FORUMS INSIGHTS BLOG VIDEOS PHYSICS NEWS INFO SEARCH Forum Phyi Claial Phyi PF thrive by your haring of our ommunity with your friend, family and oial media. If you do, tell me and I'll give you Gold Memberhip. MY PF INBOX ALERTS Sharing i Caring Link to PF PF Inight B The phyi of the inlined puhup Tag: Add Tag Page 1 of 2 1 2 Next > Unwath Thread We Turner #1 Aug 15, 2018 I am trying to analyze the phyi of the inlined puhup. I have attahed a PDF howing what I have o far. I would appreiate any ritique. I don't ee the attahment in the preview, o I hope it i viible. 43 / 3 I am epeially onerned with Figure 5. I have plaed the load (W) at the enter of ma and eparated the omponent. I then adjuted W, the weight in the diretion of the fore applied by the arm at the houlder by the mehanial advantage to get F, the fore needed to raie the body. I thi all orret? Thank Attahed File: Puhup Geometry File ize: 318 KB View: 34 Orodruin #2 Aug 15, 2018 Firt of all, a orretly performed puh-up doe not have the hand at the 1 of 20 09/06/18 18:36
2 of 20 09/06/18 18:36 level of the houlder. Partiularly not in the bottom poition. See, e.g., from 4.32 of thi video: 13,663 / 4,542 Staff Emeritu Siene Advior Homework Helper Inight Author Gold Member 2017 Award At the top of the puhup, your hand might be eentially below your houlder due to your houlder moving in a irle and therefore bakward, but it hould not be the etup in the bottom poition and the angle between your arm and your body will be le than 90 degree in the top poition. Another effet that you eem to be negleting i the fat that the entre of ma of your body will typially be behind your toe (whih you are tanding on). Thi will affet the moment arm of your entre of ma and thereby the moment that your hand puhing will need to generate to lift you. I ugget you look at the fore that need to be generated by your hand on the ground rather than the fore on the houlder. At the houlder relative to the arm there will be many fore ating, induing torque around the houlder in order to let the hand generate the appropriate amount of fore. "You did not take relativity of imultaneity into aount." - The anwer to 99% of all paradox thread in the relativity forum My PF Inight, inluding The Birth of a Textbook You like thi.
3 of 20 09/06/18 18:36 Orodruin aid: #3 Aug 15, 2018 43 / 3 Firt of all, a orretly performed puh-up doe not have the hand at the level of the houlder. Partiularly not in the bottom poition. See, e.g., from 4.32 of thi video: At the top of the puhup, your hand might be eentially below your houlder due to your houlder moving in a irle and therefore bakward, but it hould not be the etup in the bottom poition and the angle between your arm and your body will be le than 90 degree in the top poition. Great video, thank for that. Ye, I know that the hand are not exatly below the houlder, but I thought it would be loe enough for what I am trying to do. How muh differene would there be between the hand being exatly below the houlder and a few inhe lower? How would I go about alulating that differene? Another effet that you eem to be negleting i the fat that the entre of ma of your body will typially be behind your toe (whih you are tanding on). Thi will affet the moment arm of your entre of ma and thereby the moment that your hand puhing will need to generate to lift you. I don't undertand thi. I thought the enter of ma of any body i a fixed property of that body. In the own poition, whih i where I am doing my alulation, the body i at ret upported by the toe and hand. Can you provide a keth howing that the enter of behind the toe? I ugget you look at the fore that need to be generated by your hand on the ground rather than the fore on the houlder. At the houlder relative to the arm there will be many fore ating, induing torque around the houlder in order to let the hand generate the appropriate amount of fore. I gue that wa what I wa trying to do. I might have jut drawn the Fp arrow in the wrong plae. I wa trying to alulate the fore on the hand of the weight of the body redued by the mehanial advantage of the lever.
