# Q about Bodies in Motion **URL:** \ **Category:** Factual Questions\ **Created:** [April 20, 2025, 11:05am UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254 "2025-04-20T11:05:37Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Horatio\_Hellpop](https://avatars.discourse-cdn.com/v4/letter/h/e5b9ba/32.png) [@Horatio\_Hellpop](https://boards.straightdope.com/u/Horatio_Hellpop)\ **Post date:** [April 20, 2025, 11:05am UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/1 "2025-04-20T11:05:37Z")
When two identical (or at least really similar) objects collide, and one is moving and the other is stationary, which is at greater danger of breaking? Does physics have a preference, or are they pretty much equally at risk of damage? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 20, 2025, 11:18am UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/2 "2025-04-20T11:18:09Z")
Which one is moving sounds symmetric to me; what is relevant will be the relative velocity, and the masses of the two objects, and their mechanical moduli. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [April 20, 2025, 11:45am UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/3 "2025-04-20T11:45:33Z")
There shouldn’t be any preference, unless the moving object was travelling at a speed that had already done structural damage to it along the way. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [April 20, 2025, 1:32pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/4 "2025-04-20T13:32:56Z")
If you’re talking truly uniform symmetrical things like e.g. billiard balls, then as both posters said above the situation is identical; both experience the same forces and “experience” the collision identically. And so have identical risk of deformation or breakup. If they are rotating at different rates that changes things slightly. If they’re not symmetrically shaped, even if identically shaped, that potentially changes things little or lots depending on how they’re oriented when they impact. etc. It only gets messier as we introduce more variation from the idealized case of uniform spherical e.g. billiard balls. But big picture, the idealized case represents the starting point from which all those variations in set-up induce deviations in outcome away from the idealized “identical”. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 20, 2025, 1:54pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/5 "2025-04-20T13:54:59Z")
> [@DPRK](#): > > Which one is moving sounds symmetric to me Would that depend on whether the moving object is accelerating? ---
**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:** [April 20, 2025, 2:09pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/6 "2025-04-20T14:09:01Z")
> [@Thudlow\_Boink](#): > > Would that depend on whether the moving object is accelerating? How would you apply a force to just one of the balls and keep the “sameness” property. E.g., if you ball is being pushed from behind by mechanical system, some sort of piston, then that ball is experiencing two _forces_ at the time of collision. (If you pull back the piston just before the collision then that’s the same as if the piston was never there and the ball was moving at collision speed all along.) Ditto any other force being applied that would cause the acceleration of just one ball while keeping the “sameness” property.. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [April 20, 2025, 2:11pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/7 "2025-04-20T14:11:16Z")
Mathematically, I doubt it. The kinetic energy equation has no factors that can be related to acceleration. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 20, 2025, 2:14pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/8 "2025-04-20T14:14:20Z")
> [@ftg](#): > > How would you apply a force to just one of the balls and keep the “sameness” property. I was looking at the OP, which didn’t say anything about balls or a “sameness” property, just two identical objects (which could be a moving car colliding with a stationary car, for example). ---
**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:** [April 20, 2025, 2:41pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/9 "2025-04-20T14:41:19Z")
> [@Thudlow\_Boink](#): > > (which could be a moving car colliding with a stationary car, for example). If those cars are floating in space, the argument applies. If they are sitting on a road then the one whose wheels are moving is clearly different than the one whose wheels are stationary. All replies so far have not interpreted that situation as being as applying to the OP. If the OP did mean to include such situations, then they would have needed to provide a _LOT_ more information about the scenario. E.g., what type of objects, what type of surface/whatever they are in contact with, and on and on in order to get plausible answers. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [April 20, 2025, 3:06pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/10 "2025-04-20T15:06:45Z")
Under the covers one can quickly get to questions where there is an intuitive sense of a universal frame of reference. This happens a lot when people start implicitly thinking that the Earth provides a special inertial frame everything is operating in. Then things stationary wrt the Earth become somehow special. We are a hair’s breadth from airplanes on treadmills. That we spend the vast majority of our lives dependant upon traction on the Earth’s surface for mobility has a lot to do with this misguided intuition. Flat Earther’s tend to take it a step worse. ---
**Author:** ![Horatio\_Hellpop](https://avatars.discourse-cdn.com/v4/letter/h/e5b9ba/32.png) [@Horatio\_Hellpop](https://boards.straightdope.com/u/Horatio_Hellpop)\ **Post date:** [April 20, 2025, 3:26pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/11 "2025-04-20T15:26:02Z")
Thudlow\_Boink > I was looking at the OP, which didn’t say anything about balls or a “sameness” property, just two identical objects (which could be a moving car colliding with a stationary car, for example). The question was inspired by the “straw through a tree in a hurricane” scenario, where the straw was going really fast and the tree of course was not, but these are profoundly dissimilar objects. ---
**Author:** ![Marvin\_the\_Martian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/marvin_the_martian/32/2898_2.png) [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)\ **Post date:** [April 20, 2025, 3:34pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/12 "2025-04-20T15:34:06Z")
When you say “stationary” do you mean “not moving but able to move” or “fixed in place”? ---
**Author:** ![yendis](https://avatars.discourse-cdn.com/v4/letter/y/ed8c4c/32.png) [@yendis](https://boards.straightdope.com/u/yendis)\ **Post date:** [April 20, 2025, 3:42pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/13 "2025-04-20T15:42:54Z")
> [@Horatio\_Hellpop](#): > > The question was inspired by the “straw through a tree in a hurricane” scenario, In which case the key factors are the materials making up the tree and straw, the shapes of the objects, and the force being applied at the point of contact. The dynamics that lead to the force are irrelevant to the calculations from this point. ---
**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:** [April 20, 2025, 10:33pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/14 "2025-04-20T22:33:31Z")
> [@ftg](#): > > If they are sitting on a road then the one whose wheels are moving is clearly different than the one whose wheels are stationary. Slightly, yes, though in an actual car collision, the difference will be very small. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 21, 2025, 1:27am UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/15 "2025-04-21T01:27:48Z")
> [@Horatio\_Hellpop](#): > > The question was inspired by the “straw through a tree in a hurricane” scenario, where the straw was going really fast and the tree of course was not, but these are profoundly dissimilar objects. It doesn’t matter if the straw is going at 200 mph or the tree. If the straw was more substantial (like a car), it would matter because the tree is rooted to the ground normally. But for a straw, the mass difference is so large that the tree hardly changes velocity when the straw hits it. It behaves the same as if it were anchored to the ground. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [April 25, 2025, 1:30pm UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/16 "2025-04-25T13:30:50Z")
But if the straw is moving at 200mph because of a hurricane, that tree is being affected by that hurricane. It’s probably bending pretty good, putting a lot of stress on the fibers. That may affect the outcome. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 26, 2025, 5:30am UTC](https://boards.straightdope.com/t/q-about-bodies-in-motion/1017254/17 "2025-04-26T05:30:23Z")
The exact degree of sphericity of our cow is important, yes.