Prove that v(t) is any vector that depends on time

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SUMMARY

The discussion centers on proving that if the velocity vector v(t) of a particle has a constant magnitude, then its derivative \dot{v}(t) is orthogonal to v(t). Participants confirm that the derivative of the dot product \vec{v} \cdot \vec{v} = |\vec{v}|^2 leads to the conclusion that \vec{v} \cdot \vec{a} = 0, indicating orthogonality. Conversely, if \dot{v}(t) is orthogonal to v(t), it is established that the magnitude of v(t) remains constant, leading to the equation |v|^2 = C, where C is a constant.

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  • #31
Oblio said:
I need to work it to v^2?

Was the multiplying by two only by the knowledge of doing the first part?

yeah sort of... for the second part you somehow need to figure out that \frac{d(\vec{v}\cdot\vec{v})}{dt} = 2\vec{\frac{dv}{dt}}\cdot\vec{v}

we already proved this in the first part.

so yes, v.v = |v|^2

|v|^2 = C, so |v|=sqrt(C), which is a constant.
 
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  • #32
Alright, I wasn't sure if multiplying by two was also a reasonable 'step' without the preknowledge.
 

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