Which of these statements best explains why models are used to study atoms

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.)

In science and physics what is the standard unit of measure for speed?

In an experiment, how can i change human errors? be specific.