koti Posted September 12, 2019 Share Posted September 12, 2019 Obviously S ≠ F in the below example due to efficiency loss in friction. How do I calculate the exact efficiency of this simple system? Lets say F=1kg, gravity is 1G, the rope is non elastic and the angles are straight like in the graphic. Lets assume the pulley and rope diameter to be some values which are easiest for calculations. Link to comment Share on other sites More sharing options...
swansont Posted September 12, 2019 Share Posted September 12, 2019 11 minutes ago, koti said: Obviously S ≠ F in the below example due to efficiency loss in friction. How do I calculate the exact efficiency of this simple system? Lets say F=1kg, gravity is 1G, the rope is non elastic and the angles are straight like in the graphic. Lets assume the pulley and rope diameter to be some values which are easiest for calculations. I assume you are defining efficiency for this in terms of force or work. So take the ratio of the weight and the force you need to exert Link to comment Share on other sites More sharing options...
studiot Posted September 13, 2019 Share Posted September 13, 2019 https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/1096 Key words in this context are Velocity Ratio Mechanical Advantage Efficiency Link to comment Share on other sites More sharing options...
koti Posted September 13, 2019 Author Share Posted September 13, 2019 10 hours ago, swansont said: I assume you are defining efficiency for this in terms of force or work. So take the ratio of the weight and the force you need to exert 1 hour ago, studiot said: https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/1096 Key words in this context are Velocity Ratio Mechanical Advantage Efficiency Lets assume S=1kg and I am gripping the F rope and pulling it down - how much more of force than 1kg do I need to apply with my arm to lift. i understand that there are factors to take into account Im just not sure if I get all of them (what I listet in initial post) Link to comment Share on other sites More sharing options...
swansont Posted September 13, 2019 Share Posted September 13, 2019 1 hour ago, koti said: Lets assume S=1kg and I am gripping the F rope and pulling it down - how much more of force than 1kg do I need to apply with my arm to lift. 1 kg isn't a force. The weight would be 9.8 N (F = W = mg) Absent any friction, only an infinitesimal amount more initially, and after, 9.8 N. There doesn’t need to be a net force to move at constant velocity. So you need to exert a force equal to the weight while lifting. If there is friction, you need to exert the weight + frictional force Link to comment Share on other sites More sharing options...
koti Posted September 13, 2019 Author Share Posted September 13, 2019 2 hours ago, swansont said: 1 kg isn't a force. The weight would be 9.8 N (F = W = mg) Absent any friction, only an infinitesimal amount more initially, and after, 9.8 N. There doesn’t need to be a net force to move at constant velocity. So you need to exert a force equal to the weight while lifting. If there is friction, you need to exert the weight + frictional force This is clear. How do I calculate the frictional force in this particular case? Link to comment Share on other sites More sharing options...
Mordred Posted September 13, 2019 Share Posted September 13, 2019 (edited) Frictional force is typically negligible in this instance. What you need is the mechanical advantage of the pulley. Calculate the force needed to lift the weight then divide by the mechanical advantage. That will get your effort force the first calc your load resistance force then apply your efficiency equation. Edited September 13, 2019 by Mordred Link to comment Share on other sites More sharing options...
swansont Posted September 13, 2019 Share Posted September 13, 2019 4 hours ago, koti said: This is clear. How do I calculate the frictional force in this particular case? If it’s the only other force involved, then compare the force needed to lift vs the weight. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted September 16, 2019 Share Posted September 16, 2019 On 9/13/2019 at 11:44 AM, Mordred said: Frictional force is typically negligible in this instance. What you need is the mechanical advantage of the pulley. Calculate the force needed to lift the weight then divide by the mechanical advantage. That will get your effort force the first calc your load resistance force then apply your efficiency equation. As the only pulley is fixed, there is no mechanical advantage. Link to comment Share on other sites More sharing options...
Mordred Posted September 16, 2019 Share Posted September 16, 2019 1 hour ago, J.C.MacSwell said: As the only pulley is fixed, there is no mechanical advantage. I left that for the OP to determine, Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted September 16, 2019 Share Posted September 16, 2019 1 minute ago, Mordred said: I left that for the OP to determine, It seemed he knew that, and was focused on the friction, which would account for the difference between the S and F force. Link to comment Share on other sites More sharing options...
Mordred Posted September 16, 2019 Share Posted September 16, 2019 I felt it was more appropriate to not assume he was aware of the pulley ratio to number of ropes on load side rule of thumb. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted September 16, 2019 Share Posted September 16, 2019 7 minutes ago, Mordred said: I felt it was more appropriate to not assume he was aware of the pulley ratio to number of ropes on load side rule of thumb. Fair enough. I don't expect this is homework though. Koti has been around for quite a while. Assuming he is looking for a realistic estimate, it comes down to making the right assumptions and some engineering tables. ...or experimenting. Link to comment Share on other sites More sharing options...
koti Posted September 16, 2019 Author Share Posted September 16, 2019 58 minutes ago, Mordred said: I felt it was more appropriate to not assume he was aware of the pulley ratio to number of ropes on load side rule of thumb. I know this might come in as a surprise to you but I knew that 48 minutes ago, J.C.MacSwell said: Fair enough. I don't expect this is homework though. Koti has been around for quite a while. Assuming he is looking for a realistic estimate, it comes down to making the right assumptions and some engineering tables. ...or experimenting. Technically its' never too late for homework but youre right, t's not for my school project Yes, experimenting - I think its my inner self trying to subcionsciously get me back into a gym. Link to comment Share on other sites More sharing options...
Phi for All Posted September 16, 2019 Share Posted September 16, 2019 39 minutes ago, koti said: Yes, experimenting - I think its my inner self trying to subcionsciously get me back into a gym. Are you building a Nautilus machine, or is this to help get you up off the couch? Link to comment Share on other sites More sharing options...
Mordred Posted September 16, 2019 Share Posted September 16, 2019 40 minutes ago, koti said: I know this might come in as a surprise to you but I knew that Lol no it's not a surprise, but I hate assumptions. I lost track of the number of times I assumed something should be obvious to an OP then surprised when they didn't know the basics. Link to comment Share on other sites More sharing options...
Sensei Posted September 16, 2019 Share Posted September 16, 2019 41 minutes ago, Mordred said: Lol no it's not a surprise, but I hate assumptions. I lost track of the number of times I assumed something should be obvious to an OP then surprised when they didn't know the basics. ...a few years ago, I had to explain decay modes of pion meson to Nobel prize winner in physics... Link to comment Share on other sites More sharing options...
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