# math (vectors) & physics

Posted by sam on Wednesday, March 28, 2007 at 7:32pm.

In unit-vector notation, what is the net torque about the origin on a flea located at coordinates (-2.0, 4.0 m, -1.0 m) when forces F1 = (-4.0 N) k and F2 = (-5.0 N) j act on the flea?
_____________________________________

* math (vectors) & physics - drwls, Thursday, March 29, 2007 at 12:13am

Torque is the cross product of the radius vector and the net force vector. In three dimensions, it can be computed as fallows.
|i... j... k|
|0... -5... -4|
|-2... 4... -1|

Compute the determinant

~~~

this is what i've done and im stuck.

= i[(-5)(-1)-(-4)(4)] + j[(-4)(-2)-(0)(-1)] + k[(0)(4)-(-5)(-2)]

= 21i + 8j - 10k

and that's the wrong answer.. .what am i doing wrong? help?

***there's an example in my book of a similar problem with different numbers:
-In unit-vector notation, what is the net torque about the origin on a flea located at coordinates (0, –4.0 m, 5.0 m) when forces F1=(3.0 N)k and F2=(-2.0 N)j act on the flea?
~~~~~and the answer to that question is: (–2.0 i) N · m

I agree wth your numbers. However if the right hand rule is used for torque, then it should have been F x R instead of R X F. This would change the sign of the answer.

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