Students are presented with physics equations describing various phenomena, and asked to do problems, which are solving equations. Equations are only half the story. The other half is boundary or initial conditions.  The equation itself does not describe everything. In addition, we specify what happens at the boundary.

 

For example, waves on a string. It is not enough to write down the equations for waves. In addition, we must specify what happens at the ends. Usually the ends are held fixed. Saying that the ends are fixed is giving boundary conditions.  Let us have two strings, one an inch long and the other five inches long. The waves are different on each string, as the boundary conditions are different. On one string, the ends an inch apart are fixed, and the other the ends five inches apart are fixed.

 

Another example is the problem of throwing a ball over a cliff, and finding out where the ball will land. It depends on the height of the cliff. This is a boundary condition.