4 of 20 09/06/18 18:36 tnih #4 Aug 15, 2018 752 / 227 Homework Helper I like your preentation. I only have one minor quibble. You orretly define W F = W in θ to be a fore parallel to the body, but ubequently refer to it a "weight on the feet". When the body i parallel to the ground, the weight on the feet i W(1 ). In general, uing your diagram and aumption, the weight upported by the feet would be W(1 o 2 θ), and the total fore exerted by the feet W(1 ) o θ. You like thi. Orodruin #5 Aug 15, 2018 We Turner aid: 13,663 / 4,542 Staff Emeritu Siene Advior Homework Helper Inight Author Gold Member 2017 Award Ye, I know that the hand are not exatly below the houlder, but I thought it would be loe enough for what I am trying to do. How muh differene would there be between the hand being exatly below the houlder and a few inhe lower? How would I go about alulating that differene? I would ay that on an adult the differene would amount to omething around 10-15 m. Thi would be a differene of around 6-10% in the moment arm relative to the feet. We Turner aid: I don't undertand thi. I thought the enter of ma of any body i a fixed property of that body. In the own poition, whih i where I am doing my alulation, the body i at ret upported by the toe and hand. Can you provide a keth howing that the enter of behind the toe? A long a that body doe not hange hape or weight ditribution. It i quite eay to undertand that the CoM i behind the toe in the traight poition. If it wa not you would be ontantly at rik of falling over when
5 of 20 09/06/18 18:36 tanding traight. You an ee that it hange the moment arm from your figure 5. Imagine tilting the puh-uper even more up. If the CoM wa not behind the toe then he would reah equilibrium only at the very lat moment. Imagine replaing the man by a long homogeneou box - you do not need to get the box to a fully upright poition for the CoM to be above the ontat point with the ground. The ame priniple applie here. So here i a fun additional tak: Argue for why puh-up are eaier when tanding on your knee. "You did not take relativity of imultaneity into aount." - The anwer to 99% of all paradox thread in the relativity forum My PF Inight, inluding The Birth of a Textbook
6 of 20 09/06/18 18:36 We Turner #6 Aug 15, 2018 tnih aid: 43 / 3 I like your preentation. I only have one minor quibble. You orretly define W F = W in θ to be a fore parallel to the body, but ubequently refer to it a "weight on the feet". When the body i parallel to the ground, the weight on the feet i W(1 ). In general, uing your diagram and aumption, the weight upported by the feet would be W(1 o 2 θ), and the total fore exerted by the feet W(1 ) o θ. Did you ome up with the equation for the weight felt by the feet by firt alulating the weight felt by the hand ( W( )) and then ubtrating that from W? The attahed diagram illutrate thi. I it orret? It make intuitive ene. If I et S=C (moving the hand/houlder to the enter of ma), then the weight felt by the hand = W and the weight felt by the feet = 0, whih i a it hould be. Similarly, if I et S=H (move the hand to the top of the head), then both the hand and feet feel a weight of W/2. Attahed File: Puhup Geometry File ize: 35.2 KB View: 5 tnih #7 Aug 15, 2018 We Turner aid: 752 / 227 Homework Helper Did you ome up with the equation for the weight felt by the feet by firt alulating the weight felt by the hand W( ( )) and then ubtrating that from W? Ye.
7 of 20 09/06/18 18:36 We Turner aid: The attahed diagram illutrate thi. I it orret? It make intuitive ene. If I et S=C (moving the hand/houlder to the enter of ma), then the weight felt by the hand = W and the weight felt by the feet = 0, whih i a it hould be. Similarly, if I et S=H (move the hand to the top of the head), then both the hand and feet feel a weight of W/2. If you draw a imilar free body diagram for the inlined puh-up, you an olve the reulting fore and torque equation for the fore exerted by the feet. I did find an error in my olution, though. The total fore exerted by the feet hould be W 1 2 o 2 θ + ( ) 2 o 2 θ You like thi. We Turner #8 Aug 16, 2018 tnih aid: Ye. 43 / 3 OK If you draw a imilar free body diagram for the inlined puh-up, you an olve the reulting fore and torque equation for the fore exerted by the feet. Here' the drawing, but I don't think I have the fore orret. The fore at the houlder/hand (=WhCo(θ)) look reaonable, but I don't know what to do with the other fore. Do I break W up into omponent like Wh and then et Wt = W - WhCo(θ) - WCo(θ)?
