State two components common to all forces. How are these indicated in a diagram?

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.)

Where is the potential energy throughout the loading, cocking, and releasing of the trebuchet?

Convection currents are caused by differences in what two things?