Internal Energy

Note: Simulation will run for 100 seconds.

A heavy ball with an initial kinetic energy of 4000 J is trapped inside a box with rigid walls containing a cylinder constructed of small light-weight spheres.  The ball crashes into the cylinder and breaks it apart.  The bar graph at the right and the table at the bottom display the kinetic energy of the ball.

Checkpoints:

Use the simulation above to answer the following questions.

• Can you define a system for which energy is constant?
• Can you define a system for which momentum is constant?
• Why does the energy of the ball decrease?  Where does this energy go?
• Do you think it is possible to recollect all the energy from the spheres and transfer it back into the red ball without an external interaction?  Would the simulation be realistic, i.e., would it model nature, if it were run in reverse?

Answers

• The energy is constant if the system is defined as all particles within the box.
• Momentum is not constant in this simulation.  The box does not move when objects collide with it so the box must be anchored to some other object.  This external object can flow momentum into or out of the system.
• The energy of the ball decreases because it is converted to random, or thermal, energy of the spheres. 
• It would be not be possible to recover all the energy of the spheres.

Credits:
Problem by Wolfgang Christian
Script by Wolfgang Christian