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View Full Version : my current challenge: bringing down vectorial physics



raaaid
08-03-2010, 08:03 AM
im trying to build a box that holds on its edge but for this i have to bring down vectorial physics, my first step:

according vectorial physics if you have a force that is on the same plane than an axe, that force cant posibly make that axe turn

in other words a force cant produce a torque unless theres an arm

this is false:

imagine an axe, on the right side theres hanging one kg wrapped clockwise and on the left side of the axe another kg but coiled counter clock wise

the center of gravity of the two weights are exactly under the axe, so the weights cant make posibly turn the axe

this is logical since one weight will unwrap exactly as much as the other wraps up when the axe turns

but now lets cheat:

we wrap one weight normally but the other we wrap it coiled, like an helix

now the weight thats wrapped in an helix if it goes down, it will go down farther than what the other weight rises since its an helix shape

so in this special case either theres a torque without an arm or energy wouldnt be conserved

do you think its even worht it make the experiment or i should no doubt conservation of energy?

WTE_Galway
08-03-2010, 04:55 PM
Ok ...

Firstly this problem is one of static mechanics NOT vector physics http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

Secondly whilst the moment of force is identical the EFFECTIVE RADIUS of the helical wound weight is greater and that needs to be taken into account in calculations.


Here is a question for you ... how does your system differ from one where the weights are on two separate identical radius shafts joined by frictionless gears at a ratio of 2:1 ?

raaaid
08-03-2010, 05:42 PM
oh actually im not that interested to disprove the stablishment, gotta be an excuse not to study it

you hit my highly secret line of work:

the effective radius

imgine the real radius of the beam is 1 cm and the effective radius due to the coil is 3 cm

put the beam with a weight hanging over a 2 cm base

will it remain stable taking into account the cog of the sytem remains right over the base?

well actually if the weight is heavy enough it will fall over due to the fact that taking into account the efective radius now the force is applied now off the base

raaaid
08-03-2010, 05:44 PM
Here is a question for you ... how does your system differ from one where the weights are on two separate identical radius shafts joined by frictionless gears at a ratio of 2:1 ?

that in this case the cog will remain one forth away form under the axe creating an arm while in my case theres no arm