# Coefficient of Restitution - two balls collide off-center **URL:** \ **Category:** Factual Questions\ **Created:** [February 10, 2019, 9:34pm UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303 "2019-02-10T21:34:34Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Patch](https://avatars.discourse-cdn.com/v4/letter/p/a183cd/32.png) [@Patch](https://boards.straightdope.com/u/Patch)\ **Post date:** [February 10, 2019, 9:34pm UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/1 "2019-02-10T21:34:34Z")
Not sure where to post this, but giving here a try. This has been a while for me, and I;m trying to see if I have this correct. Two balls headed towards each other, but rather than colliding in a direct impact (as all my textbooks seems to have them doing), they collide a bit off center. Thus, each ball rebounds at an angle from their original path of travel. As I understand it, to work through their departure velocities if we have a coefficient of restitution, _e_, we would look at the point of collision, and determine the normal line between the two balls. The velocity along the normal line is what we would be using in our CoR equation, as CoR is valid along a single line of action. Is that correct? ---
**Author:** ![DorkVader](https://avatars.discourse-cdn.com/v4/letter/d/73ab20/32.png) [@DorkVader](https://boards.straightdope.com/u/DorkVader)\ **Post date:** [February 10, 2019, 11:41pm UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/2 "2019-02-10T23:41:39Z")
Didn’t I read something somewhere about homework? It’s been a while, might have changed ---
**Author:** ![Patch](https://avatars.discourse-cdn.com/v4/letter/p/a183cd/32.png) [@Patch](https://boards.straightdope.com/u/Patch)\ **Post date:** [February 11, 2019, 12:34am UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/3 "2019-02-11T00:34:00Z")
> [@guestchaz](#): > > Didn’t I read something somewhere about homework? It’s been a while, might have changed Not homework. I’m trying to understand the concept in a bit of self-improvement, and am unclear on the concept. ---
**Author:** ![Patch](https://avatars.discourse-cdn.com/v4/letter/p/a183cd/32.png) [@Patch](https://boards.straightdope.com/u/Patch)\ **Post date:** [February 11, 2019, 12:41am UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/4 "2019-02-11T00:41:34Z")
What’s throwing me is if you read about CoR, the examples discuss a ball dropped on the ground, or two objects moving directly toward each other, but on the Wikipedia page it shows a basketball bouncing with forward motion, which isn’t discussed. So I don’t know if I’m missing something in the language in how it’s presented, and then I got thinking about something more complicated, such as I described, and thought I’d poke the internet for help. I think I’m right, but it would be nice to know. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [February 11, 2019, 1:07am UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/5 "2019-02-11T01:07:07Z")
> [@Patch](#): > > Not sure where to post this, but giving here a try. > > This has been a while for me, and I;m trying to see if I have this correct. > > Two balls headed towards each other, but rather than colliding in a direct impact (as all my textbooks seems to have them doing), they collide a bit off center. Thus, each ball rebounds at an angle from their original path of travel. > > As I understand it, to work through their departure velocities if we have a coefficient of restitution, _e_, we would look at the point of collision, and determine the normal line between the two balls. The velocity along the normal line is what we would be using in our CoR equation, as CoR is valid along a single line of action. > > Is that correct? Yes. That does, of course, assume a perfect ball. Which is, of course, slightly in conflict with the idea of a coefficient of restitution. If a real ball deforms, you’ll also get some spin, like a tennis ball when it bounces in play. ---
**Author:** ![Patch](https://avatars.discourse-cdn.com/v4/letter/p/a183cd/32.png) [@Patch](https://boards.straightdope.com/u/Patch)\ **Post date:** [February 11, 2019, 1:43am UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/6 "2019-02-11T01:43:54Z")
> [@Melbourne](#): > > Yes. That does, of course, assume a perfect ball. Which is, of course, slightly in conflict with the idea of a coefficient of restitution. If a real ball deforms, you’ll also get some spin, like a tennis ball when it bounces in play. Or like English on a pool ball? At least I got the basics right. 🙂 ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [February 13, 2019, 1:30am UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/7 "2019-02-13T01:30:51Z")
When my math lecturer was demonstrating coefficient of restitution, he used two balls. But the better ball wasn’t much better than the worse ball, so he picked a blackboard duster off the desk to demonstrate something with really poor coefficient or restitution. Just by chance, the entire room full of ~100 people was watching that blackboard duster when it hit the desk and the touch powder it had been loaded with by a couple of students went off with an impressive bang… ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [February 13, 2019, 4:25am UTC](https://boards.straightdope.com/t/coefficient-of-restitution-two-balls-collide-off-center/829303/8 "2019-02-13T04:25:24Z")
**Moderator Action** > [@Patch](#): > > Not sure where to post this, but giving here a try. This is factual, so GQ is a better choice. Moving thread from MPSIMS to GQ.