Why does the water in the glass shift backward when force is first applied, and then shifts forward when it comes to a stop? (hint: Newton’s first law of motion)

Question

Please explain it short and simple, but in an easy to understand way.

Answer 1

It is similar to other oscillations like pendulum, or weight on a spring.

The water has a "neutral" position in the absence of any force.

The initial force then causes displacement of the water from its neutral position.

If the force continued to be applied then the water would stay in its displaced position.

The moment the force is released, the water starts to return to its neutral position.

This return movement is caused by other forces, namely gravity.

Notice that the statement of the problem would not describe the behaviour of water in weightlessness.

This return movement is similar to the return movement of a pendulum, also caused by gravity.

While the return movement of a weight on a spring is caused by the force of the spring.

By the time the water reaches its neutral position, it has some speed.

And that speed, by Newton's first law, causes it to continue its movement,

which results in a displacement in the opposite direction.