[Q] Liquid motion and nullification of momentum

[paigetheoracle]

Why is that when I am carrying a mug of tea and I turn a corner, the cup turns with me but the liquid contents stay in the same position? If the Earth is spinning constantly, is this why the sea doesn't just pour over the land in a constant tsunami?:phones:

[Moontanman]

First of all, if you turn the corner fast enough the tea will spill out, I've done it, it spills. Second the ocean is rotating with the Earth, if you suddenly speed up the Earth with out speeding up the ocean as well the ocean would indeed spill out over the land.

[paigetheoracle]

First of all, if you turn the corner fast enough the tea will spill out, I've done it, it spills. Second the ocean is rotating with the Earth, if you suddenly speed up the Earth with out speeding up the ocean as well the ocean would indeed spill out over the land.

 

I've done this but that happens when you are moving very slow and carefully - the above happened when I turned 'rapidly' - maybe pivoted like a dancer or martial artist, might be a better description as it was super-quick.:phones:

[modest]

All objects (including the cup and liquid) that are at rest want to stay at rest and will only move if an outside force acts on them. All objects in motion want to remain in that motion and will only accelerate (change their speed or direction) if some outside force acts on them. This is Newton's law of inertia.

 

In order to rotate a cup that is not initially turning you'll have to affect it with a force. You accomplished this with your hand and body when you turned the corner. The liquid in the cup also resisted the rotation and it was more able to do so because it was not firmly fixed to your hand and body. There is some friction between the liquid and the cup that transfers momentum, but it is not a rigid connection. In simple terms, the liquid is free to 'slip'.

 

Now consider rotating a cup for a protracted period of time. As friction allows the transfer of angular momentum to the liquid it will rotate faster and faster until the speed of rotation of the liquid is the same as the cup. It would all end up rotating at the same speed. This is like the earth and the oceans. The water and the land are both rotating at the same speed and the law of inertia says they want to remain at that speed of rotation until or unless some outside force were to act on them.

 

~modest