Torque acting on a projectile

1. Sep 16, 2015

Molar

In class our teacher shows a problem where he finds out the torque acting on a projectile at the highest point of its trajectory.
He calculates the horizontal displacement from the vertical axis to the highest point as "r "and multiplies with "mg" as "F".
Here I am a little confused. Isn't torque associated with rotational motion like "r" is the perpendicular distance from the rotational axis ? How we can treat a projectile motion as a rotatinal motion ?

2. Sep 16, 2015

Staff: Mentor

Torque from what, relative to what?

From gravity, relative to the center of your coordinate system: well, that is true, but I don't see the relevance of that value as the projectile does not perform a typical rotation around this point.

3. Sep 17, 2015

Molar

Thanks for replying...yes...he mentioned it, moment of the force 'mg'.

4. Sep 17, 2015

A.T.

Not in general. Torque is the time derivate of angular momentum, which doesn't require rotational motion.

5. Sep 17, 2015

Molar

Yes, torque = dL/dt ; L = angular momentum
But again angular momentum is connected to rotation of object . We do not find angular momentum of linear motion.
We define an axis of rotation and take the distance from it to measure L = r × mv
We also know L = I.ω ; I = moment of inertia , ω = angular velocity
So angular momentum is about rotational motion right ? Or where have I got it wrong ?

6. Sep 17, 2015

Staff: Mentor

Sure we do, for axes that do not cross the line of motion.
This is a pointless approach in most situations, but it is not wrong.

7. Sep 17, 2015