Here are important things to do in calculating net torque.

1. Select an axis of rotation.

2. Draw the forces extending from their points of application.  (While this wasn't important for net force problems, it is important for net torque problems.)

3. Draw position vectors extending from the axis of rotation to the points of application of each of the forces.

4. For each force, identify the angle, q, from the position vector to the force.  In order to determine the correct angle, imagine r and F starting from the same point and curl the fingers of your right hand from the position vector to the force vector.  The angle between the two vectors is the one you want.  The vector may be acute or obtuse.  Be sure to identify it correctly, because the sine of the angle is used in the calculation of the moment arm.

5. Construct the moment arm, , of each force.  This may require extending the line of application of the force. 

6. Determine the length of each moment arm using .

7. Write the net torque equation, .  Substitute the magnitude of each torque using .  Give each torque the correct sign according to the convention that torques tending to produce counterclockwise are +.

8. Substitute given values and solve for the net torque.  The sign of the result will tell you whether the angular acceleration due to the net torque is clockwise or counterclockwise.

Example.  A circular disc is rotated about an axis O through its center by the application of two forces.  A force of magnitude 11 N is exerted at a distance of 0.34 m from the axis and at an angle of 58° from a radial line extending from the axis through the point of application Q of the force.  A second force of magnitude 15 N is exerted at a distance of 0.26 m from the axis and at an angle of 119° from a radial line extending from the axis through the point of application P of the force.  Determine the net torque on the disc about its center and which way the net torque accelerates the disc.
 

Given: F1 = 11 N F2 = 15 N r1 = 0.34 m r2 = 0.26 m q1 = 58° q2 = 119° Goal: Find tnet and the direction of the angular acceleration		1. The axis has already been chosen for us at the center of the disc. 2. The forces are drawn extending from points P and Q. 3. The position vectors are drawn from point O to points P and Q. 4. The angle through which the position vector rotates into the force vector is indicated for each force. We'll now add some auxiliary lines and angles in the next diagram.  (Two diagrams wouldn't normally be needed, but we're providing a second one for clarity.)
		Calculating moment arms 5.  The line of application of F1 has been extended so that the moment arm of the force can be drawn perpendicular to the line of application.  The moment arm of F2 is also drawn. 6. The moment arm of F1 is opposite the angle a, which is equal to q1.  The moment arm of F2 is opposite the angle b, which is supplementary to q2.
	7.  The net torque equation is written.  The torque due to F1 (F2) is negative (positive), because it tends to produce clockwise (counterclockwise) rotation.  Note that the symbols t1 and t2 represent the magnitudes of the torques.  The signs are added explicitly.  The expressions obtained above for moment arm are substituted. 8.  Values are substituted and the value of the net torque is solved for.  The result is positive, indicating that the angular acceleration is counterclockwise.  Note that while the units are Nm, we don't call them joules.  A joule is used to represent a unit of work or energy.  Torque is not energy.