or have infinite mass, so some slight rotation and movement along the X axis is going to occur. Also, collisions are continuous over a short period of time and varying amounts of forces are applied as the collision progressively alters the objects.

The result depends a lot on what the masses of the two objects are. If the cube is much more massive, the platform will move down and slightly to the left while rotating clockwise, while the cube will continue down and to the right and also spin clockwise.

If the platform (and whatever it's attached to) is much more massive, the cube may bounce up and slightly to the right while spinning slightly clockwise, and the platform will deform elastically and return to its approximate original position relative to the large massive body it's attached to.

Depending on surface friction, the cube may slide off the right side of the platform.

Nothing in reality is perfectly stiff, either. Depending on the stiffness of the cube (imagine Jello), it may deform and "bend" over the edge.

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If the platform (and whatever it's attached to) is much more massive, the cube may bounce up and slightly to the right while spinning slightly clockwise

I think this is a very beautifull funded answer. Would this apply for the modification of:  perfectly stiff objects, and the platform not able to move and absorb no energy (meaning platform will couse nearly perpetum mobile return of reaction energy back to the cube, that is undeformable too, and will transform entire reaction energy to kinetcs). Would cube, then, move just perfectly up, or still would have emited a  clockwise rotation and translation to the right?

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perfectly stiff objects, and the platform not able to move ...

 

 

As Nypyren mentioned, that's not a real world situation, which the subject of your thread asks about. However, as a thought problem, if a cube moving to the right strikes the edge of a platform at a point below it's center, it will travel directly back to the left, with a clockwise-rotation. It's the same scenario of an ideal cue ball being struck in an ideal collision with an ideal cue stick - the cue ball travels directly along the direction of the cue stick strike, with a rotation about its center-of-mass due to the torque of the strike.

 

Your illustration above wasn't clear to me, so, if your cube is traveling to the left, then it would rebound to the right with a counter-clockwise rotation.

Edited by Buckeye

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Your illustration above wasn't clear to me, so, if your cube is traveling to the left, then it would rebound to the right with a counter-clockwise rotation.

The cube travels down, orthogonaly onto the platform.

 

Yes, I have "eliminated" certain real-world conditions, moving it to more theoretical level, to spectate rather only few certain real-world behaviorals of rigid bodies. Those conditions though can happen (omitting even heat on contact) to such level of derivation that one could spectate "only" that certain behavioral- wheather the part of mass with free tendence would couse the cube to recieve tendence of kinetics to the right, or "not at all" (as for the derivation acceptance of those scaled up "perfect" conditions).

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The cube travels down, orthogonaly onto the platform.

 

For a rigid cube traveling down, it will rebound straight up. If a portion of the cube "overlaps" the edge of the platform, it will also rotate. That's assuming the force of impact is perfectly and evenly distributed across the collision faces.

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If the cube was at rest on the platform I'm sure one could set up a differential system and solve it to determine how much of the cube needs to stick out over the platform before it falls off, as a function of gravity, the various masses involved, and possibly friction. If the cube starts with some initial velocity it could be much harder though.

 

Otherwise I guess the experiment isn't hard to repeat with any modern 3D physics engine.

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Would this apply for the modification of:  perfectly stiff objects, and the platform not able to move and absorb no energy (meaning platform will couse nearly perpetum mobile return of reaction energy back to the cube, that is undeformable too, and will transform entire reaction energy to kinetcs). Would cube, then, move just perfectly up, or still would have emited a  clockwise rotation and translation to the right?


As long as the collision physics still use some approximation of http://en.wikipedia.org/wiki/Coulomb%27s_law , then I believe it would still have a clockwise rotation, but the bounce angle would probably have less magnitude in the rightward direction. Edited by Nypyren

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... use some approximation of [ Coulomb's law ...

 
Coulomb's Law has to do with electrical charge. I don't see how that relates to ideal rigid body collision. Can you explain?


The OP seems to be asking about what would happen if you had a physics engine that was a mix between rigid body dynamics and real-world subatomic-force-based physics, so I'm trying to mentally simulate what would happen under those conditions.

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