Answer: A force must cause a displacement in order for work to be done. A book falls off a table and free falls to the ground. Yes. This is an example of work.
Explanation:
I’m pretty sure it is an object with a net force of zero. All forces are balanced and EQUAL
Answer:
"where crests and troughs have their maxima at the same time"
Crests and troughs are 180 deg out of phase and when they have their maxima at the same time and place, their net contribution will be zero"