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Dead weight vs. live weight

  1. Mar 18, 2007 #1
    I am a 8th grade student getting ready for a science fair project. My sister, 80lbs, is impossible to lift when she is asleep. But I can pick her up when she is awake. What is the difference between dead weight ( asleep) and live weight? I know she has to weigh the same amount but why is she so much heavier when she's sleeping? Help.
  2. jcsd
  3. Mar 18, 2007 #2

    D H

    Staff: Mentor

    She helps you lift her when she is awake and hinders whe she is asleep. Awake, she might jump just a little so you don't have to lift so much weight and grab on your neck with both arms to reduce the torque. Asleep, her center of gravity is far from yours: gravity is not just exerting force on you but also torque.
  4. Mar 18, 2007 #3
    Thanks DH. Now I have to think up an experiment to show how she seems to weigh so much more asleep. When I get the experiment clearer in my mind can I ask you what you think?
  5. Mar 19, 2007 #4


    User Avatar
    Gold Member

    Even without her active assistance, the rigidity of the body makes a huge difference. Asleep, her mass is constantly shifting, which makes it very difficult to get a grip in a convenient location.
    You can demonstrate this for yourself by taking a large sack of something like potatoes and an equal size/weight of boards or similar solid material. See which is easier to lift and manoeuvre.
  6. Mar 19, 2007 #5


    User Avatar

    Staff: Mentor

    It is entirely about center of gravity. If her center of gravity (roughly in her midsection) is close to you, she is easy to lift. If it isn't, she's impossible to lift without you falling over. Very important concept for wrestling, btw - if someone is trying to pick you up by grabbing you around the waist, just lean back into them with your hips away from them and it will simply be impossible.
  7. Mar 19, 2007 #6
    Thanks Russ. What to you think would be a cool experiment to show this? I'm not sure she is going to let me weigh her awake or asleep? I'm looking at torque and moment arms but not sure how it works with this idea of dead weight vs. live weight. Just think the idea is cool cause it happens all the time with her.
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