Questions for a physicist and/or chemist

  1. My can of soup says to put in a bowl, cover loosely, microwave, and then let stand for a minute. I do all of this in a plastic bowl with a flimsy plastic lid that can be sealed shut (but I don’t seal it). When I take the soup out of the microwave, the lid is not only sealed on, but bowed in from lack of pressure (lack of atmosphere?) inside the bowl. The soup is still hot (I don’t think it has cooled much). What’s going on?
  2. My shower has a smooth, clear, glass door. When I shower, the water covers it with a smooth, even layer, so I can still see out pretty clearly. When I touch my finger to the glass, the water “runs away” from my finger and leaves the glass nearly dry in the vicinity. This is so strong that the water will actually flow up to “escape” my finger’s vicinity. New drops of water that hit the glass while my finger is touching it also move away (though at a slower pace, since they can’t flow as easily without more water around). When I remove my finger, the film of water returns to the way it was before. This isn’t dependent on something I’ve put on my finger; it works with all 10 fingers/thumbs, both elbows, my tongue, and at least one toe. I don’t have a water softener. What is happening?
  3. Galileo showed that all objects fall at the same rate, no matter how much they weigh. I have a bowl full of Cheerios and put some raisins on top. The Cheerios and raisins are about the same size, but the raisins are denser. I put my hand over the top of the bowl and shake it. I would expect all objects in the bowl to fall at the same rate when I shake, and keep the raisins on top. Instead, they gradually migrate towards the bottom. What’s going on?
  4. There is a doorway with the sun shining through it onto a wall (with the shadow of the doorframe on the wall). I stand several feet in front of the door so that the side of my shadow nearly touches the side of the door’s shadow. The part of my shadow closest to the door’s is mirrored, so it appears that the side of my arm is coming out of the doorframe. This occurs even if I move around slightly (it appears as though the door’s arm is moving in the opposite direction). The shadows need to be within a couple inches of each other to get this to work. What is going on?

The world is a strange and fascinating place.

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14 Comments

  1. mockery0 says:

    1. Heat makes the plastic soft, and then gravity deforms it.
    2. The water is afraid of you. Try turning the radio to a calming Smooth Jazz station and see that this phenomenon no longer occurs.
    3. Objects accelerate at the same speed, but they have different amounts of momentum at any given moment. The raisins can “bully past” the cheerios on their way down.
    4. This is likely an indication that you are a vampire.

    I just made all of those up, so they might be wrong. Except #2 and #4. Those ones are definitely right.

  2. 1. Magic.
    2. Surface tension.
    3. I imagine this is the same reason balloons work: when getting wiggled about, more-dense things tend to end up at the bottom.
    4. I concur with Michael. May I recommend avoiding garlic?

  3. riccobot says:

    “… it works with all 10 fingers/thumbs, both elbows, my tongue, and at least one toe.”

    Man, you must take really long showers to try all those. But, I’m a little disappointed you didn’t keep going. At least one toe? What about the others?! I’m sure they feel left out. Also, what if you have both your tongue and a thumb on the glass? Does it only work for prime numbers of things touching the glass? Just think of all the possible combinations you could try!

    As for #4, does the sun cause you to catch fire when it shines on you?

    Duke should cancel Game Theory less often, as I am obviously doing nothing useful right now. It’s raining! People can’t expect me to do work under these circumstances.

  4. 1. If the soup started frozen, the case in all cans, I know, then it would have gotten smaller when heated. This pulled the lid on tighter.
    2. This water knows what you did to the soup.
    3. Cheerios are full of air. Things that are full of air do not fall at the same rate as everything else.
    4. Parallel universe.

  5. muddernh says:

    so it seems like time for an actual chemist to try to tackle these questions…

    1) Microwaving involves hitting whatever you’re heating at the vibrational frequency of water, giving it more kinetic energy & therefore raising the temperature of the food. While microwaving, water heats up and some water vapor forms. Letting it stand for one minute will decrease the vapor pressure, and (?) cause it to seal, and as it cools the amount of actual atmospheric gas in the contain will be less than the amount there when it sealed ’cause the water vapor will condense again as it cools. Whether or not the soup has cooled much the water is no longer being made incredibly hyper by the microwaves. I’m about 75% confident in this answer, and sure that it’s least that it’s due to water vapor effects… The 2 sentences are most definitely true.

