Why do liquids in a cup remain in the same place while the cup is being rotated?
In: Physics
Newton’s First Law of Motion. Paraphrased, it is “Objects in motion will remain in motion and object at rest will remain at rest unless acted upon by an unbalanced force.
This means that when you turn a cup the force of that cup needs to be transferred to the fluid in some way to make it move as well, otherwise it won’t. Except of course a classic feature of a fluid is that it flows which means the moving inner cup wall isn’t able to effectively transmit motion to the non-turning fluid, so the bulk of the fluid doesn’t turn right away.
Because there isn’t enough force to force them otherwise. When you spin a glass the edges of the glass and the outer most layer of liquid touch and then the edge exerts a force on that liquid which then exerts its force to the liquid next to it and so on. Because each time this happens there is less and less force being exerted the liquid usually stays still but if you would rotate the glass really fast then even the liquid would start spinning.
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