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Homework Help: Gravity on the human body

  1. Apr 22, 2012 #1
    1. The problem statement, all variables and given/known data

    i measured myself before going to sleep and after waking up over several nights and found out i'm ~1.5cm taller in the morning then at night. I know this question is related to human biology, but i'm equally (if not more so) interested in the physics behind the problem i.e. how gravity affects the body.

    2. Relevant equations

    3. The attempt at a solution

    I understand that cartilage in the body has an elastic / flexible / resilient nature and that gravity causes compression of cartilage in the spinal cord / knees. Gravity on Earth does not change, so the body is exposed to the same force of gravity regardless of wether one is standing or lying down: why then does this difference in height occur?

    I'm going to take into account hydrostatic pressure because the body is 60% fluid. So, pressure = F / area. Gravity acts in one direction (downwards) so if you change the surface area for gravity to act on then pressure exerted on the body changes accordingly, i.e. if you lay down at night you increase your surface area for which gravity can act on, pressure on the body decreases and the cartilage of the body is able to expand again.

    i feel like there should be more to it. I'm curious if theres anything else to consider here or if i'm completely thrown off :S
  2. jcsd
  3. Apr 22, 2012 #2
    When you are standing, gravity acts on your vertebrae, compressing your cartilage. This is what causes the loss in height. When you are lying down, gravity still acts downwards. There is no way for gravity to act sideways to force your vertebrae together, which gives time for your cartilage to rebound. Thus, you are taller in the morning, when gravity hasn't had a chance to force your vertebrae together yet.
  4. Apr 22, 2012 #3
    thankyou for your reply :) !!

    i understand that gravity is acting downwards with the same force in both cases (and not horizontally), but that doesnt explain why the cartilage expands when the body is lying down if the force of gravity is exactly the same as when the body is standing. The independant variable here is the position of the body. I feel it is because when you are standing gravity acts downwards on a vertical spinal column. Then when you are lying down its acting on a horizontal spinal column.
  5. Apr 22, 2012 #4
    http://www.hergoods.info/avatar1.jpg [Broken]When you are standing, gravity acts on your vertebrae, compressing your cartilage.
    Last edited by a moderator: May 5, 2017
  6. Apr 22, 2012 #5
    That's what I'm getting at.
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