Describe a situation where you know the mechanical
energy was not conserved. That is,
you know that total energy at the
start is not the same as at the end.
Why do you think it was not
conserved?
If you are struggling to think of
something, imagine a scenario where
there's some energy in the system at
the start, but then 0 J of energy in
the system by the end. Or vice versa.

It has previously been mentioned that there is a relationship between work and mechanical energy change. Whenever work is done upon an object by an external force (or nonconservative force), there will be a change in the total mechanical energy of the object. If only internal forces are doing work (no work done by external forces), then there is no change in the total amount of mechanical energy. The total mechanical energy is said to be conserved. In this part of Lesson 2, we will further explore the quantitative relationship between work and mechanical energy in situations in which there are no external forces doing work.

The quantitative relationship between work and the two forms of mechanical energy is expressed by the following equation:

KEi + PEi + Wext = KEf + PEf

a longitudinal wave moves energy that is to the direction of the wave’s movement. - true