Rotation of a body around which axis

  • Context: Undergrad 
  • Thread starter Thread starter jd12345
  • Start date Start date
  • Tags Tags
    Axis Body Rotation
Click For Summary

Discussion Overview

The discussion revolves around the mechanics of rotational motion, specifically the axis around which a body rotates when torque is applied. Participants explore the relationship between torque, angular momentum, and the center of mass, considering both theoretical and practical implications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that a body rotates around its center of mass when torque is applied, questioning the reasoning behind this assumption.
  • Another participant describes the motion of a rigid body as a combination of rotation around the center of mass and translational motion, depending on the application of torque and net force.
  • A third participant challenges the assertion that the rotation axis is always aligned with the torque direction, noting that an irregularly shaped body is characterized by a tensor of inertia, which complicates the relationship between torque and rotation axis.
  • This same participant clarifies that the axes of torque and angular momentum may not coincide unless the torque is applied along one of the principal axes of the body.
  • A later reply specifies that the previous statement referred to the axis of angular momentum, indicating a need for clearer communication regarding terminology.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between torque, angular momentum, and the rotation axis, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

The discussion highlights the complexity of rotational dynamics, including the influence of body shape and inertia on the rotation axis, as well as the need for precise definitions in the context of torque and angular momentum.

jd12345
Messages
251
Reaction score
2
When i apply torque on a body, it will rotate around whcih axis? I guess its the axis along centre of mass but i don't know why?

Also if the answer is centre of mass what will be the orientation of the axis- I mean there are infinite lines passing through a single point so which of those axis will be the one around which body rotates?
 
Physics news on Phys.org
You can always describe the movement of a rigid body as a rotation around its center of mass (with a certain orientation of the angular momentum) plus a velocity of the center of mass (in some direction).

If you apply a torque without any net force (which means that the sum of forces (not torques!) is 0) to an unmoved object, it will begin to rotate at the position where it was before. If you apply a torque with a net force (for example, kick one side of the object), it will do a combination of rotation and global movement in the direction of the kicking force.

what will be the orientation of the axis
The same as the axis of your torque (F x r).
 
mfb said:
The same as the axis of your torque (F x r).
This statement is not true in general. An irregularly shaped body is described by a tensor of inertia, which means that the body will not necessarily rotate around the axis defined by the torque. The two axes will only coincide if the torque is applied along one of the principle axes. Otherwise, the angular momentum will be along the axis of the torque, but the angular velocity will be around a different axis, as described by the equation L = I w.
 
I meant the axis of angular momentum with this reply.
Maybe I should have said this explicitly.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 6 ·
Replies
6
Views
1K
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 32 ·
2
Replies
32
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 35 ·
2
Replies
35
Views
4K
  • · Replies 21 ·
Replies
21
Views
4K
  • · Replies 7 ·
Replies
7
Views
2K