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 help by explaining it short and simple, but in an easy to understand way.

Answer 1

Newton's first law of motion states that any object at rest will stay at rest and any object in motion will stay in motion unless acted upon by an unbalanced force. What this means is that objects like to keep doing whatever it is that they are already doing. Imagine a glass of water. When the glass isn't moving, the water isn't moving. When the glass begins to move, is there a force being applied to the water inside? There really isn't until it reaches the edge and touches the glass, at which point the glass pushes the water forwards. The water isn't shifting backwards, it just is staying still! Similarly, when the glass and water are both in motion and the glass stops moving, the water will keep moving until it hits the glass. This is the same as when you are in a car, and the reason we wear seatbelts. When the car accelerates, you feel like you are being pushed back against your seat, and when it brakes suddenly, you feel like you are jerking forwards.

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Alim Merchant