# Basic Mechanics Question and Some Mythbusters **URL:** \ **Category:** Factual Questions\ **Created:** [March 17, 2014, 8:22pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945 "2014-03-17T20:22:04Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 19, 2014, 11:13pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/21 "2014-03-19T23:13:40Z")
Another potentially-relevant asymmetry is that the steering column is on one side, and might be less compressible than the rest of the crumple zone. If that’s the case, then you should favor the head-on collision. When we say “identical car”, we really ought to say “mirror-imaged car”. Suppose that the other car is the across-the-pond version of your same model. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [March 20, 2014, 7:19am UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/22 "2014-03-20T07:19:38Z")
I read this thread and find it of interest that the swords are treated as point objects or the angular momentum is being neglected. I would have modeled the two swords as rigid long objects, with fixed ends (pivots), swung in two different planes and intersecting at somewhere in the middle of the swords. Now real swords are curved , the weight distributed near the tips - all for the rotational inertia to be a certain way. I am not sure if the bending moment experienced in the two cases (one moving and the other stationary, both moving) when the swords intersect in the middle is the same. Also, I am not sure if the bending forces and the shear forces are directionally the same in both cases. Since the sword is not a round bar with weight uniformly distributed. I am thinking of a H-Beam where it has different bending characteristics depending on the direction of the bending moment. ---
**Author:** ![Dead\_Cat](https://avatars.discourse-cdn.com/v4/letter/d/9fc29f/32.png) [@Dead\_Cat](https://boards.straightdope.com/u/Dead_Cat)\ **Post date:** [March 29, 2014, 1:54pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/23 "2014-03-29T13:54:23Z")
> [@Chronos](#): > > As well you should. You should, however, have no preference between the 25 MPH head-on and a 25 MPH collision with a wall. Now you’ve really confused me! But I think I get it - the fact that the closing speed in the first case (25mph head-on with a mirror-image car also travelling at 25mph) is twice that in the second case (25mph collision with solid wall) is irrelevant because in the first case, the other car absorbs half the energy of the impact (whereas the theoretical perfectly solid wall absorbs no energy). Do I have it right now? It just seems very counter-intuitive. Sorry I didn’t come back to this at the time but I hope no-one minds if I resurrect this fortnight-old thread. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [March 29, 2014, 2:03pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/24 "2014-03-29T14:03:36Z")
> [@Dead\_Cat](#): > > Now you’ve really confused me! But I think I get it - the fact that the closing speed in the first case (25mph head-on with a mirror-image car also travelling at 25mph) is twice that in the second case (25mph collision with solid wall) is irrelevant because in the first case, the other car absorbs half the energy of the impact (whereas the theoretical perfectly solid wall absorbs no energy). Do I have it right now? It just seems very counter-intuitive. > > Sorry I didn’t come back to this at the time but I hope no-one minds if I resurrect this fortnight-old thread. Yes, you’ve got it. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [March 29, 2014, 6:25pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/25 "2014-03-29T18:25:59Z")
> [@Chronos](#): > > […] > When we say “identical car”, we really ought to say “mirror-imaged car”. Suppose that the other car is the across-the-pond version of your same model. Chronos, you genius you, this fixes everything if you carry it just a tiny bit further. Let’s epoxy a very durable mirror to an enormous stiff massive block of tungsten, and watch the collision. I think that if the mirror remains sufficiently clean and flat and perfect, you can’t tell whether our car is hitting another one exactly like it in the transposed drive- on - the - other - side - of - the - road sense, or a shiny wall. You can’t tell by watching, or by listening, or with your magical telescopic stress and strain gage. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [March 29, 2014, 7:03pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/26 "2014-03-29T19:03:15Z")
> [@Napier](#): > > You can’t tell by watching, or by listening, or with your magical telescopic stress and strain gage. There would be twice as much noise with two cars. (Including before the crash as well.:)) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 29, 2014, 7:59pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/27 "2014-03-29T19:59:58Z")
There might be less total noise hitting the mirror, but you’d still hear the same amount, since you’d also get noise reflected back at you (assuming that the perfect mirror reflects sound as perfectly as it does light). ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [March 29, 2014, 10:10pm UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/28 "2014-03-29T22:10:49Z")
> [@Dead\_Cat](#): > > It just seems very counter-intuitive. Physics can be counter-intuitive. That’s why you have to go to school for it. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [March 30, 2014, 2:52am UTC](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945/29 "2014-03-30T02:52:22Z")
> [@Chronos](#): > > There might be less total noise hitting the mirror, but you’d still hear the same amount, since you’d also get noise reflected back at you (assuming that the perfect mirror reflects sound as perfectly as it does light). This. See? Chronos is looking at this thing perfectly! [Previous page](https://boards.straightdope.com/t/basic-mechanics-question-and-some-mythbusters/683945.md?page=1)