# Physics question : is this graphic accurate ? **URL:** \ **Category:** Factual Questions\ **Created:** [July 9, 2015, 2:31am UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634 "2015-07-09T02:31:23Z")\ **Posts on this page:** 10\ **Page:** 3
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [July 10, 2015, 4:46am UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/41 "2015-07-10T04:46:10Z")
> [@Joey\_P](#): > > Assuming it can physically support them, the weight is evenly distributed across it’s length and the two end people are strong enough to hold half of it (more or less) plus the weight of the hanging person, it shouldn’t make a difference if it’s a 5 pound carbon fiber rod, a 10 pound wood 2x4 or a 200# steel I-Beam. Incorrect. It matters if the weight of the beam is a significant fraction of the load to be carried. A 300 lb beam wiyh 3 200lb workers is going to be different than a 25 lb beam, even if it is rigid and strong enough to hold the weight. > [@JimSC](#): > > Actually, the maximum required strength would only be 3X. Why? The beam, of course. The guy in front with feet in air. The guy in back, also with feet in air to act as counterweight. And the center person’s own weight, which is not being carried on the shoulder, but it’s there. 4x. > [@cmyk](#): > > All things being equal, when the first guy is over the gap, couldn’t the guy in the rear lower his center of gravity by crouching, thereby enforcing some leverage? Crouching but holding onto the beam to create counterweight is also called hanging from the beam. > [@Joey\_P](#): > > While I hadn’t considered that, that’s exactly that kind of stuff that gets ignored, and is meant to be ignored when we say “in a perfect world” in physics class. The same way you ignore air resistance, friction, heat and just about every other force that you’re not directly testing. > The middle guy might actually be able to support 800 pounds on his shoulder, but he went out drinking last night and has a migraine and his muscles are really shaky, so it’s not going to work tonight. I disagree. The graphic assumes all three support the load from underneath, and when one dangles, the other two still support from underneath. No. The two in back do not evenly share the load from the one in front. Just consider the guy in front letting go and stepping aside, and let the other two carry the beam without him. The second person carries a lot more load than the third, because of the cantilever. Now put more weight on front. > [@](#): > > Imagine it’s just three big wheels, make them as heavy and strong as you want or three people sized pillars with wheels at the bottom that can be pushed across the gap. The system is strong enough that it won’t collapse. Would it work then? Assuming _that’s_ what’s really being tested, I vote yes. If the beam is sufficiently rigid, and the wheels/columns are sufficiently rigid, then yes out well work. You know what “sufficiently rigid” in this case means? It means the beam is an insignificant weight so it doesn’t flex and doesn’t cause the wheels/columns to flex. The middle wheel is sufficiently strong to support the weight of the column and first wheel and not flex so the hanging wheel rolls onto the surface, not dips in the gap. You ideal case does not resemble this presented case because the assumptions aren’t valid. > [@TruCelt](#): > > My one question becomes the last member. Would three at the other end allow the beam to be supported and stable enough to act as a cantilever? Perhaps, with two people together acting as fulcrum and sharing the load, and the bricklayer as counterweight. Depends on the weight of the beam. > [@Saint\_Cad](#): > > BUT what if the woman in the middle moves her hand backwards enough to create a moment arm and pulls down on the beam? Now the torque will hold the guy in front up and help the guy in back stay down. She still has to lift all the weight. She’s fighting her owm strength. Yes, it helps the dynamic wiggling aspect, but doesn’t solve the fundamental problem that the load imbalance creates. ---
**Author:** ![Ornery\_Bob](https://avatars.discourse-cdn.com/v4/letter/o/da6949/32.png) [@Ornery\_Bob](https://boards.straightdope.com/u/Ornery_Bob)\ **Post date:** [July 10, 2015, 4:57am UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/42 "2015-07-10T04:57:51Z")
_No. Unless person in centre is a weight lifter with super strength, the answer is NO._ Let’s assume the 165 lbs weight class - so three times that (two people and the beam) is 495 lbs. The benchpress record for the 165 lb weight class is like 500+ lbs. The deadlift record is somewhere in the 700+ lbs range… simply supporting 495 lbs? Not a problem. Hell, the woman’s deadlift record for the 165 lb class is 535 lbs! If she can LIFT more than three times her weight, she can support it. So, maybe not possible for people of “average” strength, but for trained weightlifters, the weights involved would not be unusual. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [July 10, 2015, 5:02am UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/43 "2015-07-10T05:02:13Z")
> [@Learjeff](#): > > Both happen because the end person on the land is not lifting the beam, he’s pulling down on it. With two people supporting a beam with single point of contact each, there’s no way for two to help in supporting a third who’s on either end. Only the guy in the middle gets supported by two people. I don’t think it’s an either/or situation. You don’t have to exert all your force in one direction. You can just hold the beam in place. So Carl could exert just enough downward force on the beam to keep Al from dropping. And because Carl is further from the fulcrum he can apply less downward force than Al is and still balance him. In fact, if Carl lifted his feet off the ground and applied all of his weight downward, he’d cause Al to rise up. ---
