A boxcar traveling at 10 m/s approaches a string of three identical boxcars sitting stationary on the track. The moving boxcar collides and links with the stationary cars, and the four move off together along the track. What is the final speed of the four cars immediately after the collision? // If the boxcar, in the exercise above, instead bounces back with a speed of 1 m/s after the collision, find the speed of the three boxcars that were initially stationary