enter image description here

I saw this on a youtube video. The answer is that the one in the middle is carrying 25 % more than the other two.
Can someone explain this?


There is no way to know, just from the picture, because the problem of body equilibrium with 3 upward forces is underdetermined. The middle man could choose to be slacking and pushing only a little, while the two other men carry most of the weight. Or he could choose to carry the load himself, so the other two men will not have to push up at all.

In case you're thinking "they are not pushing, the log is just put on their shoulders" then the problem is still underdetermined; the actual forces will depend on the details of the shape of the log, and the way the log and the bodies of the men deform.

| cite | improve this answer | |
  • $\begingroup$ i think we can assume that all of them are trying equally hard $\endgroup$ – Milan Jul 24 '18 at 18:28
  • $\begingroup$ @MilanStojanovic, that is hard to translate into physics, because they actually do not have to be trying at all - the log could be just put on their shoulders. Then, see the second part of my answer. $\endgroup$ – Ján Lalinský Jul 24 '18 at 18:30
  • $\begingroup$ are u saying that the same force acts on each one of them $\endgroup$ – Milan Jul 24 '18 at 18:30
  • $\begingroup$ No. I'm saying the forces cannot be determined without further assumptions about the log and the way it interacts with the men. $\endgroup$ – Ján Lalinský Jul 24 '18 at 18:31
  • $\begingroup$ About the only thing that can be said with certainty is that the frontman and the backman will carry roughly the same weight, but what weight it will be, we cannot determine. $\endgroup$ – Ján Lalinský Jul 24 '18 at 18:32

Not the answer you're looking for? Browse other questions tagged or ask your own question.