Q: In class you went over the direction of torque to clarify it,
and when you were talking about it I thought I understood, but I’ve been thinking about it more and I don’t think I do. I understand that torque is positive if it acts counterclockwise and negative if it acts clockwise. However, you also said that torque actually points straight upward and I don’t understand how that works. If it points upward, then how does it change the angular acceleration of the rotating object? Wouldn’t the torque have to act in the same direction as the force does?
A: This a good question! To fully understand the direction of Torque, we need to recall the direction of Angular velocity and Angular acceleration first. Also we always use the Right-hand rule.
Just use a speed-up counterclockwise rotation of a disk as an example. The angular velocity is counterclockwise, so positive/upward. Speed-up means the angular acceleration is also counterclockwise, so positive/upward. According to Newton’s 2nd law in Rotation, the net Torque must be counterclockwise/positive/upward, too. It is self-consistent since the Right-hand rule is applied to each of the above angular quantities.
Going a little deeper, Torque and Force are in different directions indeed. The Right-hand rule comes from the Cross product of vectors.
