Having a vehicle get pushed backwards can be done by doing the following. We will be setting up the yellow vehicle to be pushed back.
1. Create a motion path for the yellow vehicle/
Diagram 1
2. Set the first event on the yellow vehicle's path to be a stop event for the duration of the
time before the collision. For instance if the blue vehicle that hits it has a
pre impact path 1.27 seconds long, have the stop event last for 1.27
seconds. (Diagram 2,3)
Diagram 2
Diagram 3
3. Select the path of the vehicle that is being pushed back (in this example the yellow car). For event
one, use the green ball to orientate the vehicle so it is facing the
way you wish. (Diagram 4)
Diagram 4
4. Select the end event. Again, use the green ball to orientate the yellow vehicle. (Diagram 5, 6, 7)
Diagram 5-7
Once you have done this, the yellow vehicle should remain at rest until the blue vehicle hits it, then appear to roll backwards. You can input the speeds and the stop times over on the left under the properties area.
If you find that the yellow vehicle has an unwanted spin, just ensure that the yaw direction is identical for both the start and the end event for the vehicle.