Is Torque Relevant in Projectile Motion?

[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 ?

 

[mfb]

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.

 

[Molar]

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

 

[A.T.]

Isn't torque associated with rotational motion

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

 

[Molar]

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

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 ?

 

[mfb]

We do not find angular momentum of linear motion.

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.

 

[A.T.]

We do not find angular momentum of linear motion.

https://en.wikipedia.org/wiki/Angular_momentum#Definition

So angular momentum is about rotational motion right ?

There is nothing in the definition that requires rotational motion.