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Physics Questions

  1. Oct 13, 2009 #1
    Pls. answer as many as you can in the following questions. I'm not sure in all of my answers. I just answer them using my best.

    1. The problem statement, all variables and given/known data

    1. Why are persons confined to bed less likely to develop bedsores on their bodies if they use water bed than ordinary mattress?

    2. Which do you suppose exerts more pressure on the ground, an elephant or a lady on spike heels? Which most likely to make dents on a linoleum floor?

    3. Why do your body get more rest when you're lying than when you're sitting? Why is blood pressure measured in the upper arm at the elevation of your heart? Is blood pressure on your leg greater?

    4. The weight of a human brain is 15 N. The buoyant force supplied by the fluid around the brain is about 4.5 N. Does this mean that there must be at 14.5 N of fluid surrounding the brain? Prove your answer.

    5. When floating why do you tend to float lower in the water when you exhale?

    6. Will breaking a T.V. picture tube cause it to implode or explode? Why?

    7. Would it be more slightly difficult to draw soda through a straw at sea level or on top of very high mountain? Why?

    8. When a shower water turned on full blast, the shower curtain move in. Why?

    9. Why does the fire in fireplace burn more briskly on a windy day?

    10. Small bubbles of air are released by a scuba diver deep in the water. As the bubbles rise, do they become larger, smaller, or stay about the same size? Explain.

    2. Relevant equations

    3. The attempt at a solution

    1. Because it is softer than the ordinary mattresses. Normal mattresses are a very unforgiving surface, and they force your body into an unnatural position. They put undue pressure on bony areas and do not allow the spine or joints to adopt a natural relaxed shape. Your weight is distributed in water bed.

    2. Lady wearing spike heels because small area means great pressure. I don't know what's the answer in the follow-up question.

    3. Because while you're in vertical position, your heart has to pump harder.
    It is a place representative of the pressure that your heart is experiencing and also a standard place that yields consistent results.
    Pressure increases as depth increases or distance from heart to leg if you are vertical.

    4. (I don't know)

    5. Because air is less dense than water. When you inhale, you have air in your lungs that will also make the buoyant force increase. When you exhale, you'll release the air and the BF will decrease.

    6. It will implode because the tube is a vaccuum. When you put a hole on it, The air will go through that hole until the pressure outside and inside are equal.

    7. Drinking through a straw is slightly more difficult atop a mountain. This is because the
    reduced atmospheric pressure is less effective in pushing soda up into the straw.

    8. It's Bernoulli's Principle. There are two equal pressure in the room. One is outside the curtain and the other is inside the curtain. When you turn on the shower, it will distract the pressure inside the curtain that will make the pressure inside the curtain reduce. Now, the pressure outside the curtain is now greater than the other one so the pressure outside can now push the curtain.

    9. On windy days, the wind passing over the top of the chimney draws more air up the flue. This draws more air from the house into the fireplace, providing more oxygen to the fire, causing it to burn more intensely.

    10. It will become large because in pressure, when the depth is great, the pressure is great. When the deeper you go in an ocean, the temperature is low. Since the bubble is going up, the temperature goes up that will make the bubble expand because of the characteristics of a matter. And also the pressure will be less because the depth is not that great anymore.
    Last edited: Oct 14, 2009
  2. jcsd
  3. Oct 13, 2009 #2


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    Why don't you propose an answer for each and we could provide comments?
  4. Oct 14, 2009 #3
    Now I put answers
  5. Oct 14, 2009 #4


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    For 3a, your heart has to work harder to pump blood while vertical.
    For 3b, it is a place representative of the pressure that your heart is experiencingy and also a standard place that yeilds consistent results.
    For 3c, pressure increases as depth increases or distance from heart to leg if you are vertical.

    For 6, a TV picture tube (cathode ray tube) contains no air (vacuum). Now how would you answer?
  6. Oct 14, 2009 #5
    I updated the answers.
    Last edited: Oct 14, 2009
  7. Oct 14, 2009 #6


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    For 2b, linolium is a semi-hard flooring material that will deform of enough pressur is applied. I don't know what that is, offhand, but the question is asking about likelihood. Say the weight of an elephant per foot is 500lbs and the surface area of an elephants foot is 100in² (this does not need to be realistic). You can calculate the elephants pressure in lbs/in². Compare this value to a man, 200lbs (it's possible--Hallowe'en is coming up), wearing truly spiked heels (surface area of heel approaches zero). Now which is more likely?

    For 4, this is an oddly formulated question. Where does 14.5N come from (typo??)? I suppose there is an assumption that the brain is free-floating in brain fluid. If this is true then the brain must be displacing 4.5 N of brain fluid. You should wiki buoyancy to get a better background. I need to think about how to "prove" anything about this question.
  8. Oct 15, 2009 #7
    Area can't be zero because if you divide 200 by 0 is undefined.

    14. 5 N is not typo. It's the given question. The biggest problem here is how to prove it.
  9. Oct 15, 2009 #8


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    What happens if you divide it by something close to zero like 0.0000001 in² ?
  10. Oct 15, 2009 #9
    Lady on heels is the answer.

    What about the question # 1. I really didn't know. And also the question #4.
  11. Oct 15, 2009 #10


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    #1 seems reasonable.
    For #4, "Does this mean that there must be at 14.5 N of fluid surrounding the brain? " --if an apple floats in water the same way if it is in a bucket full of water or a bathtub full of water, what does that tell you about the amount of fluid required to make something float? By the way "N", is this case, "Newtons" is a confusing way to refer to the "amount" of something (which is how I am reading this question. Let's assume the brain in question is located near the surface of the earth).
  12. Oct 15, 2009 #11
    Yes it means that there must be 14.5 N of fluid...
    Both of us already knew that the answer is no but what is the reason.

    In question # 3. Why it is harder for the heart to pump in vertical than horizontal? Also, can you elaborate your answer in the 3b?
  13. Oct 15, 2009 #12


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    3a: The heart has to pump blood upwards to the brain (do work) which is not so hard to do when you are lying down. We are getting into biomechanics of the heart as a pump, which I cannot help with. I'd ask Mr. Google to find more background on this.

    3b: It is a standard place for taking the measurement. If you strapped the band around your neck, you would probably get a lower pressure than if the doctor strapped the band around your ankle.
  14. Oct 15, 2009 #13
    okay. what about the # 4 and #6?
  15. Oct 15, 2009 #14


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    4: Not sure what else I can sea about that. Wether or not an object is buoyant is only dependant upon the weight (force) of the object and the weight (counterforce) of the fluid displaced by the object. The amount of fluid is not relevant by the bucket/bathtub analogy.

    5. You'd get a better implosion if you hit it with a baseball bat ;)
  16. Oct 15, 2009 #15
    in #7?
  17. Oct 15, 2009 #16


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    7, 9 10: no comments--they look reasonable.

    8: The phrase "it will distract the pressure inside the curtain that will make the pressure inside the curtain reduce" needs some work. You should wiki "shower-curtain effect". Apparently there are competing theories about this.
  18. Oct 16, 2009 #17
    My project is already finished. My answer in 4 is because the brain is just nearly the size of our head. Then, it displaced all the liquid in our head. When the fluid is 14.5 N, the buoyant force will be also 14.5 N. Since the original weight of the brain is 15 N, the new weight will be .5 N when you less the buoyant force. And our brain will float because the buoyant force is now greater than the weight. Floating of brain and too much liquid in our head cause hydrocephalus.
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