8 of 20 09/06/18 18:36 I did find an error in my olution, though. The total fore exerted by the feet hould be W 1 2 o 2 θ + ( ) 2 o 2 θ I don't follow that at all. Can you provide a diagram? tnih #9 Aug 16, 2018 We Turner aid: OK 752 / 227 Homework Helper Here' the drawing, but I don't think I have the fore orret. The fore at the houlder/hand (=WhCo(θ)) look reaonable, but I don't know what to do with the other fore. Do I break W up into omponent like Wh and then et Wt = W - WhCo(θ) - WCo(θ)? Your free body diagram doe not how all of the fore. The firt thing you need to deide i, what i the (ingle) objet that the fore at on. (In more omplex problem with multiple objet, you may need to draw a FBD for eah one.) In thi ae you have one objet - the peron doing the puhup. I have drawn thi a a blue line in my diagram. Thi aume that all of the fore at on point on the ame line, whih i an approximation.
9 of 20 09/06/18 18:36 Thi diagram how all of the fore ating on the body - the fore on the enter of gravity W, the fore on the houlder S, and the fore on the feet F. I am auming the objet i not moving (yet), o the net fore and the net torque hould be zero. Uing thoe fat and the diagram, you hould be able write the equation for the net fore in both the x diretion and the y diretion, and for the net torque. Notie that to get a net fore of zero in the x diretion, you a need a fore to balane N x. The only plae thi fore an be exerted i on the feet. So 0 = N x + F x I point thi out to how that uing the FBD and the net fore/torque equation together, you an uually figure out what all of the fore are and in whih diretion they at. We Turner aid: I don't follow that at all. Can you provide a diagram? Uing thi diagram, you hould be able to write out the net torque/fore equation and olve for F.
10 of 20 09/06/18 18:36 We Turner You like thi. #10 Monday at 1:13 PM Hi, I am jut getting bak to thi after leaning up ome more preing work. tnih aid: 43 / 3 Thi diagram how all of the fore ating on the body - the fore on the enter of gravity W, the fore on the houlder S, and the fore on the feet F. A ouple of quetion: 1. I do not ee any fore labeled S in your diagram. Did you mean N or hould there be another down arrow at the houlder labelled S? 2. The fore at the houlder are all labeled N. Are thee the Normal fore exerted by upport urfae where the peron i reting their hand? 3. The fore at the feet are all labeled F. Are thee not alo Normal fore? Should they be labeled omething like N f, N fx, and N fy? And if o, then the Normal fore at the houlder alo need an extra ubript, omething like N, N x, and N y, no? 4. The fore at the feet labeled F i pointing at what appear to be an angle of Θ above the body line. I that orret? How did you arrive at that angle? Doe that mean that the feet are exerting an equal and oppoite fore on the ground? Notie that to get a net fore of zero in the x diretion, you a need a fore to balane N x. The only plae thi fore an be exerted i on the feet. So 0 = N x + F x Do you mean on the feet or by the feet? If it' on the feet, what i exerting that fore? Do we alo need a fore to balane N y or are the two y fore Normal fore exerted by the floor and the elevated upport?
