What units for time must be used in calculating power

Aboxcar traveling at 12 m/s approaches a string of 5 identical boxcars sitting stationary on the track. the moving boxcar collides and links with the stationary cars and they all move off together along the track. what is the final speed of the cars immediately after the collision? (you may take the mass of each boxcar to be 18,537 kg.)

J. j. thomson’s experiment disproved the theory that an atom

Select the correct answer. what does the process of natural selection involve?