# Homework Help: Car crash Problem

1. Jun 26, 2010

### Simo43

1. Can you be of any help, I need a simple (if that is possible) equation to help
me determine the weight increase of a 12 stone occupant of a car doing 30mph

2. Don't have a clue

3. This is a very general question to put into a slide show presentation on Road Traffic Accidents, you may have to dumb down a bit but any help is appreciated.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jun 26, 2010

### Uku

The traveling persons weight as such does not increase (talking non-relativistic speeds), rather his momentum.

Last edited: Jun 26, 2010
3. Jun 27, 2010

### Simo43

Thank you for that but its not what I meant, I realise that there are many forces which act on a body on impact I was looking for something to work out the max weight that will as you say with the momentum of a 30mph dead stop will tear out a passenger seat from its floor mountings, say rated to 50st max.

4. Jun 27, 2010

### Uku

Well, I have yet to see a car where the seatbelt is connected to the chair, usually (in European cars) I see them connected to the cage frame.
As I see it applied, there is usually a stopping distance. If you take the stopping distance to 0, then all the kinetic energy is applied to the person via the seatbelt (no work is done), and that is not the real case.
Check out the examples here:

http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html
http://hyperphysics.phy-astr.gsu.edu/hbase/carcr2.html#cc1

You can use the work equation:
$$W=\Delta E_{kin}=E_{kin2}-E_{kin1}=-E_{kin1}$$ since the car is stopped.

Now on the other hand
$$W=Fs \Rightarrow Fs=-E_{kin1} \Rightarrow F=\frac{-E_{kin1}}{s} \Rightarrow F=-\frac{mv^{2}}{2s}$$

But that is all explained with examples in the links provided.