    2) Some kinda coating on your shower door? blah blah surface tension of water.. blah blah afraid of you…

    3) Density, and size allows the raisins to fall to fit into the small spaces. Whether or not they would fall at the same rate if dropped in air, the raisins will fall to fill the holes… Shape makes a difference too

    4) Ask fireshadowed…

    • nematic says:

      1: as muddernh explained. I have a further hypothesis about it, though. From how I understand microwaves, water vapor would be less effecient at absorbing microwave energy than liquid water, so that the temperature of the steam would be only slightly above the boiling point – thus it would not take very much cooling time for the vapor to recondense.

      2: might be explained as follows: When you put your finger to the glass, you are breaking the flow of water. Gravity acts to “pull” the water around your finger. However, parting a stream of water creates two new water/air boundaries, which have a higher energy than if they were not present – the act of the water “moving away” up the glass may be it just finding the energy minima for the process of separating and going around your finger

      3: most people miss the part of this law, that all objects fall at the same rate IN VACUUM

    • Alan says:

      Whoops! When I wrote Cheeries, I meant Cheerios (cereal; raisin-sized pieces) rather than cherries (fruit; larger than raisins), so size probably isn’t a big factor in what I had intended to write about. I’ve corrected this in my original post.

      but that’s a good answer to the question I didn’t mean to ask.

    • hmcmodelt says:

      I think I just confirmed Rachel’s answer to #2. I made rice and took some out to eat and recovered the pot. When I came back to get more, the lid was difficult to remove, almost as if it was attached through some sort of pressure gradient. Now rice, as you know, is a solid. The coefficient of thermal expansion for a solid such as rice is low, thus it did not change volume much as it cooled slightly. The gas in the pot, however, did.

  6. hmcmodelt says:

    I really liked Michael’s answer to #1, until Rachel gave a better one.

    1. What Rachel said
    2. This is a specially engineered coating designed originally for the space shuttle. They coat the insides of the windows with it to prevent moisture from the air from condensing on them. This is especially harmful during reentry when it gets warmer and the pilot really needs to see out the window. It has been adapted to all kinds of terrestrial applications, such as your shower door. Its purpose there is to make the shower door not get all water-spotted when it dries.

    3. The raisins don’t want to be eaten, so they run away. In fact, they’re probably having a party at the bottom of the bowl where you can’t see them. They’re probably related to the California Raisins (of notable fame during the 1980s), they even played music – Google it.

    4. Most likely it’s some person from the future in your apartment wearing a cloaking device. Everybody knows that warps light around the person. And don’t say those don’t exist . . . If they have time travel, why can’t they have cloaking technology. Besides that, I’ve heard they almost have one that works now.

  7. mockery0 says:

    If it’s not clear, my answer to number two was intended to be serious, and I stand by it. I’m confident it doesn’t have to do with the presence of air, (buoyancy or otherwise.)

    To rephrase my explanation, consider two spheres falling into a funnel from opposite edges, one made of lead and one made of balsa wood. If they start at the same height they will accelerate at the same rate (barring friction) and eventually collide somewhere near the bottom of the funnel. However, after the collision the lead sphere will have a lower rebound velocity and thus lower rebound distance, and then will have a head start in the post-collision race down the funnel.

    Momentum is the key!

    • Alan says:

      I rather hope you mean your answer to number three. but I’ll try the smooth jazz thing this morning just in case!

    • nematic says:

      I wonder if the raisins are more dense than the cheerios. If they are, then the raisins are simply sinking through the cereal.

      • Alan says:

        Yeah, they’re definitely denser (see original post). but neither one is particularly heavy, so I don’t think they can easily sink through the Cheerios (if I don’t shake the bowl, they stay on top). mockery0 might be on to something with this momentum stuff…

        • hmcmodelt says:

          I stumbled upon this old post today for some reason and I have another explanation for the cheerio/raisin thing. Since the raisins are more dense, they would in equilibrium end up beneath the cheerios (as with two liquids of differing density). However, because of friction the heavier particles cannot move past each other. I imagine that if you could do this experiment with frictionless raisins and cheerios you would see the raisins quickly squirming to the bottom of the bowl. When you shake the bowl the frictional forces are momentarily decreased allowing the raisins to subvert the cheerios.

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