1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Lifting Weight VS Pushing a Scale

  1. Oct 27, 2008 #1
    Is the concept the same? If I am able to bench press a maximum of 200lbs, and I were to push with full force a scale bolted in a stationary position at the weakest point of my bench press motion, would the scale read 200lbs?
     
  2. jcsd
  3. Oct 27, 2008 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Yes, with the caveat that stability plays a role in weightlifting. If you can bench 200 lb, can you dumbell press 2x 100 lb?
     
  4. Oct 28, 2008 #3
    At the "weakest point of my bench press motion" the scale would read 0, as no force is being applied.
     
  5. Oct 28, 2008 #4

    russ_watters

    User Avatar

    Staff: Mentor

    No, at the weakest point, you are just barely able to get the bar off your chest. If the bar+weights is 200lb, then the force is 200 lb. If you couldn't apply any force at the weakest point, you'd choke yourself to death.
     
  6. Oct 28, 2008 #5
    the dumbell question doesn't really come into play because I am only comparing a straight bar with 200lbs on it to the same straight bar being pushed against the scale.
     
  7. Oct 28, 2008 #6
    Thanks Bobby. I'm not sure if my answer is correct(though I think it is), but I do understood your question.
     
  8. Oct 28, 2008 #7
    You can probably push more with a setup like that since its more stable.
     
  9. Oct 28, 2008 #8

    russ_watters

    User Avatar

    Staff: Mentor

    So you are pushing on the barbell and weights and a scale? Or a barbell only? The OP doesn't suggest you are pushing against anything except a scale.

    If you are pushing against a bar (and no weights) bolted to a scale, then the force is 200-45= 135.
     
  10. Oct 28, 2008 #9

    russ_watters

    User Avatar

    Staff: Mentor

    That was my point with the dumbell comment. You can lift more with a bar than with dumbells because you have 1 axis of stability to deal with instead of 2. And if the setup described by the OP is completely immoble, then you have 0 axes of stability to deal with instead of 2. Essentially, you can push as hard as you want and not have to worry about the force being perfectly vertical.
     
  11. Oct 29, 2008 #10
    Well he says its bolted in the first posting. What is 200-45 about?He can probably push 300 lbs with a bolted scale.
     
  12. Oct 29, 2008 #11

    russ_watters

    User Avatar

    Staff: Mentor

    It didn't say bolted to what, but fair enough.
    I don't know how much more force he could apply due to the added stability, but if he can only apply 200 lb, the 200-45 is the 200 lb he is capable of generating minus the 45 lb of the bar.
     
  13. Oct 30, 2008 #12
    let me clarify:
    senario 1 - a man lifts a 200lbs plate off his chest. we will call the position of the starting point on his chest '0'. his muscles are only capable of lifting this 1 time and he is not capable of lifting any more weight than this. this is called his 1 rep max.
    senario 2 - the same man has a scale the exact size and shape as the plate in senario 1 bolted in place at position '0'. he pushes on the scale with all his force as if to try and do the same motion as from senario 1.

    question: does the scale reach 200lbs? does it pass the 200lbs mark? etc..?

    assumption: this is a perfect world and balance does not come into play.

    Thanks for all the responses so far!
     
  14. Oct 30, 2008 #13
    Like i said, the scale will pass 200 lbs mark, probably up to 250-300.
     
  15. Oct 30, 2008 #14
    Thanks Emreth! Could you please explain? I saw one mention of stability, but I mentioned in the repost that that is not a factor as both are sompletely stable/balanced. And I saw a mention of the 45lbs of bar, but that has also been removed in the repost.
     
  16. Oct 30, 2008 #15
    The weight of the bar is irrelevant in this case, only the total weight matters, which is 200 lbs. When you're lifting free weights, some of the muscles are used just for balancing. When it's secured as in your case, you can use all the muscles in the single direction, you can push with more force which would show up on the scale as something larger than 200 lbs. If you bench press in a machine in a gym for example, which are stable compared to free weights, you can lift more for the same reason.
     
  17. Oct 30, 2008 #16
    So let's say the weights are guided in senario 1, so the lifter doesn not have to control stability on his own. Would it the scale then stop at 200lbs?
     
  18. Oct 30, 2008 #17
    no it will go higher. you can push more in that case.
     
  19. Oct 30, 2008 #18
    Force = mass x acceleration
    1kg = 2.2 lbs, 200 lbs = 91kg

    91 kg barbell resting on a scale on your hand without any movement will show 91 kg X 9.8m/s^2 netwons (equal to 891.8N or 91kg or 200lbs)

    Now when you accelerate uniformly it at say 0.2 m/s the force will now become 91 kg x 10 newtons which is also equal to 93kg. A 3 kg or 6.6lb increase reading on the scale

    So in other words acceleration has everything to do with it. The faster you try to lift it, the heavier it will be until velocity is constant.
     
  20. Oct 30, 2008 #19

    stewartcs

    User Avatar
    Science Advisor

    200 lbf is 200 lbf so the scale will read the same thing in a perfectly balanced situation. Different muscles are used for stability when performing a flat bench press (shoulders primarily whereas the pectorals do most of the work in a flat bench press) so in the real world the reading would be essentially the same.

    CS
     
  21. Oct 30, 2008 #20

    stewartcs

    User Avatar
    Science Advisor

    Most everyone lifts the weight at a constant velocity so the additional acceleration doesn't exist.

    CS
     
  22. Oct 30, 2008 #21

    stewartcs

    User Avatar
    Science Advisor

    If, as assumed in the OP's statement, the two scenarios are balanced conditions, the scale will read the same and not higher. All of the muscles' force would be applied in the same direction on both the lifts.

    CS
     
  23. Oct 30, 2008 #22
    But from rest to that velocity there will be acceleration, until the velocity is constant like i mentioned.
     
  24. Oct 30, 2008 #23

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Assuming the person doesn't try to cheat, it should weigh 200 pounds, since that has been identified as the most he can push. OTOH, if he slammed his hands into the scale, he might be able to get a higher reading (I mention this solely to forewarn you if you're applying this to an actual device)
     
  25. Oct 30, 2008 #24
    No. A 200lb barbell resting on a table would have the table exerting a 200lbs (x9.8) normal force back onto the weight. The ending result is no movement. Newtons first law: every object will stay at rest or constant velocity if no force acts upon it. So for a weightlifter to hold a 200lb weight and then accelerate it upwards, he would need an additional force to start the acceleration
     
    Last edited: Oct 30, 2008
  26. Oct 30, 2008 #25

    stewartcs

    User Avatar
    Science Advisor

    Sorry I didn't see that last part before. However, the initial acceleration would be negligible since the person would hit their constant velocity almost immediately.

    CS
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook