# Physics 101: Please settle a bar bet (collision physics) **URL:** \ **Category:** Factual Questions\ **Created:** [January 18, 2023, 10:09pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429 "2023-01-18T22:09:36Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [January 19, 2023, 1:14pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/41 "2023-01-19T13:14:07Z")
> [@Sitnam](#): > > I see now my error was in adding the ‘impenetrable’ part of the wall. All walls, even brick take SOME force. > > But you guys are right, wall or opposing-equally sized and sped car it’s the same, shit. > > Hopefully he forgets tomorrow. If you looked at the question, came to understand it and learned something and all the other guy got was a chance to be right, I’d say you won. ---
**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:** [January 19, 2023, 1:33pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/42 "2023-01-19T13:33:27Z")
Thanks for this, and to everyone else (including the Mythbusters video) who helped me to reach a better understanding of how this works. At least I caveated my first attempt. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 2:06pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/43 "2023-01-19T14:06:33Z")
> [@Sitnam](#): > > But you guys are right, wall or opposing-equally sized and sped car it’s the same, shit. It’s the same concerning the end result for car A, but it still is different wrt the question as you posted it in the OP: “At the point of impact, are the forces the same?” The force _on car A_ is the same, _the total force at the point between car A and car B_ is double that, as it is the force on car A and the force on car B. Maybe the question to ask is easier to visualize if we look at the total energy balance instead of the force. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 19, 2023, 4:58pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/44 "2023-01-19T16:58:04Z")
> [@Pardel-Lux](#): > > The force _on car A_ is the same, _the total force at the point between car A and car B_ is double that, as it is the force on car A and the force on car B. The forces on car A and car B are equal and opposite forces; there is nothing being doubled. If car A experiences an impact force of 60,000 pounds and car B experiences an impact force of 60,000 pounds, a force gauge placed between them at the impact point will measure a force of 60,000 pounds. > [@Pardel-Lux](#): > > Maybe the question to ask is easier to visualize if we look at the total energy balance instead of the force. the total energy for two cars colliding head-on is twice the energy of one car colliding with a stationary immovable object. But since there are two cars, there is twice as much crumple zone. The crumple zone on each car will deform the same amount as it would for a single-car-versus-immovable-object scenario, and the force will be the same. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 5:12pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/45 "2023-01-19T17:12:31Z")
> [@Machine\_Elf](#): > > But since there are two cars, there is twice as much crumple zone. The crumple zone on each car will deform the same amount as it would for a single-car-versus-immovable-object scenario, and the force will be the same. Isn’t that what I wrote? Right in the second post in this thread? And in the 12th, and in the 16th, and in the 20th, and in the 26th… Of course the force will be the same _for each car_, so double in total. But the work this force has to do is also double: there are two cars to crumple. Where do you think are we in disagreement? I think @am77494 suggested a very good way to visualize that with his diminishing wall in post 34. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 19, 2023, 5:22pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/46 "2023-01-19T17:22:01Z")
> [@Pardel-Lux](#): > > Of course the force will be the same _for each car_, so double in total. It’s the “double in total” that I think is wrong. If I drop one ball, it experiences the force of gravity. If I drop two balls, they both experience the force or gravity. Has the force of gravity “doubled in total”? ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 5:31pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/47 "2023-01-19T17:31:20Z")
The force that the gravity potential exercises on two balls is double the force that it exercises on one ball, as per F = G (m1 m2)/r2, if m2 is Earth, then m1 will be one ball or two balls, two balls weighing double as much as one ball, the force exercised on them by the same gravity potential will double. No need to double the “force” of gravity, it doubles the potential energy if you double the balls, and it doubles the “force” they liberate if dropped a distance d (potential energy → kinetic energy. So yes, it is perhaps better to speak about energy than force). ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 19, 2023, 6:15pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/48 "2023-01-19T18:15:57Z")
> [@Pardel-Lux](#): > > Where do you think are we in disagreement? You said there is twice the force. There is not. If a car pushes against an immovable, undeformable wall with X pounds of force, the wall pushes back with X pounds of force. If a car pushes against an oncoming car with X pounds of force, the other car pushes back with X pounds of force. There’s no “doubling” of force going on. A load cell placed at the plane of collision in either of the above scenarios will record X pounds of force. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [January 19, 2023, 6:17pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/49 "2023-01-19T18:17:17Z")
It always annoys me that in news reports or other descriptions of head-on collisions that commentators talk about a “combined impact of (the two speeds added together)” The result of such collisions is that the occupants of the vehicles are subjected to sudden deceleration. The impact of that will depend largely on _their_ speed and the design of their vehicle. Modern cars are capable of absorbing a considerable amount of energy without too much deformation of the passenger compartment. This is why we commonly see people walking away from destroyed cars with minor injuries, ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 19, 2023, 6:20pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/50 "2023-01-19T18:20:02Z")
> [@bob\_2](#): > > It always annoys me that in news reports or other descriptions of head-on collisions that commentators talk about a “combined impact of (the two speeds added together)” This is something where I’m not sure you can be too harsh on the journalists, unlike they purport to be a science correspondent. I would guess that \>99% of people don’t understand this - it’s quite counterintuitive. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 6:20pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/51 "2023-01-19T18:20:05Z")
