[rlovatt]

I’ve been having a crack at assisted lock offs and assisted one arm pulls up on a bar.

I can manage a one armer with 7kg on a pulley and a 30 second 90 degree lock off with 4kg.

Does this mean that I am only 7kg away from a one arm pull up or 4KG away from a 90 degree lock off, or does the pulley take more load?

I suppose this is a question for a Physicist!

[Ru]

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.

[rlovatt]

should it be attached to a harness or is ok to hold the pulley with the the free hand? 

[dave]

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.

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.

[Ru]

True. Thanks real world Dave.

[Teaboy]

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

[Jim]

nuff science....   adding numbers doesn't get you strong
get cranking   

[dave]

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.

Another way to think about it is image 2 scenarios. One is where the pulley has zero friction and the string travells over it freely - this i rupert's physics exam case. The other scenario is the pulley has so much friction you can't overcome it by pulling, i.e. the string does not move over the pulley at all. In other words the string is fixed. therefore the "assist" is to all intents and purposes a 2-arm pullup, i.e. lots of assistance!

Now imagine the real word scanario that lies somewhere between the 2 above cases. I.e. more friction than zero, but not enough to stop the string moving entirely. This is what you've got in real life. somewhere between 7kg of assistance and a 2-armed pullup.

[Ru]

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.

Depends how you do it. You're suggesting pulling with both hands so the weight moves up, I presume Teaboy presumes you hold the other hand still as you pull up so effectively the weight is pulling you up - ie moving down from the pulley.

[dave]

Shirley you'd need an enormous weight for that to happen? I would have thought most people don't do assisted pullups with 50kgs on a pulley.

[jwi]

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.

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.

On the other hand, if you attach the cord to your harness the assist would be a bit less that 7kg or 69 N.  Oh, and when pulling down with your arm on the free end of the rope the amount of assist will be considerable more than 69 N at your stick-point, since you're probably accelerate the weight upwards at that point.

[richdraws]

How very boring.

[andy_e]

adding numbers doesn't get you strong

...but can have O'Conor like results: Today I did 2 V7s, V7 + V7 = V14!

[GCW]

Today I did 2 V7s

I don't believe that at all   

[andy_e]

It wasn't meant to be me, it was just an example... Can't decide where to go tonight...