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When you have a drink with an ice cube and twirl the glass, the liquid itself seems to twirl but the ice cube stays roughly in the same place. Why is this?

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When you say "twirl," are you just rotating the glass about it's center (ie. spin it)? Or something more complicated? –  tpg2114 Jul 22 '13 at 20:52
    
@tpg2114 Yes, just rotating it about it's center. –  Jop Vernooij Jul 22 '13 at 20:57
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I think that this would first only induce the surface of the water to tilt, since it is trying to orientated to the normal of the resulting acceleration. However the water is not rotating at first, just as the ice cubes. –  fibonatic Jul 22 '13 at 23:33

3 Answers 3

When you stir the contents, everything in the cup gains angular velocity including the ice cubes. Centripetal acceleration causes less dense materials to accumulate at the center of rotation as the denser materials force their way to the edge of rotation. If the angular velocity is high enough, this edge can rise against gravity, causing the water surface to curve into a parabola. All of this is exploited in the laboratory equipment known as the centrifuge. Since ice is less dense than water it will tend to collect in the center of a stirred cup.

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when u rotate a glass of water about its axis, only the water at the sides of the glass which is in direct contact with the glass rotates along with it initially as water has a high force of adhesion with glass which is stronger than the forces of cohesion between water molecules. the rest of the water which is not in direct contact with the glass rotates with a far lesser angular velocity whose magnitude decreases as we move towards the centre of the water surface. at the very centre, the velocity is zero. this is similar to the fact that the "eye" of a hurricane is the safest place to be if you are caught in one! the surface of the water aligns itself perpendicularly to the resultant acceleration vector so the surface of the water tilts inwards towards the centre and hence an ice cube placed in a glass of water, on rotation of the glass, tends to move towards the centre of the glass where the angular velocity is almost zero(i am saying almost zero since the ice cube isnt a point mass. it covers some surface of the water so it may acquire a small angular velocity which is negligible at the beginning but after some time, becomes observable). i must also add that the rotation of the ice cube becomes visible only after some time because the coefficient of friction between ice and water is very less hence the angular acceleration is very small.

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The coefficient of friction between the glass and water is high enough so that by twirling your glass, the water gets dragged along and starts twirling too.

However, this seems not to be the case for the coefficient of friction between the water and the ice cube. Thus, the water cannot drag the icecube with itself (or cannot drag it aswell as the glass drags the water : maybe if you keep twirling long enough the ice cube will eventually start spinning too)

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This doesn't seem like a likely explanation. If that were true, you would get flow around the cube with the cube remaining stationary. This (1) doesn't seem intuitive at all and (2) doesn't match what I've observed playing with glasses of water+ice. –  Kyle Jul 23 '13 at 0:28

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