11 of 20 09/06/18 18:36 We Turner aid: #11 Monday at 4:16 PM Hi, I am jut getting bak to thi after leaning up ome more preing work. 752 / 227 Homework Helper A ouple of quetion: 1. I do not ee any fore labeled S in your diagram. Did you mean N or hould there be another down arrow at the houlder labelled S? 2. The fore at the houlder are all labeled N. Are thee the Normal fore exerted by upport urfae where the peron i reting their hand? 3. The fore at the feet are all labeled F. Are thee not alo Normal fore? Should they be labeled omething like N f, Nfx, and Nfy? And if o, then the Normal fore at the houlder alo need an extra ubript, omething like N, Nx, and Ny, no? 4. The fore at the feet labeled F i pointing at what appear to be an angle of Θ above the body line. I that orret? How did you arrive at that angle? Doe that mean that the feet are exerting an equal and oppoite fore on the ground? Do you mean on the feet or by the feet? If it' on the feet, what i exerting that fore? Do we alo need a fore to balane N y or are the two y fore Normal fore exerted by the floor and the elevated upport? You are right, there i no S in the diagram. I alled it N in the diagram. The anwer to the ret of your quetion i that eah fore ha an equal and oppoite oppoing fore. The weight on the hand and feet would add up to W. But ine I already have W in the diagram, putting vetor for weight on the houlder and feet would be double ounting. A it i, the diagram how eah fore one, with the fore on the houlder and feet broken out into x and y omponent. Everything ele an be derived from what i hown in the diagram, inluding the angle of F. Try uing the diagram to write the equation for net fore in the x and y diretion. Lat edited: Tueday at 7:04 AM You like thi.
12 of 20 09/06/18 18:36 tnih #12 Monday at 4:23 PM tnih aid: 752 / 227 Homework Helper You are right, there i no S in the diagram. I alled it N in the diagram. The anwer to the ret of your quetion i that eah fore ha an equal and oppoite oppoing fore. The weight on the hand and feet would add up to W. But ine I already have W in the diagram, putting vetor for weight on the houlder and feet would be double ounting. A it i, the diagram how eah fore one, with the fore on the houlder and feet broken out into x and y omponent. Everything ele an be derived from what i hown in the diagram, inluding the angle of F. Try uing the diagram to write the equation for net fore in the x and y diretion. Oh, and the net torque equation. Lat edited: Tueday at 7:04 AM tnih #13 Monday at 4:36 PM tnih aid: Oh, and the net torque equation. 752 / 227 Homework Helper One of the key onept you need to draw a free body diagram i that you need to deide what body it repreent. In thi ae it i the body of the peron doing the puhup, not the thing he i puhing againt. In the diagram you only inlude the fore ating on that body, not reation fore. In thi ae, there are only three fore ating on the body - the fore of gravity W ating downward, the normal fore at the houlder N, and the fore on the feet F. You like thi. #14 Tueday at 1:14 AM
13 of 20 09/06/18 18:36 tnih aid: 43 / 3 One of the key onept you need to draw a free body diagram i that you need to deide what body it repreent. In thi ae it i the body of the peron doing the puhup, not the thing he i puhing againt. In the diagram you only inlude the fore ating on that body, not reation fore. In thi ae, there are only three fore ating on the body - the fore of gravity W ating downward, the normal fore at the houlder N, and the fore on the feet F. Thi wa very helpful. I ee now that I wa getting myelf onfued by having fore ating all over the plae. Let me tart over with the fore in the down poition. Conider thi diagram. I am uing an atual peron image, rather than a blue line, beaue I am writing thi for normal (non-tehie) people. C and