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# Rotational motion to linear motion

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### #1Lars-Kristian  Members   -  Reputation: 158

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Posted 20 July 2012 - 07:04 PM

Hi, like my title says I want to transform rotational motion in to linear motion.
In my game I have a circle that spins, and when it collides with something, it shall transform some of that spin in to a linear motion.

I have tried alot of different things, and I'm out of ideas. Does someone have a solution?
And I'm using a verlet integration for physic's if it helps.

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### #2medevilenemy  Members   -  Reputation: 326

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Posted 21 July 2012 - 10:42 AM

I'm assuming what you want is for the circle to roll along the surface. I think what you need is to model the friction between the object and the surface, and the torque with which the circle spins. On contact with a surface of nonzero coeficcient of friction, the friction generated will tend to produce a torque opposite to that being generated by the circle (or whatever is causing it to spin). When the circle's torque overcomes the frictional torque, the circle rolls including a certain linear component. Unfortunately I don't have the math handy, but a physics textbook or google should help.
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### #3Postie  Members   -  Reputation: 1115

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Posted 21 July 2012 - 06:44 PM

The counter intuitive thing about rolling vs sliding is that initially when the circle is stationary relative to the surface the friction force that comes into play uses the dynamic (or moving) friction coefficient. That force causes an unbalanced torque on the circle that causes an acceleration along the surface. Eventually, the speed of motion along the surface opposes the tangential velocity of the point of contact and that point can be considered stationary relative to the surface, so the dynamic friction coefficient is replaced by the static friction coefficient. This removes the torque, so the circle stops accelerating.
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### #4Lars-Kristian  Members   -  Reputation: 158

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Posted 22 July 2012 - 05:08 AM

Thanks for the reply, but im still not sure how to achieve this effect.

This is my ideas:

### #5Kyall  Members   -  Reputation: 287

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Posted 23 July 2012 - 02:24 AM

Angular momentum beecomes linear momentum dependent upon the friction pretty much so look up andlgular 'moment' i think is the techinical term in physics stuff to get the equation that gives you that angular momentum

just googled it, take a look at moment of inertia on wikipedia, that should point you in the correct dorection; id.post.the equations but my physics book is not at.the location i am currently chillin sorry - also phone so srry about the typos
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### #6Bartpab  Members   -  Reputation: 109

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Posted 23 July 2012 - 09:23 PM

With a simple kinematic way, V = k*R*W where 0 < k < 1 which can be a sort of transmission factor, R for radius and W for the rotation speed. :3

### #7Lars-Kristian  Members   -  Reputation: 158

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Posted 24 July 2012 - 02:50 AM

Thanks for all the help everyone, but i did it my way, and it ended up working the way I wanted to.

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