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Sledge hammer lever

  1. Jan 22, 2013 #1
    The feat is performed by holding a 20lb sledgehammer from the bottom, shoulder level and parallel to the ground. The hammer should end up being right over your head. The length of the handle is 36 inches, and the width of the hand is 4 inches. How can I figure out the force it takes to hold the hammer in place. Let me know if other measurements are required.
    Thanks
     
  2. jcsd
  3. Jan 22, 2013 #2

    sophiecentaur

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    This looks like a candidate for the Homework / Coursework forum, actually. Rather than giving you a short course in mechanics, I must ask what you have done, so far, towards solving the problem. What facts can you write down about the various forces in play? (We tend not to come back with 'the answer' on PF. We expect people to demonstrate that they have made some effort first.)
     
  4. Jan 22, 2013 #3
    This isn't homework, I dropped out of school in 9th grade and have very little physics knowledge. If you have the time to walk me through the steps to solve this or can just answer this question I would be grateful.
     
  5. Jan 22, 2013 #4
  6. Jan 23, 2013 #5

    sophiecentaur

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    That article is correct but, as it's a practical example, it may be a bit fuzzy about the actual basics. The basic principle of moments is that length times force needs to be the same on both sides - the "Clockwise Moments equal the Anticlockwise Moments" when balanced (in Schoolboy language). Only a minimal extra force is needed to make the lift and, of course, as the weight is raised, it comes closer in to the pivot so the radius gets less. The angle of the muscles on the skeleton will change and could even give a greater mechanical advantage*. Fair enough. But how do you actually define where the pivot is, inside your wrist or where the forces are acting in there? Levers don't need to be just in a straight line and what counts is the radius (called the perpendicular distance) around which the forces act. When you step on a bicycle pedal, you are pushing down on the crank but the chain is pulling horizontally - the mechanical advantage is still the ratio of the pedal length to the sprocket diameter. (Is this too basic? Sorry if it is.)
    In the case of your hammer exercise, you have three pivots - wrist, elbow and shoulder. You can ignore the elbow joint, probably, because you can rotate the arm so the joint is locked.
    Does it matter what you do to get the hammer vertical? No swinging allowed, presumably but what about dropping your arm to allow the wrist to point the hammer vertically and then lift your arm? That would make the job easier as your wrist could rotate the hammer to the vertical much more easily (like in clean and jerk) - hardly any force needed at all if you get the timing right. Then you'd be bringing the hammer weight a lot closer to your shoulder for the final lift.
    If it has to be a straight arm lift, the actual muscle force needed to hold the hammer horizontal would be around:
    Hammer weight X (total length of hammer plus arm) ÷ (one tenth of the length of your upper arm)
    Sounds horrendous (around 1500 lbs)!!! But, as you lift it (haha) the muscle force should get less.

    If you wanted to know the equivalent weight held directly in the hand, that would be
    Hammer weight X (length of hammer plus arm) ÷ (length of arm)
    About 40 lbs.

    *Mechanical advantage: for muscles, working on bones and lifting a load, the mechanical 'advantage' is actually a fraction (less than one) - around 1/10, so it's a 'disadvantage' haha and the muscles need to be ten times stronger than the load they need to deal with. But it does mean that the end of your limbs can move a long way for a small muscle contraction.
     
  7. Jan 23, 2013 #6
    OK finally found a good video. Hope its OK to post links

    http://m.youtube.com/#/watch?v=96pKz0rzxko&desktop_uri=/watch?v=96pKz0rzxko

    Thank you for your reply I found it very helpful. The actual pivot point is in between pointer finger and thumb, I think. That being said most of the force is put upon upper wrist and forearm.

    What you mentioned about lowering the wrist to below the elbow and then back up isn't aloud in the feat. Sorry I should have mentioned that the wrist must stay above the , but for the record attempting to swiftly move the hammer head into position that way would likely result in a broken wrist when the momentum came back with a vengeance.
    Thank you again for your time. Is there a way to +1 people for helping on this forum?
     
  8. Jan 23, 2013 #7

    sophiecentaur

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    Nah. We just ride off into the sunset. :smile:
    (Mind your wrist!)
     
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