How to determine what to subtract from what inhttp://www.youtube.com/watch? F=MA

[imperfectluck]

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http://www.youtube.com/watch?v=32X5Udm57bU&feature=plcp


^starting from 8:21 of this video, why is it 1/2mg- T=1/2m *a rather than T-1/2mg=1/2m*a.
Basically I would like to know how do you determine what to subtract from what in the Force=ma equation??

 

[Fivo]

Hmm, didn't see the video, but....

T-mg = ma

 

[circulus vitios]

It's a matter of reference, I think. Try reading this page, they do a pretty good job of explaining it: http://www.studyphysics.ca/newnotes/20/unit02_circulargravitation/chp07_2d/lesson31.htm

 

[wazgood]

In the problem we are assuming that both 0.5mg and 0.5ma have the direction. Whereas the tension is in the opposite direction. By same direction it means essentially that they have the same sign.

Your two possibilities are either:

1: T + (-mg) = (-ma) [for the force of tension going in the positive direction]
or
2: -T + mg = ma [for mg going in the positive direction], which is the same as mg - T = ma