Problem in choosing axis in rotational mechanics?

Click For Summary

Discussion Overview

The discussion revolves around the selection of the axis of rotation in problems related to rotational mechanics. Participants explore various scenarios involving a falling rod and a stationary rod subjected to a force, questioning how the choice of axis affects the calculations of angular velocity and angular acceleration.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant expresses confusion about choosing the appropriate axis of rotation for different problems in rotational dynamics, specifically regarding a rod falling onto pads and a rod held vertically.
  • Another participant asserts that angular velocity is a vector quantity and suggests that the choice of axis does not affect its value.
  • A different participant proposes taking the axis around the point where the rod touches the surface for the second question, providing calculations for angular acceleration based on that choice.
  • Another participant notes that if torques are calculated about the center of mass, the torque due to the normal force from the ground must also be considered.
  • A further suggestion is made to use the parallel axis theorem when computing inertia about the end of the rod.

Areas of Agreement / Disagreement

Participants express differing views on the implications of choosing different axes for calculations, with no consensus reached on the best approach or the effects of these choices on the results.

Contextual Notes

Participants highlight the importance of considering the torque contributions from various forces and the need to account for inertia depending on the chosen axis, indicating that assumptions about the system's constraints may affect the outcomes.

vkash
Messages
316
Reaction score
1
In rotational mechanics i am mostly confused with the axis of rotation. Let me give you some examples of question.
A rod of length l falls on two pads of same height from height h(rod is initially horizontal to plane when it is at height h). The coefficient of restitution of the metal pads are e1 and e2(e1 > e2). The angular velocity of the rod after it recoils is__________
about which axis. Is that instantaneous axis of rotation.
OK this may seem to be solved here but what about this question.
A uniform rod of mass length l is held stationary in vertical position on a smooth horizontal surface.Find angular acceleration of the rod in this position when horizontal force F=mg is applied at it's topmost point.
Now once again. which axis should i choose.
IAOR(instantaneous axis of rotation). I can;t find that.
However when i see solution of this example they take torque around the center of the rod and solve it. Why they take center of the rod. Why not any other point. If any other point on the plane is taken then answer may vary.
this point is really confusing me in rotational dynamics. which axis should i choose when i do questions.

please help me.!

I think general physics is correct place for this question.
 
Physics news on Phys.org
angular velocity is a vector, and it doesn't matter what axis you chose, it will be the same.
 
Curl said:
angular velocity is a vector, and it doesn't matter what axis you chose, it will be the same.

In second question, Take axis around point where rod touches the surface.
I=\frac{ml^2}{3}
T(Torque)=mgl

T=I\alpha
\alpha=\frac{T}{I}
\alpha=\frac{mgl}{\frac{ml^2}{3}}
\alpha=\frac{3g}{l}
Now take the center as the axis. Your answer will \frac{6g}{l}. That's different.
How will you explain this with our answer.
 
If you take torques about the CM, then you must also account for the torque by the ground (the normal force).

If you sum torques about the ground, make sure you compute the inertia about the end of the rod. Use the parallel axis theorem.
 

Similar threads

Undergrad Is angular momentum perpendicular to fixed axis of rotation constant?
  • · Replies 1 ·
Replies
1
Views
1K
Undergrad Energy of spinning objects as axis of rotation moves
  • · Replies 1 ·
Replies
1
Views
1K
Undergrad Tangential velocity of an object rotating around a rotating axis
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad A rod that falls while rotating from the end of a table
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Confused about the axis of rotation in rotational motion w/o slipping
  • · Replies 10 ·
Replies
10
Views
4K
Graduate Tension of a rod rotating on a horizontal table about a vertical axis
  • · Replies 12 ·
Replies
12
Views
2K
Undergrad Finding the Axis of Rotation with Given CoG and Force Point
  • · Replies 32 ·
2
Replies
32
Views
3K
High School Impulse and distance from the axis of rotation
  • · Replies 9 ·
Replies
9
Views
3K
Undergrad Writing an equation for torque around an arbitrary point on a body
  • · Replies 6 ·
Replies
6
Views
1K
High School The Effect of Linear and Rotational Motion on Measured Weight
  • · Replies 10 ·
Replies
10
Views
1K