# Physics-related question involving car crash scenario **URL:** \ **Category:** Factual Questions\ **Created:** [October 1, 2022, 1:51am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528 "2022-10-01T01:51:24Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [October 2, 2022, 10:24am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/21 "2022-10-02T10:24:03Z")
> [@Snipe70e](#): > > A car traveling at 50mph striking another car of equal mass and design would be the same as one car hitting a solid brick wall. No, two cars have between them two crumple zones. One car and one solid brick wall have one crumple zone. > [@Snipe70e](#): > > The 25 mph car is going to stop in an instant and start to roll back wards. From 25mph which is less damaging. The other car is going to stop from 75mph over a longer period but from a higher speed. It all works out the same. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 2, 2022, 10:46am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/22 "2022-10-02T10:46:16Z")
> [@Princhester](#): > > > [@Snipe70e](#): > > > > A car traveling at 50mph striking another car of equal mass and design would be the same as one car hitting a solid brick wall. > > No, two cars have between them two crumple zones. One car and one solid brick wall have one crumple zone. I agree with Snipe, if the question is what the effect feels like for the one car. Two crumple zones, yes, but each is crumpling around their meeting point, which is the point where a single car meets the wall. ---
**Author:** ![Danger\_Man](https://avatars.discourse-cdn.com/v4/letter/d/6de8d8/32.png) [@Danger\_Man](https://boards.straightdope.com/u/Danger_Man)\ **Post date:** [October 2, 2022, 10:46am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/23 "2022-10-02T10:46:49Z")
> No, two cars have between them two crumple zones. One car and one solid brick wall have one crumple zone. They have two crumple zones, but those two crumple zones have to be divided between the two cars. So assuming identical cars and speeds, it’s like each car hitting a solid wall. ---
**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:** [October 2, 2022, 10:48am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/24 "2022-10-02T10:48:39Z")
> [@Sam\_Stone](#): > > I’m just saying that while momentum is conserved, felt impulse may be different. > [@Sam\_Stone](#): > > The deceleration impulse will be less in the faster car than in the slower one, right? Assuming both cars have the same mass, in your 50 mph vs 50 mph scenario both cars go from 50 to 0 mph, the difference is 50. In the 75 mph vs the 25 mph scenario one car goes from 75 to 25 mph (difference: 50 mph) and the other goes from 25 mph to -25 mph (difference: 50 mph). To be precise: in the first example one of the cars goes from -50 mph to 0 mph, in the second one of the cars goes from -25 to +25 mph. Kinetic energy is conserved if you consider it negative when it goes in the opposite direction. ---
**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:** [October 2, 2022, 12:04pm UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/25 "2022-10-02T12:04:50Z")
> [@Sam\_Stone](#): > > Impulse is the integral of change in momentum _over time_. > > If you hit a brick wall and it takes you .5 seconds to stop, the impulse is much higher than if it takes you 1 second to stop. The change in momentum is the same. Impulse is the integral of force over time. Force is the derivative of momentum with respect to time. By the Fundamental Theorem of Calculus, then, impulse is the change in momentum. If you hit a brick wall and it takes you half a second to stop, that’s a greater force than if it takes you one second, but the same impulse. > [@Snipe70e](#): > > Lets go simple. A car traveling at 50mph striking another car of equal mass and design would be the same as one car hitting a solid brick wall. When both cars hit they should stop at the point of impact with a little bounce back. > > Now one car going 75 mph head on to another going 25 mph. The 75 mph car is going stop over a longer time and is going to push the slower car backwards. The 25 mph car is going to stop in an instant and start to roll back wards. Ther will be a greater change of velocity. more deacceleration. In the brick wall example (or in the head-on, equal speed example), the car will go from 50 MPH to 0 MPH, a change of 50 MPH. In the head-on example with one at +75 MPH and the other at -25 MPH, the fast car will go from +75 MPH to +25 MPH, and the slow car will go from -25 MPH to +25 MPH, which is still a change of 50 MPH for both. > [@Snipe70e](#): > > A test. take two balls the same mass each tied to a string. One stationary the 2nd ball pulled back and let swing into the 1st ball what happens. Now make one of the balls 1/2 the mass and repeat. I’m not sure what this test is supposed to illustrate, since all of the cars we’re considering are assumed to be the same mass. > [@Kedikat](#): > > Kinetic energy. > If two identical objects collide head on at different velocities, one has more kinetic energy. > A