A single pulley offers no mechanical advantage, so if you have 7kg on one side and you attach the other side to yourself, it is pulling up with a force of 70N, or reducing your weight by 7kg.
Wouldn't it mean that the effect would be less than 7kg assitance on the upward as some of the 7kg would be used to overcome the friction in the system but on the lowering down bit it would offer more than 7kg assistance \ resistance
No. If you think about it, unless your bodyweight minus the amount of weight you can take on your hanging arm is less than 7kg then when doing the pullup the weight is going to be moving upwards toward the pulley, and the string at the pulley is going to be moving over the pulley and down towards your assisting hand. this mean the fiction at the pulley is going to be resisting this motion, just like the weight on the other end is resisting the motion of your pull on the string.at you've got in real life. somewhere between 7kg of assistance and a 2-armed pullup.
Quote from: Ru on July 18, 2007, 10:24:54 amA single pulley offers no mechanical advantage, so if you have 7kg on one side and you attach the other side to yourself, it is pulling up with a force of 70N, or reducing your weight by 7kg.This assumes that the pulley is frictionless of course, and that the cord used is the famous "light inextensible string" of A-level physics exam-paper fame. Otherwise its probably giving marginally more than 7kg - if its a really wack pulley it could actually be taking off 8 or 9 kg or whatever.
adding numbers doesn't get you strong
Today I did 2 V7s