S are a before, the ditane to the enter of ma and the houlder. D i the ditane to the upport truture where the hand are plaed. Here it i the ame a S, but when we raie the hand, it will be horter. I have three fore, N f, the normal fore at the feet, W, the weight of the body, and N, the normal fore applied at the houlder. I am happy with the firt two, but N bother me. I have labeled it a being applied at the houlder, but I how it being applied at the hand, whih are loated lightly below the houlder. Should I rename it N h, the normal fore applied at the hand? In that ae, what i the moment arm? I it the ditane from the feet to the hand or the feet to the houlder? I have been uing the houlder. And if it' to the
14 of 20 09/06/18 18:36 houlder, do I need to do another moment arm orretion between the hand and the houlder? On me, the differene i only about 2 inhe, o I am inlined to rename it N h and ignore the mall differene between the hand and the houlder. I want to keep thi a imple a poible. I am not deigning omponent for a uperoni jet. I'm jut trying to get a good general ene of the weight the peron will need to be able to lift at different elevation. Suggetion? tnih #15 Tueday at 7:28 AM We Turner aid: Thi wa very helpful. I ee now that I wa getting myelf onfued by having fore ating all over the plae. 752 / 227 Homework Helper Let me tart over with the fore in the down poition. Conider thi diagram. I am uing an atual peron image, rather than a blue line, beaue I am writing thi for normal (non-tehie) people. View attahment 230196 C and S are a before, the ditane to the enter of ma and the houlder. D i the ditane to the upport truture where the hand are plaed. Here it i the ame a S, but when we raie the hand, it will be horter. I have three fore, N f, the normal fore at the feet, W, the weight of the body, and N, the normal fore applied at the houlder. I am happy with the firt two, but N bother me. I have labeled it a being applied at the houlder, but I how it being applied at the hand, whih are loated lightly below the houlder. Should I rename it N h, the normal fore applied at the hand? In that ae, what i the moment arm? I it the ditane from the feet to the hand or the feet to the houlder? I have been uing the houlder. And if it' to the houlder, do I need to do another moment arm orretion between the hand and the houlder?
15 of 20 09/06/18 18:36 On me, the differene i only about 2 inhe, o I am inlined to rename it N h and ignore the mall differene between the hand and the houlder. I want to keep thi a imple a poible. I am not deigning omponent for a uperoni jet. I'm jut trying to get a good general ene of the weight the peron will need to be able to lift at different elevation. Suggetion? A ouple of uggetion. 1) Stik with the fore on the hand and do not how the fore on the houlder. If you are onidering the body to inlude the arm and hand, then you have a normal fore ating on the hand and the ditane D will be the ame throughout the exeution of the puhup. If not, then you have lateral fore ating on the houlder that you probably don't want to onider. 2) Ue fore label that will alo be appropriate for the inlined poition. The normal fore on the hand hould work for both the horizontal and inlined poition. The fore on the feet will not be normal in the inlined poition. You like thi. A.T. #16 Tueday at 8:19 AM We Turner aid: View attahment 230196 8,907 / 1,178 Siene Advior Should I rename it N h, the normal fore applied at the hand? In that ae, what i the moment arm? I it the ditane from the feet to the hand or the feet to the houlder? The external fore i at the hand, o feet to hand i the relevant lever arm for the external moment balane around the foot ontat. The houlder poition i relevant for the moment balane around the houlder joint (houlder mule v. external fore at hand).
