# A car crashes into a concrete wall

1. Oct 21, 2004

### envscigrl

Here it is:
A 2000-kg car traveling at 85.0 km/h crashes into a concrete wall that does not give at all. Estimate the time of the collision, assuming that the center of the car travels halfway to the wall with constant deceleration. (Use 5.75 m for the length of the car.)

I know I need to use the equations of kinematic motion to find acceleration and then find time from that. But I am so confused as to what I use for delta x in the equations. I know it somehow involves the center of mass of the car but I dont know how. I have tried solving with just the lenghth of the car as it is given (5.75) and have also tried dividing it by two and four but neither worked. How do I solve this????

2. Oct 21, 2004

### Tide

If the decleration is constant then the starting speed is related to the distance required to stop by

$$v = \sqrt {2 a d}$$

from which you can work out the acceleration since you know the initial speed and the distance.