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?
_____________________________________
For Further Reading
* 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.
0 answers