# Car crash force

1. Nov 9, 2005

### vamp00

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?

2. Nov 10, 2005

### Staff: Mentor

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.

3. Nov 10, 2005

### vamp00

What would it be if you took out the crumple variable?

4. Nov 10, 2005

### Staff: Mentor

Infinite....

5. Nov 10, 2005

### vamp00

infinite??

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

6. Nov 10, 2005

### Stingray

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.

7. Nov 10, 2005

### vamp00

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? Im 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...

8. Nov 10, 2005

### pmb_phy

If you know the accleration 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

9. Nov 10, 2005

### Staff: Mentor

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.