For Torque, when do you use (F x r) and (Fd)?
Torque, or moment
F x r
Fd (d is the perpendicular distance)
how do you know which one to use?
- Randy PLv 71 decade agoFavorite Answer
r x F is the definition of torque, where r and F are vectors and "x" means the vector cross product. The magnitude of this is equal to F*r*sin(theta) where theta is the angle between the force vector and the radius vector.
r*sin(theta) is the same as the perpendicular distance (meaning the shortest distance to the line along the direction of the force vector). So Fd is also always correct for the magnitude of the torque.
In other words, they're both always correct, but the first is a vector expression and the second is the magnitude of that vector.