Internal Energy

(Illustration)

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In the above animation, a red 80-kg ball with an initial kinetic energy of 360 J is trapped inside a box with rigid walls containing a cylinder constructed of 80 small 1-kg spheres (position is given in meters and time is given in seconds).  The ball crashes into the cylinder and breaks it apart.  The bar graph displays the kinetic energy and the table displays the time momentum, and kinetic energy of the red ball.

This animation is meant to simulate a collision between two solid objects, one of which is stationary.  The stationary object is a loose collection of smaller objects and approximates a larger solid object.  This is only an approximation as this object should stay together, not break apart. (When you do collision experiments in lab the objects colliding do not usually deform this much!)  Nevertheless, we can learn a lot from this simulation.  As the red ball hits the blue object, the blue object deforms absorbing some of the red ball's kinetic energy and momentum.  If the blue object was indeed solid, the deformed object---the entire object---would move to the right.    We can imagine this by considering the average motion of the small blue balls which make up the larger solid object.  We note that the general motion if these blue balls is to the right.  Where does all of the initial kinetic energy go?  Well some of it goes into the kinetic energy of the small blue balls, but some of it goes to breaking apart the blue cylinder into the small blue balls.