No, two cars have between them two crumple zones. One car and one solid brick wall have one crumple zone.
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.
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.
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.
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.
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.
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.
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.
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.
This is exactly wrong. Kinetic energy is never negative. Momentum, however, can be.
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.
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.
“Assume the car is traveling at an imaginary velocity…”
Could a car traveling at imaginary velocity collide with a car traveling at real velocity?
They would be out of phase with one another so would pass though one another.
At least that’s how I would imagine it.
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.
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.
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.
It’s genius! (click here to see why)