There is double the work done by the double amount of energy: two cars are crumpled here, only one there. Where does the energy come from to do this crumpling? ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 19, 2023, 6:24pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/52 "2023-01-19T18:24:27Z")
Each car brings enough energy to crumple its own crumple zone. One car, one wall, one crumple zone, 0.5_m_V2. crumple zone experiences force F, deforms by distance d, total energy absorbed = F \* d = 0.5_m_V2. Two cars, two crumple zones, 2 \* 0.5_m_V2, so each crumple zone absorbs 0.5_m_V2, same as in the first scenario, so each crumple zone experiences force F, deforms by distance d, total energy absorbed = F \* d = 0.5_m_V2. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 6:26pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/53 "2023-01-19T18:26:17Z")
I give up. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 19, 2023, 7:05pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/54 "2023-01-19T19:05:21Z")
> [@Pardel-Lux](#): > > I give up. Would it help if I clarified my previous post? What I originally said: > [@Machine\_Elf](#): > > Two cars, two crumple zones, 2 \* 0.5_m_V2, so each crumple zone absorbs 0.5_m_V2, same as in the first scenario, so each crumple zone experiences force F, deforms by distance d, total energy absorbed = F \* d = 0.5_m_V2. That last bit should have read “total energy absorbed **by each crumple zone** = F \* d = 0.5_m_V2.” Other than the question of whether the force is doubled, can you point to which of my assertions you disagree with? ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 7:26pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/55 "2023-01-19T19:26:32Z")
**By each crumple zone** makes all the difference. As there are two crumple zones in two cars but only one in the wall case the energy is doubled. And this energy, which does the crumpling (work), is mediated by forces. Is it not? Let me try a last example, a different one, a thought experiment. First case: a meteorite hits planet Earth. It has a mass of 100 metric tons and a velocity of, say, 10 km/s at the moment of impact (round numbers for convenience). When it hits earth it has enough force/speed/energy/whatever-you-call-it to vaporize 1 km³ of rock in the Earth crust. Scientists calculated afterwards that the energy the meteorite released on impact was equivalent to 10,000 hiroshima bombs. Second case: the meteorite just misses planet Earth, but when it has been accelerated by Earth’s gravitiy to 10 km/s tangentially to the surface of the planet, just above the atmosphere, by an incredible coincidence it collides head on with another meteorite which also has a mass of 100 metric tons and happens to be travelling at 10 km/s in exactly the opposite direction. Scientists knew of this event in advance and could set up all instruments money could buy to measure this extraordinary clash. When they measure the energy output of the perfect smashing of two meteorites, what is the energy liberated in hiroshima bomb equivalents they will measure? 10,000 or 20,000? Needless to say, in this example one meteorite against the Earth is one car against an immovable wall and two meteorites are two cars. And I claim the energy released would be 20,000 hiroshima bombs. Because the “m” in E = 1/2 mv2 has doubled. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 19, 2023, 7:40pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/56 "2023-01-19T19:40:16Z")
> [@Machine\_Elf](#): > > …can you point to which of my assertions you disagree with? > [@Pardel-Lux](#): > > Let me try a last example, a different one, a thought experiment. I’m not sure your example will help, because I’m fairly sure that there is no confusion or dispute here about the underlying physics. Force is an extensive property, so one could make an argument defending @Pardel-Lux that it’s _logical_ that when the same force is applied to two different things and twice as much work is being done it sort of makes sense to say that the force has “doubled in total”. But we just don’t say it that way because it’s confusing. It obviously leads to confusion about whether the _magnitude_ of the force has doubled (which we all agree hasn’t happened here). And it can be even more misleading and confusing when we’re just talking about action and reaction. This is not even a situation like dropping two identical balls vs dropping one ball, where we could maybe say “twice as much force is acting” without being misunderstood, although “the same force is acting on two balls” would be much clearer. In this case we’re just talking about action and reaction, which is _always_ present. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 8:49pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/57 "2023-01-19T20:49:36Z")
This is perfect with me, but what shall we tell the OP? I would suggest we say the question was not clear enough to have a definitive answer, it is undecidable as is. Not undecidable in the mathematical sense, but undecidable in the sense that it would not be accepted as a question in a school exam. Depending on the interpretation of the concepts involved both points of view are valid enough to consider he bet void. Let’s see if that settles it. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 19, 2023, 8:51pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/58 "2023-01-19T20:51:50Z")
I think the answer to the OP is that at the point of impact, the forces are the same. The action-reaction force between car and wall is the same as the action-reaction force between car and car. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 19, 2023, 9:04pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/59 "2023-01-19T21:04:13Z")
The only thing that gives me pause about whether you have the physics right is this thing you said: > [@Pardel-Lux](#): > > But in the second case, the _point of impact_ will experience itself the double amount of force, as it will be compressed from both sides. Did you later acknowledge that that this is wrong? If you dangle something small in between (small meaning that the amount of energy it absorbs is negligible) it will experience the same forces and the same fate in both cases. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [January 19, 2023, 9:11pm UTC](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429/60 "2023-01-19T21:11:07Z")
No, I have not acknowledged it, I drew a veil of silence over it and looked the other way hoping nobody would come back to mention it. But if you force me to, I am ready to acknowledge that by virtue of actio = reactio that was badly worded. [Previous page](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429.md?page=2) [Next page](https://boards.straightdope.com/t/physics-101-please-settle-a-bar-bet-collision-physics/978429.md?page=4)