16 of 20 09/06/18 18:36 We Turner #17 Tueday at 9:20 AM tnih aid: 43 / 3 A ouple of uggetion. 1) Stik with the fore on the hand and do not how the fore on the houlder. If you are onidering the body to inlude the arm and hand, then you have a normal fore ating on the hand and the ditane D will be the ame throughout the exeution of the puhup. If not, then you have lateral fore ating on the houlder that you probably don't want to onider. Great, thank. 2) Ue fore label that will alo be appropriate for the inlined poition. The normal fore on the hand hould work for both the horizontal and inlined poition. The fore on the feet will not be normal in the inlined poition. I don't undertand. In your FBD, you how N, whih I aume i the normal fore. But it i not orthogonal to the ground, whih i what i needed to oppoe gravity, no? Are normal fore alway orthogonal to the body they are ating on? If the fore on the feet in the inlined poition i not normal, then what i it? What i F in your FBD? I it frition? I that why you labeled it "F" and I jut aumed it meant "feet"? And if o, then i there no normal fore on the feet? Or i it F y, but that i not orthogonal to the body either. Thi may be too ompliated for my feeble brain... tnih #18 Tueday at 12:12 PM We Turner aid: In your FBD, you how N, whih I aume i the normal fore. But it i not orthogonal to the ground, whih i what i needed to oppoe gravity, no? Are normal fore alway
17 of 20 09/06/18 18:36 orthogonal to the body they are ating on? 752 / 227 Homework Helper If the fore on the feet in the inlined poition i not normal, then what i it? What i F in your FBD? I it frition? I that why you labeled it "F" and I jut aumed it meant "feet"? And if o, then i there no normal fore on the feet? Or i it F y, but that i not orthogonal to the body either. Thi may be too ompliated for my feeble brain... A normal fore i normal to the urfae that applie the fore. So if the peron doing the puhup i leaning againt a urfae, the normal fore i the fore of the urfae on the hand that i normal to the urfae. In the ae of the feet, if the feet ret on the floor, then the normal fore would be perpendiular to the floor. The horizontal fore on the feet would be due to frition. To keep the body from moving horizontally, there need to be an equal and oppoite fore to ounter the horizontal fore on the hand. So to alulate it, you would jut hange the ign of the horizontal omponent of the normal fore on the hand. Thi all aume that the fore on the hand i a normal fore. That i, that the peron doing the puhup i not uing frition of the urfae to keep from liding along it. That eem like a reaonable aumption, but it i not a neeary one. You like thi. We Turner #19 Tueday at 2:08 PM 43 / 3
18 of 20 09/06/18 18:36 tnih aid: A normal fore i normal to the urfae that applie the fore. And "normal" mean orthogonal or perpendiular, right? So if the peron doing the puhup i leaning againt a urfae, the normal fore i the fore of the urfae on the hand that i normal to the urfae. In the ae of the feet, if the feet ret on the floor, then the normal fore would be perpendiular to the floor. Where on your FBD i the normal fore on the feet? I it F y? The horizontal fore on the feet would be due to frition. To keep the body from moving horizontally, there need to be an equal and oppoite fore to ounter the horizontal fore on the hand. So to alulate it, you would jut hange the ign of the horizontal omponent of the normal fore on the hand. Thi all aume that the fore on the hand i a normal fore. That i, that the peron doing the puhup i not uing frition of the urfae to keep from liding along it. That eem like a reaonable aumption, but it i not a neeary one. OK, I think I have it. In your FBD, F y i the normal fore on the feet that keep them from falling through the floor due to gravity. F x i the fritional fore that ounterat (balane) N x, the x- omponent of the inlined normal fore on the hand. And F i jut the ombination of F y & F x. Right? I got onfued beaue at the hand (formerly houlder), we tarted with N and broke it into x and y omponent. At the feet, we worked the other way around. We tarted with the omponent and ontruted F. Or at leat that' how I viewed it. A you ay, I have F x. It' jut the oppoite of N x. For my purpoe, I really don't need F y, unle that' how I get to N, whih i what I am intereted in.
19 of 20 09/06/18 18:36 I believe that W = F y N y for all θ. Corret? I think I an alulate N y at any inlination angle from the tandard equation of a lever, a I did for the down poition. A the angle inreae, N y will dereae to zero and F y will inreae to W and they will alway add up to W. Right? So, unle you ee ome error in the above, I will do that work and ome bak with my reult. Cheer... Lat edited: Tueday at 3:18 PM We Turner #20 Tueday at 3:30 PM 43 / 3 One more quetion. You mentioned that it would be good if I ould hooe variable name that ould be onitent and meaningful both in the down and inlined poition. What if I all the fore in the down poition N F & N H? When I inline it, N F will beome what you all F y and will alway point traight up. N H will beome what you all N and will be normal to the plane of the body. I an break N H up into N H x & N H y. I'm not ure what to all the omponent of N F, maybe F F x & F F? Thank
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