# 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