Uniform Circular Motion: some help with the math proof?

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Discussion Overview

The discussion revolves around the mathematical proof related to uniform circular motion, specifically focusing on the components of velocity in relation to angle θ. Participants are examining the relationships between the velocity components and trigonometric functions.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant questions the assignment of the sine and cosine functions to the velocity components, suggesting that the angle θ should be considered differently when visualizing the vector.
  • Another participant agrees with the initial claim about the angle but clarifies that using the angle between the velocity vector and the x-axis leads to different expressions for the components.
  • There is a request for a mathematical demonstration to clarify the relationship between the angle and the velocity components.
  • Participants reference trigonometric identities to derive the components, specifically using the formulas for cosine and sine of angle sums.
  • A later reply acknowledges the initial confusion and expresses gratitude for the clarification provided through the mathematical proof.

Areas of Agreement / Disagreement

Participants express differing views on the correct interpretation of the angle θ in relation to the velocity components, leading to a lack of consensus on the initial understanding. However, there is agreement on the validity of the trigonometric identities used to clarify the relationships.

Contextual Notes

Participants are working through the implications of their assumptions regarding angle definitions and trigonometric relationships, which may affect their conclusions. The discussion does not resolve all uncertainties regarding the initial claims about the angle and its relationship to the velocity components.

babaliaris
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I can not understand why ##v_x = -|v|sin(θ)## and ##v_y = |v|cos(θ)##
I'm asking about the θ angle. If i move the vector v with my mind to the origin
i get that the angle between x'x and the vector in anti clock wise, it's 90+θ not just θ. So why is he using just θ? Does the minus in v_x somehow make the result the same when using just θ?

Also I thought that sin goes with the v_y and cos goes with v_x not the opposite...
 

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babaliaris said:
Also I thought that sin goes with the v_y and cos goes with v_x not the opposite...
If you are using the angle between v and the x-axis, that is right. So vx = |v|cos(90+θ) and vy = |v|sin(90+θ).
Do you see how that turns into -|v|sinθ and |v|cosθ?
 
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mjc123 said:
If you are using the angle between v and the x-axis, that is right. So vx = |v|cos(90+θ) and vy = |v|sin(90+θ).
Do you see how that turns into -|v|sinθ and |v|cosθ?

No I don't, this is what I want to understand. Can you show me mathematically how is this true? I need to see it written in math (this is important) else I can't understand why.
 
Do you know the formulae cos(A+B) = cosAcosB - sinAsinB
and sin(A+B) = sinAcosB + cosAsinB ?
 
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mjc123 said:
Do you know the formulae cos(A+B) = cosAcosB - sinAsinB
and sin(A+B) = sinAcosB + cosAsinB ?

Oh, I forgot about that... Now i can see why.

$$
cos(90+θ) = cos(90)cos(θ) - sin(90)sin(θ) = 0 - sin(θ) \\
sin(90+θ) = sin(90)cos(θ) + cos(90)sin(θ) = cos(θ) + 0
$$

Thank you! I was truly struggling finding this one!
 
Here is my proof:
IMG-20190322-204937.jpg
 

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