This is a 2 part question, and I'm fine on part 1. However, for background info, here is part 1: A motorcycle racer traveling at 145km/h loses control in a corner of the track and slides across the concrete surface. The combined mass of the rider is 243kg. The steel of the motorcycle rubs against the concrete road surface. (a) What is the fricional force between the road and the motorcycle and rider (714N) (b) What would be the acceleration of the motorcycle and rider during the wipeout (-2.94m/s^2) and (c) Assuming there were no barriers to stop the motorcycle and rider, how long would it take the bike and the rider to slow to a stop? (13.7s).

Question

This is the part I need help with:

The motorcycle and rider are sliding with the same acceleration as found above (-2.94m/s^2). If the motorcycle and rider have been sliding for 4.55s, what will be the force applied to the motorcycle and the rider when they strike the barrier and come to rest in another .530s? I have the answer (-1.23 x 104N), but have no idea how to arrive at the answer.

Any help would be greatly appreciated!