Is Torque Relevant in Projectile Motion?

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    Projectile Torque
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Discussion Overview

The discussion centers around the relevance of torque in the context of projectile motion, particularly examining whether torque can be applied to a projectile at the highest point of its trajectory. Participants explore the relationship between torque, angular momentum, and linear motion, questioning the applicability of rotational concepts to projectile motion.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about the application of torque in projectile motion, noting that torque is typically associated with rotational motion and questioning how a projectile can be treated as undergoing rotation.
  • Another participant asks for clarification on the reference point for torque, suggesting that while torque can be calculated relative to a coordinate system, its relevance to projectile motion is unclear.
  • A participant acknowledges the mention of torque as the moment of the force 'mg' at the highest point of the trajectory.
  • One participant argues that torque is the time derivative of angular momentum, which does not necessarily require rotational motion, challenging the conventional association of torque with rotation.
  • Another participant asserts that angular momentum is indeed connected to rotation, emphasizing the need for a defined axis of rotation to measure angular momentum.
  • A counterpoint is raised that angular momentum can be defined for linear motion relative to axes that do not intersect the line of motion, although this approach may be considered impractical.
  • Participants reference the definition of angular momentum, suggesting that it does not inherently require rotational motion, thus questioning the traditional view of its application.

Areas of Agreement / Disagreement

Participants do not reach consensus on the relevance of torque in projectile motion, with multiple competing views regarding the relationship between torque, angular momentum, and linear motion remaining unresolved.

Contextual Notes

Participants highlight limitations in their understanding of how torque and angular momentum apply outside of traditional rotational contexts, indicating a potential dependence on definitions and interpretations of motion.

Molar
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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 ?
 
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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.
 
Thanks for replying...yes...he mentioned it, moment of the force 'mg'.
 
Molar said:
Isn't torque associated with rotational motion
Not in general. Torque is the time derivate of angular momentum, which doesn't require rotational motion.
 
A.T. said:
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 ?
 
Molar said:
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.
 

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