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Centripetal force discussion question (Washing machine).

  1. Dec 21, 2011 #1
    Note: This discussion question is from my University Textbook, as it so happens you can only get the solutions to half of the odd numbered exercise questions... :grumpy:

    Disclaimer: I have been trying to wrap my head around this answer for a good 5 hours now, so it's not like I'm coming here to be spoon fed.

    Anyway, the question:

    "What is the purpose of the spin cycle of a washing machine? Explain in terms of acceleration components"

    My answer:

    The purpose of a spin cycle of a washing machine is to remove water from clothes.

    The drum of radius, r, rotating with speed, v, will have a centripetal acceleration a = V^2/r which is provided by a centripetal force, acting toward the centre of the drum, which in turn has an equal and opposite reaction (centrifugal?) force.

    The inertia of the clothes and the water particles causes them to keep moving in a straight line, that with the combination of the reaction force, causes them to move around the edge of the drum and the water particles move out through the holes in the drum, which then fly off at a tangent to the drum's rotation.

    But then I watched this lecture from MIT and he explained a similar situation with water particles in a test tube and said that they experience no centripetal force:

    Please watch from 17:30 to 19:06

    Immediately after he then does an identical experiment to my question (lettuce in a siv) and links it to his previous explanation.

    So now I feel like an idiot, even though he has gone through almost a identical experiment I can not find a way to intuitively explain what is happening.

    If anyone could talk me through this I would greatly appreciate it. I have 25 more lecture topics from this past semester to fully understand before the end of the christmas break!
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Dec 21, 2011 #2


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    The marble slides toward the outward end of the tube until it hits the end. At that point, if there's a hole big enough for the marble to escape out of it just keeps going.

    Same thing with the washing machine, except it's droplets of water and the holes are big enough for the water but your clothes stay inside.

    There's a confounding factor, which is that water tends to cling to your clothes, so it takes some time for the water to make it through the layers of clothes. Since the water (at least some of it) doesn't just go flying out, it means it does experience some centripetal acceleration.

    While all that's going on, the clothes also get smashed up against each other on the inside of the drum. You can imagine that also helping to squeeze the water out of the clothes.
  4. Dec 21, 2011 #3
    Thanks, I was going to write something along the lines of that bout the 'explain in terms of acceleration components' was throwing me off.
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