structure has mass. Speed adds kinetic energy to that mass. > So the object going at lower speed has less kinetic energy at every point of its mass. At any point of contact, the equal mass with lower kinetic energy, will deform more in response to the same mass with more kinetic energy. I think… > The two objects are not equal in all aspects of energy. How much kinetic energy each car has, and which one has more, depends on your frame of reference, but how much damage a car takes will be independent of frame of reference. The damage a car takes cannot, therefore, depend on which car has the greater kinetic energy. > [@Pardel-Lux](#): > > Kinetic energy is conserved if you consider it negative when it goes in the opposite direction. This is exactly wrong. Kinetic energy is never negative. Momentum, however, can be. ---
**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:** [October 2, 2022, 12:20pm UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/26 "2022-10-02T12:20:13Z")
Yes, that is what I should have written. The change in momentum is the same (but opposite) for both cars, and that is what kills the drivers. Whether one car is accelerating or decelerating does not change the outcome. It’s the speed change at the moment of impact and immediately afterwards, not immediately before, that counts. If Suicide Girl survives and the other driver does not it will not be due to braking or accelerating before impact. It will be due to better/worse airbags and safety belts. And the whims of the writer who spun this story. ---
**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:** [October 2, 2022, 12:28pm UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/27 "2022-10-02T12:28:25Z")
A simple problem with the original story is that it implies that the faster you drive the safer you are if you plough into another car. Avoid dying in a car crash with this one weird trick. ---
**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:** [October 2, 2022, 9:32pm UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/28 "2022-10-02T21:32:49Z")
> [@Chronos](#): > > Kinetic energy is never negative. “Assume the car is traveling at an imaginary velocity…” ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [October 3, 2022, 11:32am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/29 "2022-10-03T11:32:19Z")
Could a car traveling at imaginary velocity collide with a car traveling at real velocity? ---
**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:** [October 3, 2022, 11:37am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/30 "2022-10-03T11:37:45Z")
They would be out of phase with one another so would pass though one another. At least that’s how I would imagine it. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [October 3, 2022, 12:37pm UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/31 "2022-10-03T12:37:14Z")
I’m with Chronos, the forces of acceleration or braking would add/subtract from the collision forces, but are negligible in comparison. The real benefit/downside is that braking/accelerating changes the relative velocity at impact, that is the thing that will save you or kill you, not which way you’re accelerating at the moment of impact. This idea is like jumping just before your falling elevator hits the ground. ---
**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:** [October 4, 2022, 1:00am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/32 "2022-10-04T01:00:08Z")
> [@Francis\_Vaughan](#): > > A simple problem with the original story is that it implies that the faster you drive the safer you are if you plough into another car. One thing that _is_ true is that the more massive your car is, the safer you are if you plow into another car (all other things being equal). And the less safe is the person they hit. Unfortunately this causes many selfish morons to buy huge behemoths because they are safer–for them. ---
**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:** [October 4, 2022, 1:03am UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/33 "2022-10-04T01:03:19Z")
> [@Sam\_Stone](#): > > Also, if two cars are going 50 mph each and collide, they both come to rest and momentum is conserved. But kinetic energy is not - kinetic energy goes to zero, and it is converted to heat, deformation of materials, noise, damage to the bodies, etc. My first job was software development for crash test analysis for Ford. The engineers said that vehicle and occupant damage was proportional to the change in kinetic energy. The crash tests there were one car into a barrier; we didn’t test two cars crashing where the COG of the system still had some momentum after the crash. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 9, 2022, 6:14pm UTC](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528/34 "2022-10-09T18:14:50Z")
> [@Francis\_Vaughan](#): > > Avoid dying in a car crash with this one weird trick. It’s genius! (click here to see why) [Previous page](https://boards.straightdope.com/t/physics-related-question-involving-car-crash-scenario/972528.md?page=1)