Mousetrap Car-Find Average Force of Snap-Please Help?

We were assigned a project in my high school physics class for which we had to build mousetrap cars and take data on their motion. We performed 12 trials in which we measured the time elapsed and distance traveled by the car before the mousetrap snapped, and the time elapsed and distance traveled after the snap until the car stopped. There was a string wound around the wheel which pushed the car forward until the mousetrap snapped.

We needed to calculate:
- Average velocity before snap
- Velocity at snap
- Average velocity after snap
- Acceleration before snap
- Acceleration after snap
- Average force during snap
- Coefficient of friction

*For the first trial, the total distance traveled was 27.7 meters, the distance traveled before the snap was 12.2 meters, the total time from start to end was 38.08s, and the time elapsed before the snap was 11.70s.

I found the average velocity before the snap like this:

#v=(Deltax)/(Deltat)=(12.2m)/(11.7s)=#

#=1.043m/s#

For the acceleration before the snap I used this kinematic equation and assumed that the initial velocity was zero:

#Deltax=v_oDeltat+1/2a(Deltat)^2#

Which became #a=(2(Deltax-v_oDeltat))/(Deltat)^2#

And I got

#a=(2(12.2m-0))/(11.7s)^2=0.178m/s^2#

For the velocity at the snap, I assumed that the initial velocity was zero and used this kinematic equation:

#v_f=v_i+aDeltat#

so #v_f=aDeltat#

#v_f=(0.178m/s^2)(11.7s)=2.09m/s#

But this is right before the mouse trap snaps. Here is where I get stuck!

I know that I can find the average velocity after the snap in a similar way, by subtracting the distance to the snap from the total distance.

#v=(27.7m-12.2m)/(38.08s-11.70s)=15.5/26.38=0.589m/s#

For acceleration after the snap, I assumed this would be the acceleration due only to friction slowing the car down after the mousetrap snaps. But how can I find this value if I don't know the initial velocity of the cart directly after the snap? And if I don't know that value or the time or distance for the actual snap, how can I find the average force applied by the mousetrap?