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This is a question from a past paper

The drum is a hollow metal cylinder in a washing machine with a series of holes through its surface. When the drum spins, explain how it separates water from wet clothes. (2 marks)

I talked about how a centripetal force acts on the wet clothes so they are seemingly forced to the walls of the drum alongside the water molecules and as the water molecules are small enough to pass through the "series of holes", they will separate out.

When I looked at the marking scheme, this is what they wanted

(1) Water has no resultant/centripetal force
(2) Water continues its motion in a straight line

I see how the second point follows after reading about the law of inertia and how circular motion is what it is. But the first does not seem correct. I mean, isn't it precisely because the water feels a centripetal force, that it continues in its tangential straight line motion allowing it to seep out through the holes?

The info I read on this site would suggest so, but I can't be sure if I've misinterpreted anything, so I need some clarification.

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1 Answer 1

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When the drum rotates, both the water and clothes would naturally fly off at a tangent. The clothes can't fly off, because there are constrained by the walls of the drum applying a centripetal force to them. The free water isn't completely constrained by the centripetal force the drum applies to the clothes, because the drum is perforated and the water can pass through it.

What you have mixed up in your answer is the direction of the centripetal force- it acts inwards, and is applied by the walls of the drum to the clothes.

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  • $\begingroup$ Ah, that last point is what I was missing. Thanks! $\endgroup$
    – Doobius
    Jul 21, 2021 at 11:09

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