I am confused by the sign and direction torques and forces in linked/attached photo especially that the gravitational force acting on the counter weight on left is having positive sign in torque equation below and gravitational force acting on the propeller actuator on the right has negative sign in below torque equation?
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4 Answers
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Convention is to use the right hand rule. Your right thumb points in the direction of the torque (moment) vector produced by the rotation associated with the curl of your fingers. Thumb pointing out of the page is positive torque due to a counter clockwise rotation.
But as already noted it’s really arbitrary. As long as you are consistent when setting the sum of the moments equal to zero, the results will be the same.
Hope this helps.
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It looks as though the torques about $O$ are being considered.
A convention has to be decided on and with no further information it looks as though anticlockwise torques are defined to be positive and clockwise torques are negative.
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You can choose arbitrary direction for the sum of the torques.
$\sum \tau_i=0$
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The effect of torque is rotation. Although the forces are in same direction, but since they are acting on opposite ends about O, their effect of rotation is opposite. So if torque of one is positive, the other has to be negative. Usually, CCW is chosen as positive.
