Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can you push down more than your weight?

  1. May 15, 2007 #1
    quick question.......is it possible to press down more than you weigh without being strapped in ?
     
  2. jcsd
  3. May 15, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I don't see how, not without accelerating. You, of course, could jump in the air and hit the floor with a force greater than your weight, but I assume that's not what you are talking about. Pressing down on a bar (attached to a pulley and weights, say) smoothly and continuously, the best you can do is lean all your weight on it.
     
  4. May 15, 2007 #3
    im talking about in a seated tricept machine. with a cable and pullies. if you are seated there and without the strap around your waist. lets say you weigh 200lbs and you have 275lbs selected on the machine.....can you press 275lb down when you weigh 200 without leaving the ground.....thanks for your help on this
     
  5. May 15, 2007 #4

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, not in smooth controlled movements unless you are strapped in.
     
    Last edited: May 15, 2007
  6. May 15, 2007 #5
    thanks for your help on this
     
  7. May 15, 2007 #6

    Doc Al

    User Avatar

    Staff: Mentor

    (I think I'll move this little sidebar to its own thread, lest it confuse the original topic.)
     
  8. May 15, 2007 #7
    Well, I don't know what I can say. What I do know is I was not strapped in and I selected 270 pounds and was able to press it.
     
  9. May 15, 2007 #8

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In smooth controlled movements, or fast explosive presses? Without wrapping your legs around anything, or pushing yourself back into the seat?

    P.S. You've got your own thread now :biggrin:
     
  10. May 15, 2007 #9

    turbo

    User Avatar
    Gold Member

    You have not described the machine adequately, but if you are pressing a bar attached to a cable, and that cable loops around a pulley at the top of the weight stack, you are actually pressing 135# (disregarding mechanical drag).
     
  11. May 15, 2007 #10
    I did smooth controlled movements. As a matter of fact, I did negative presses on the upswing. So, it was very controlled. the seat in which I sat is not straight back. It is at a slight forward angle so my body is in a slightly forward position. My feet flat on the ground and not wrapped on anything and I was not strapped into the seat.
     
  12. May 15, 2007 #11

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Okay, fair enough. However, as turbo said the configuration of the pulleys can reduce the amount of force required to lift the nominal mass.
     
  13. May 15, 2007 #12
    thanks......so there is now way to push down more than you weigh, with smoth controlled movements without leaving the ground?
     
  14. May 15, 2007 #13

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not without restraining yourself with a strap or bar, no. That is why Lat Pull-down machines have knee bars, since your lats are stronger than your triceps, you can lift/pull a lot more weight with them, many times your bodyweight. Placing your legs under the bars stops you coming flying out of your seat when you pull ala chin ups.
     
  15. May 15, 2007 #14
    thanks ....you guys are awesome
     
  16. May 15, 2007 #15

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    My Pleasure :smile:
     
  17. May 15, 2007 #16
    I'm the guy doing the seated Tricep Pushdowns

    Although my friend Jace positioned my original questions, we had a healthy disput this morning about this aspect of working out specifically with seated Tricep Pushdowns. I simply stated that I was able to select 270 lbs. on the machine. His response was you would be lifted from the seat. True enough if my back were not pushing in a seat at approximately 75 degrees. Additionally, my hands are grippping and situated to each of my sides. Granted there is a pully system but it behooves me to believe that 270 pounds would be reduced to approximately 135 lbs. I weight approximately 197 lbs. at 6 feet tall. Please advise.
     
  18. May 15, 2007 #17

    turbo

    User Avatar
    Gold Member

    Look at the configuration of the pulley system. In general with weight machines, if there are only fixed pulleys, their function is to change the direction of the applied force and there is no mechanical advantage. If there is a pulley on top of the weight stack and the cable wraps around that, and that pulley rides up and down with the weights, you can safely assume that it is cutting the force required to lift the weights by 50% (disregarding friction, etc), so no matter how much weight you select, you could balance that weight by putting half that amount of weights on the bar. If you've got some free-weights in that gym, you can prove it for yourself by attaching plates to the bar.
     
  19. May 15, 2007 #18
    ok lets say the pully is just changing the direction of the applied force, and there is no mechanical advantage with the pullies.......would you now have to consider the leverage. where your hands grip to where the cable is attached?
     
  20. May 15, 2007 #19

    Doc Al

    User Avatar

    Staff: Mentor

    Clearly how you grip the bar will determine how much force you can apply--but never more than your weight, unless something else besides gravity is holding you down.
     
  21. May 15, 2007 #20
    You can lift more weight than you weigh, but only with some sort of mechanical advantage. These things include lever arms, screws, and pullies. If this machine uses any of these (most likely pulleys), then you can lift more weight than you weigh. This is slightly decieving, however, because if you're using a 1-pulley system to lift 270 lbs, then you only need to apply 135 lbs of force to begin accelerating the weight upwards.

    You could, with frictionless pullies and a lot of massless string, lift millions of pounds with a pinky finger. This would require an extremely large distance traveled by your pinky finger for a small amount of distance moved by the weights.

    This is due to the fact that the amount of work done by your hands is equal to the amount of work done on the weights. If W = Fd(cosx), and if you're applying 135 lbs of force to the bars, then you can plug in to find that the weights should move exactly 1/2 the distance which your hands moved.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?