# Problem with vector additon
**URL:** https://boards.straightdope.com/t/problem-with-vector-additon/693042
**Category:** Factual Questions
**Created:** [July 14, 2014, 2:23am UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042 "2014-07-14T02:23:18Z")
**Posts on this page:** 7
**Page:** 2
### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [July 15, 2014, 3:20pm UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042/21 "2014-07-15T15:20:17Z")
A car hitting a wall is likely to be extremely inelastic, because even though the wall doesn’t deform, the car does. In fact, in practice, it’ll be quite close to perfectly inelastic: The car won’t bounce at all, but will come to rest relative to the wall and still in contact with it. If, however, we had a car-sized lump of wall material hitting our ideal wall, that would probably be very elastic: The lump would bounce off.
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### Author:  [@Isilder](https://boards.straightdope.com/u/Isilder)
#### Post date: [July 15, 2014, 11:15pm UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042/22 "2014-07-15T23:15:43Z")
> [@Chronos](#):
>
> If, however, we had a car-sized lump of wall material hitting our ideal wall, that would probably be very elastic: The lump would bounce off.
??? Which material ??? I’m confused.
A car sized lump of solid steel is very elastic …thats a wrecking ball.
solid steel an other metals are very elastic.( The sheet metal is easily crumpled)
Ceramics, very inelastic .
Wood, inbetween , eg archers bow…
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### Author:  [@Polar\_Iceman](https://boards.straightdope.com/u/Polar_Iceman)
#### Post date: [July 16, 2014, 12:14am UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042/23 "2014-07-16T00:14:44Z")
Irish:
Thanks for the link to momentum balance equations. I have been working through those, and they do seem to answer my question. Chronos’ two cents helped as well.
Now all I have to do is figure out how to explain it to the grandson, and make it seem like I knew it all along…!
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [July 16, 2014, 2:25am UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042/24 "2014-07-16T02:25:39Z")
> [@](#):
>
> Quoth **Isilder** :
>
> ??? Which material ??? I’m confused.
The material our hypothetical perfectly rigid indestructible walls are made out of. I don’t know what material it is, but very rigid materials tend to be very elastic.
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### Author:  [@EdwinAmi](https://boards.straightdope.com/u/EdwinAmi)
#### Post date: [July 16, 2014, 4:38am UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042/25 "2014-07-16T04:38:57Z")
> [@](#):
>
> but very rigid materials tend to be very elastic.
More specifically, the elastic response (going back to the original shape) happens much more rapdily than with bendy materials. This makes for good elastic collisions, unless the material breaks or cracks.
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### Author:  [@EdwinAmi](https://boards.straightdope.com/u/EdwinAmi)
#### Post date: [July 16, 2014, 4:43am UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042/26 "2014-07-16T04:43:51Z")
for the record, cars don’t work like that at all since we design them with crumple zones, and in general all that collision stuff they teach you is highly restricted in its actual scope compared to the complexity with of the real world, and isn’t used that often.
That being said, always just do the stupid m\*v calculations and whatever. Real world comparisons and explanations are harder; does the teacher even ask them? I can tell you that the example described is NOT like hitting a brick wall. If you calculate the momentary forces based on the change in speed in however much time it takes for the speed to change, you’ll find the forces are much greater for the slower driver than if he just hit a wall and stopped. Going 150 mph one way then suddenly 25 mph the other way (as the other poster calculated) is a much bigger momentum change and involves much greater accelerations/forces
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [July 16, 2014, 3:05pm UTC](https://boards.straightdope.com/t/problem-with-vector-additon/693042/27 "2014-07-16T15:05:52Z")
> [@](#):
>
> for the record, cars don’t work like that at all since we design them with crumple zones
Crumple zones mostly have the effect of making collisions inelastic. They do not change the fact that momentum and total energy are conserved. Those are true of everything, no matter how cleverly we design it.
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