# Just need a little explaining

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In Chris Hecker's article, he gives an equation for finding the impulse needed for collision response. I just can't seem to "solve for j/Impulse" given this equation RelativeVelocity + Impulse/MassOfObjectA + Impulse/MassOfObjectB = -e * RelativeVelocity which, once its has been rearranged should be Impulse = -(1 + e) * RelativeVelocity * (MassOfObjectA + MassOfObjectB) It does make sense to me, but I just don't know how (1 + e) gets there. I've been trying for some time to solve for "j" but I just can't! It would really help if someone could do it step by step for me. I feel as if it is actually really simple, but for some reason I just can't figure it out on paper! I probably wrote it wrong on here, but i hope that someone just gets the point.

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It has -(1 + e) which is the same as this -1(1 + e) which when you multiply -1 and 1 you get -1 + e which equals -e.

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Let me shorten the names of your varibles so that the form of the equation is easily seen:

V + I/Ma + I/Mb = -e * V

For this you simply move all terms without I to the right hand side

I/Ma + I/Mb = -e * V - V

Factor I out on the left. (We can simplify the right a little too by factoring out -V)

So: I (1/Ma + 1/Mb) = -V(1 + e)

We can divide both sides by that factor of ( 1/Ma + 1/Mb ) to isolate I

I = -V (1+e) / (1/Ma + 1/Mb)

It looks like the answer is a little different than you expected.

If you prefer we can simplify a little more by putting 1/Ma + 1/Mb into a single fraction by expressing it over a common denominator:

1/Ma + 1/Mb = (Ma + Mb)/(Ma*Mb)

So that:

I = -V (1+e) * (Ma*Mb)/(Ma + Mb)

Hope that helps.

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RelativeVelocity + Impulse/MassOfObjectA + Impulse/MassOfObjectB = -e * RelativeVelocity

RelativeVelocity + e * RelativeVelocity = - Impulse / ( MassofObjectA + MassofObjectB )

- Impulse = ( 1 + e ) * RelativeVelocity * ( MassofObjectA + MassofObjectB )

Impulse = - ( 1 + e ) * RelativeVelocity * ( MassofObjectA + MassofObjectB )

Simple math..

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Quote:
 Original post by mattnenterpriseIt has -(1 + e) which is the same as this -1(1 + e) which when you multiply -1 and 1 you get -1 + e which equals -e.

Hmm? -1 + e = -e? is that right!?

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Quote:
 Original post by Sleek...RelativeVelocity + e * RelativeVelocity = - Impulse / ( MassofObjectA + MassofObjectB )...Simple math..

This line is incorrect since I/Ma + I/Mb = I*(1/Ma + 1/Mb) = I * (Ma + Mb)/(Ma*Mb) which is not the same as I /(Ma + Mb), as you have written (try it with numbers). Remember that when adding fractions you must put them over a common denominator.

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Quote:
Original post by dustydoodoo
Quote:
 Original post by mattnenterpriseIt has -(1 + e) which is the same as this -1(1 + e) which when you multiply -1 and 1 you get -1 + e which equals -e.

Hmm? -1 + e = -e? is that right!?

No, it isn't. But what i think he was trying to say is that -(1+e) = -1(1+e) = -1 + -e, which is correct.

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Thanks Alfred! I would say thanks to the person that replied after you, but I was a little insulted by "Simple math.." I always seem to get offended when people use "..." I really need to get better at these kinds of things. I guess its because I don't practice much.

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Quote:
 Original post by dustydoodooThanks Alfred! I would say thanks to the person that replied after you, but I was a little insulted by "Simple math.." I always seem to get offended when people use "..." I really need to get better at these kinds of things. I guess its because I don't practice much.

No problem. Are you familiar with vector arithmetic? I took a look at the Chris Hecker article and you should watch out for the vectors that come up in these equations. Vab is a vector and Vab dot n gives the component of the velocity vector in the direction of n. You'll run into trouble treating them as pure numbers. Here's a little tutorial incase you're a bit rusty:

http://www.math.hmc.edu/calculus/tutorials/vectoranalysis/

Good luck.

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Quote:
Original post by AlfredR
Quote:
 Original post by dustydoodooThanks Alfred! I would say thanks to the person that replied after you, but I was a little insulted by "Simple math.." I always seem to get offended when people use "..." I really need to get better at these kinds of things. I guess its because I don't practice much.

No problem. Are you familiar with vector arithmetic? I took a look at the Chris Hecker article and you should watch out for the vectors that come up in these equations. Vab is a vector and Vab dot n gives the component of the velocity vector in the direction of n. You'll run into trouble treating them as pure numbers. Here's a little tutorial incase you're a bit rusty:

http://www.math.hmc.edu/calculus/tutorials/vectoranalysis/

Good luck.

Thanks for the link! I do know a little about vector math (cross and dot product, addition/subtraction) But I've yet to really do much with it. So having this to look to is excellent! thanks.

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