Momentum vector always parallel to velocity

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Discussion Overview

The discussion revolves around the relationship between momentum and velocity vectors, particularly in the context of objects in rotational motion. Participants explore the implications of momentum being parallel to velocity, the definitions and roles of linear and angular momentum, and the mathematical representations involved.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that for an object in rotation, the momentum vector, defined as p = mv, is always parallel to the velocity vector.
  • Others clarify that this parallelism holds true in uniform circular motion, but in non-uniform motion, additional components of velocity may exist.
  • There is a discussion about whether momentum can be viewed as a scalar product of mass and velocity, with some participants agreeing that for massive objects, this is valid.
  • One participant introduces the concept of light momentum, stating that it can be expressed as p = ħk, where k is the wave-vector, which also points in the direction of travel.
  • Concerns are raised about the interpretation of the cross product of velocity and momentum, with some participants noting that the product equals zero when the vectors are parallel.
  • Angular momentum is introduced, with a participant expressing confusion regarding its relationship to linear momentum and the conditions under which the cross product is considered.

Areas of Agreement / Disagreement

Participants generally agree that momentum is parallel to velocity in certain contexts, but there are competing views regarding the implications in non-uniform motion and the interpretation of mathematical relationships. The discussion remains unresolved regarding the nuances of angular versus linear momentum.

Contextual Notes

Participants express uncertainty about the definitions and relationships between linear and angular momentum, as well as the conditions under which certain mathematical expressions hold true. There is also a lack of consensus on the clarity of notation used in the discussion.

negation
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For an obejcting in a state of rotation, the velocity is always tangential to the distance acceleration to the axis of rotation.
From what I read, the momentum vector, p = mv, is always parallel.
Would it then be right to state that the momentum vector, p, is nothing more than a scalar product of m and v?
In other words, the momentum vecotr is "superimposed" onto the velocity vector and therefore parallel.

In looking at this from the cross product, v x vm (where both v are vector), the product = 0 because the angle between them is zero. Sin(0°) = 0.

Am I looking at this from the right angle or is there a better way to look at it?
 
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negation said:
For an obejcting in a state of rotation, the velocity is always tangential to the distance acceleration to the axis of rotation.
Only in uniform circular motion - the object could be increasing or decreasing it's distance from the center of rotation, then it will have a radial component to it's velocity as well as a tangential component.

If it's angular velocity increases, then it's total acceleration will no longer point towards the center either.

From what I read, the momentum vector, p = mv, is always parallel.
Would it then be right to state that the momentum vector, p, is nothing more than a scalar product of m and v?
For objects with mass ##\vec p = m\vec v## yes.
For light ##\vec p = \hbar \vec k## ... where ##\vec k## is the wave-vector - it points in the direction the light travels and has magnitude ##k=2\pi/\lambda##.

I don't know what you mean by "nothing more", this is quite sufficient.

It is also valid to say that the velocity is nothing more than the momentum multiplied by a scalar too. However, momentum is important because it is a conserved quantity.

In other words, the momentum vector is "superimposed" onto the velocity vector and therefore parallel.
It points in the same direction as the instantaneous direction of travel yes.

In looking at this from the cross product, v x vm (where both v are vector), the product = 0 because the angle between them is zero. Sin(0°) = 0.
... this is for linear momentum.
There is also angular momentum.

note:
in general $$\vec\omega = \frac{\vec v \times \vec r}{r}$$

Am I looking at this from the right angle or is there a better way to look at it?
You seem to be overthinking the concepts here.
 
Simon Bridge said:
... this is for linear momentum.
There is also angular momentum.

From what I gathered from the lecture.

An object in a state of rotation can be decomposed into torque and angular momentum.

Let L→ be angular momentum

L→ = r→ x p→ = r→ sin Θ x mv→

Taking the derivative of L:

dL/dt = dr/dt x p + r x dp/dt

v x mv + r x F

I'm a little confused as to why you said vx mv is true only in linear momentum. I did worked out v x mv = 0 from angular momentum. I might be missing something here.
 
I'm a little confused as to why you said vx mv is true only in linear momentum. I did worked out v x mv = 0 from angular momentum. I might be missing something here.
... Oh I think I got confused by your notation.
 
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