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B Physics of moving heavy objects

  1. Apr 26, 2017 #1
    trying to apply some physics to the 4 major power lifts.

    the key area I wish to analyse is bar path in a squat and deadlift.

    the work done against gravity is independent of path?

    a lifter can either have a straight as possible body path and deviate the bar path or the lifter can alter his body path and have a straight lift of the bar.

    my position is bend your body around the bar to give the straightest bar path possible, preferably a vertical bar path.

    how could I prove this is the most efficient lift?

    talking maximum possible lifts.
  2. jcsd
  3. Apr 26, 2017 #2


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    Right. The amount of energy you expend depends on the path, but the work is just finish vs start. If you get it over your head you've done positive work. If you then lower it back to the floor you are again expending energy but now you are doing negative work.

    I think it's ENERGY that you want to be concerned with since that's what wears a lifter out.
  4. Apr 27, 2017 #3
    Work is energy tho??
  5. Apr 27, 2017 #4


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    The issue in these problems is almost entirely biomechanical. The external work done on an object is not really the issue, but which muscles you use and how you use them. There is a minimum amount of external work to be done to raise a weight, but this would be achieved by moving the weight as slowly as possible (minimum acceleration of the weight). But, biomechanically, that would be a bad idea. Compare this, for example, with lifting a weight using mechanical apparatus, in which case raising the weight slowly would use less energy, hence less fuel.

    Other biomechanical techniques include getting your arms straight and lifting the weight with your legs; and, not having your muscles in an awkward position: such as not lifting a weight with your arm extended in front of you. None of these techniques change the external work done, but how efficiently and effectively your body can do that work.
  6. Apr 27, 2017 #5
    These questions are essentially not reduced to mechanics. The law of energy changing is a general physics law and for this case one must take in to account somehow energy due to chemical processes in muscles, thermodynamics equations are also needed. It is a very complicated problem.
  7. Apr 27, 2017 #6


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    While holding weights stationary above your head you are doing no work on them. But it doesn't feel like that because the human body is an inefficient machine.

    It's not clear to me that "the most efficient lift" is what you should be aiming for. For example the faster you lift the less time it takes so the less energy is wasted by the bodies overheads (thinking, digesting food, pumping blood, heat loss etc). So a faster lift could be "more efficient". However I suspect what matters more is the profile of the load on one particular muscle or group of muscles.

    It's not really a physics problem.
  8. Apr 27, 2017 #7


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    The maximum possible lift is set by the geometry of the mechanics of the lift , available muscle forces and the breaking strength of body parts .

    Not much different to a JCB .
  9. Apr 27, 2017 #8


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    You turn up at the Olympic weight-lifting with a JCB and see what they say!
  10. Apr 27, 2017 #9

    Now I'm am not sure if efficiency is the right thing.
  11. Apr 30, 2017 #10
    as was said, this is a biomechanical answer. the boy will sometimes not use the most efficient path, to create the most optimal application of force . (Angles of joints, length of limbs, etc).
  12. Apr 30, 2017 #11
    Lot harder than I thought.
  13. May 1, 2017 #12


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    It could be fairly important, in that it involves what you get out over what you put in.
    With our bodies, the 'dead weight' is relevant, if you are looking for an efficient way of lifting an object, it is better to reach down just with your arms and raise it than to squat down and then have to lift your whole body weight. But weight lifting actually involves the body as well as the load.
    Many exercises are designed to use 'pointless' energy (the most efficient activity is arguably sitting in a chair) in order to use up the chocolate you have been eating or to increase your strength / appearance.
    If you are earning money by doing work, then efficiency is more relevant - good digging technique is worth developing if your food input is limited.
    So what do you want to find out about your lifts? Sadly, Physics doesn't necessarily do a lot for you when you want to become an athlete, despite what equipment manufacturers would suggest. I reckon that success can only be judged after a long period of a particular exercise regime. Perhaps your body can tell you more about how well you're doing by the aches and pains you get, afterwards.
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