**Author:** ![GuanoLad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guanolad/32/42_2.png) [@GuanoLad](https://boards.straightdope.com/u/GuanoLad)\ **Post date:** [July 10, 2015, 8:00am UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/44 "2015-07-10T08:00:08Z")
So if I had a six wheeled trolley, and it went over a channel in the floor, my guess is the front wheels would dip into the channel immediately upon their encounter. Unless the trolley was evenly weighted over the front and back wheels, but not over the middle pair of wheels. At least, theoretically. I am not a physicist. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [July 10, 2015, 4:09pm UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/45 "2015-07-10T16:09:01Z")
Sort of close answers. Assuming the weight is not a problem to carry. Each person is not a knife edge fulcrum. They have at least two points of contact on the beam, shoulder and hand. So while the front worker is going over, the center person has to pull up on the beam in front of her (or lift from below with her hand) to help keep the beam weighted to the back. The question is, does she have the upper body strength? Similarly, the back or front person can help balance by extending their leverage. The person who wants to stay on the ground (read person at first) simply crouches a little, thus pushing their weight (beer belly and fat ass) further from the balance point. They use their hand to emphasize this added distance. heck, just moving the hand back away from the fulcrum point gives added moment. But yes, as illustrated without any such “tricks” it is a risk that the least motion will imbalance the setup, since it is “dynamically neutral” ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [July 10, 2015, 8:03pm UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/46 "2015-07-10T20:03:19Z")
> [@Little\_Nemo](#): > > I don’t think it’s an either/or situation. You don’t have to exert all your force in one direction. You can just hold the beam in place. So Carl could exert just enough downward force on the beam to keep Al from dropping. And because Carl is further from the fulcrum he can apply less downward force than Al is and still balance him. In fact, if Carl lifted his feet off the ground and applied all of his weight downward, he’d cause Al to rise up. You’re right; I was wrong. Take the extreme case, where the beam is long, and A slides to just in front of B, with C at the far end. C has to exert only a small force down to keep A up, and B carries A’s weight plus the small downward force of C (plus the beam, of course). I was right that only one person can support the suspended person (as I said, the woman does the heavy lifting.) But I was clearly wrong that it’s always the full weight of both people! The higher the ratio of CB to BA, the less B has to support. The asymptote is A’s weight plus half the weight of the beam. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [July 10, 2015, 8:10pm UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/47 "2015-07-10T20:10:07Z")
> [@Learjeff](#): > > … The asymptote is A’s weight plus half the weight of the beam. Oops, A’s weight plus the weight of the beam. ---
**Author:** ![usedtobe](https://avatars.discourse-cdn.com/v4/letter/u/ec9cab/32.png) [@usedtobe](https://boards.straightdope.com/u/usedtobe)\ **Post date:** [July 11, 2015, 6:50am UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/48 "2015-07-11T06:50:40Z")
While it would be possible to stage each of the scenes, it would not work if the people are moving - the live load vs static load. With an end person over thee gap, the rear-most would need to shift away from the gap to offset the moment created by the drop of the person over the gap. IOW: it would be possible to arrange the spacing so either the front or rear person could be supported; the same configuration would fail if the action were reversed. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [July 13, 2015, 5:27am UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/49 "2015-07-13T05:27:53Z")
> [@Little\_Nemo](#): > > I don’t think it’s an either/or situation. You don’t have to exert all your force in one direction. You can just hold the beam in place. So Carl could exert just enough downward force on the beam to keep Al from dropping. And because Carl is further from the fulcrum he can apply less downward force than Al is and still balance him. In fact, if Carl lifted his feet off the ground and applied all of his weight downward, he’d cause Al to rise up. I think the original setup was equal weight, equal spacing. Yes, you can affect that with a long enough beam and creative shifting. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [July 13, 2015, 7:24pm UTC](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634/50 "2015-07-13T19:24:37Z")
> [@usedtobe](#): > > While it would be possible to stage each of the scenes, it would not work if the people are moving - the live load vs static load. If the people on the end work in concert, this problem goes away. That is, if they both lift their legs at the same time. [Previous page](https://boards.straightdope.com/t/physics-question-is-this-graphic-accurate/724634.md?page=2)