Laws of Motion in weight lifting

  1. Hi all, I'm trying to get my head round how to apply newtons laws and an exercise like a bench press, probably a bit simple to what you guys are used to.

    I want to make it as simple as possible to understand what forces would be acting during the eccentric phase of an exercise, I.e lowering the barbell during a bench press.

    Rather than viewing it as a moment I want to view it as an object moving vertically at a constant speed. So say you lower a 50K dumbell over a 3 second period, lets suppose you move it 0.6 metres. means it's travelling at 0.2 m/s
    now if it's moving at a constant speed, the forces should be equal, which means you'd be pushing up at 50K, but if that was the case, how would it be lowering in the first place. THink I'm just looking at it from the wrong angle. Anyways any help would be really appreciated, if anyone would be willing to chat on MSN it would be much appreciated.


    Regards
    Zach.
     
  2. jcsd
  3. atyy

    atyy 11,047
    Science Advisor

    Say it's stationary at the top. So you are pushing it up with 50k, and gravity is pulling down with 50k. To start it moving down, you momentarily decrease the force with which you are pushing it up, so that gravity can pull it down. It accelerates downwards. It stops accelerating, and continues moving downwards with constant velocity when you increase your upward force back to 50k.
     
  4. Cool, that makes sense, then because of the intertia, a force greater than 50K would be required at the bottom of the motion to stop it.

    Thanks for your help.
     
  5. cepheid

    cepheid 5,190
    Staff Emeritus
    Science Advisor
    Gold Member

    What do you mean "because of the inertia?" Be careful not to misuse terminology.

    The only thing inertia does is tell you that if you don't do something, the weight will keep moving downward at a constant speed. I.e., there has to be a *net* upward force on the dumbbell if you want to decelerate it. Obviously you have to push up with more than 50 kg * g if you want there to be a *net* force.
     
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