Estimating Car Crash Force: Can We Determine Impact in LBS?

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Discussion Overview

The discussion revolves around estimating the force of a car crash, specifically how to calculate the impact force in pounds when a vehicle collides with a stationary object. Participants explore various scenarios, including different speeds and the effects of vehicle crumple zones, while considering the complexities involved in such calculations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests estimating force by assuming constant deceleration and calculating based on the distance the car is crushed.
  • Another participant questions the impact force if the crumple variable is removed, leading to a discussion about the implications of infinite force in theoretical scenarios.
  • Concerns are raised about the unrealistic nature of instantaneous stops and the non-uniform deceleration experienced during a crash.
  • Several participants emphasize the importance of various factors, such as the state of the stationary vehicle (in park or neutral) and the specifics of the collision, in determining the force required to move another vehicle.
  • One participant mentions that if the stationary vehicle is in neutral, it may take very little force to move it, while being in park would require consideration of additional factors like friction and bumper energy absorption.
  • There is a suggestion that the smaller vehicle may not need to exceed 10 mph to move a larger stationary vehicle, highlighting the variability in required force based on conditions.

Areas of Agreement / Disagreement

Participants express differing views on how to accurately estimate crash forces, with no consensus reached on a definitive method or outcome. The discussion remains unresolved with multiple competing perspectives on the factors influencing the calculations.

Contextual Notes

The discussion highlights limitations in assumptions regarding deceleration, the impact of vehicle design on force calculations, and the variability introduced by different collision scenarios.

vamp00
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How would you accurately estimate the force of a car crash? Say, a 2,500lb vehicle has an impact with a wall @ 30mph.. How would you accurately estimate the force in LBS, Is it possible?
 
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The only easy way is by assuming constant decelration and calculating the deceleration based on how much the car got crushed - ie, the distance it took to stop.
 
What would it be if you took out the crumple variable?
 
infinite??Ok, let's drop the speed down to 5mph, and instant stop on a wall, vehicle no longer accelerating.
 
vamp00 said:
infinite??


Ok, let's drop the speed down to 5mph, and instant stop on a wall, vehicle no longer accelerating.

Infinite is infinite. It doesn't matter what speed you start from. Changing speeds instantly is impossible.

And in reality, the deceleration is not at all uniform. Some of the crash test databases online list the decelerations of various parts of a dummy's body for each car. I think that they test head-on crashes into a wall at 35 mph. Loads on various parts of the body are usually between 40-150 g for that type of collision.
 
I wasn't trying to get G's, I could probably find that info from accelerometer data from some crash test site... What I want to know is the force IN LBS, if that is possible. E.G: 2,500lb vehicle hits a 10,000lb vehicle, how fast would it have to be moving to move the 10,000lb vehicle? Is there no scientific way to "estimate" this? I am guessing there is a lot of variables that come into play, like if the other vehicle is in park, where it was hit, how many tires it has, etc...
 
vamp00 said:
I wasn't trying to get G's, I could probably find that info from accelerometer data from some crash test site... What I want to know is the force IN LBS, if that is possible. E.G: 2,500lb vehicle hits a 10,000lb vehicle, how fast would it have to be moving to move the 10,000lb vehicle? Is there no scientific way to "estimate" this? I am guessing there is a lot of variables that come into play, like if the other vehicle is in park, where it was hit, how many tires it has, etc...
If you know the acceleration then you know the force through F = ma. Note that the entire car cannot stop all at once. The rear end of the car will not deaccelerate at the same rate as the front of the car.

Ask yourself this - Take a 1 ton steel square brick and slam it into a massive flat wall of steel. What do you think the force exerted on the steel object will be?

Force = time rate of change of momentum. To determine the force you must take the initial momentum and subtract the final momentum and divide that by the time it took for the momentum to make that change. You seem to have in your mind that this time is zero and the change in momentum is not zero. This is why Russ is telling you that the force is infinite.

Pete
 
vamp00 said:
I wasn't trying to get G's, I could probably find that info from accelerometer data from some crash test site... What I want to know is the force IN LBS, if that is possible. E.G: 2,500lb vehicle hits a 10,000lb vehicle, how fast would it have to be moving to move the 10,000lb vehicle? Is there no scientific way to "estimate" this? I am guessing there is a lot of variables that come into play, like if the other vehicle is in park, where it was hit, how many tires it has, etc...
If the stationary vehicle is in neutral, then it takes very little force - perhaps 100lb (people push cars all the time). If the stationary vehicle is in park, then you have to know how much the stationary car can rock back in forth without skidding and how much energy the bumper can absorb (by law, a 5mph collison with a stationary object, with no damage). Remember that ultimately the amount of force required to move a car, if that force is constant, is simply equal to (well, just above) the friction force at the tires.

For this type of problem, those variables make all the difference. I doubt the smaller car, in this case, would need to be going more than 10mph to move the bigger one.
 

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