# Missing sketch in K&K Mechanics book

1. Feb 1, 2014

### Seydlitz

Hello guys,

I'm currently reading through K&K Intro to Mechanics book, and I'm on page 26 where I encounter a bit odd derivation.

The authors say:
I'm rather lost on this part. I don't know which sketch corresponds to this equation.

2. Feb 1, 2014

### Staff: Mentor

The diagram is indeed missing.

The three vectors form an isosceles triangle. The dashed line bisects it into two right triangles. To get the equation, consider one of the right triangles.

If you take the diagram at the top left of page 26, and fill in the hypotenuse which is $\vec A (t + \Delta t)$, you get a similar diagram, but with the right angle in a different location. The discrepancy disappears in the limit as $\Delta \theta \rightarrow 0$ and $\Delta \vec A \rightarrow 0$.

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Last edited: Feb 1, 2014
3. Feb 1, 2014

### Seydlitz

Ok that makes it clear. But it raises another question on why the vector $\Delta A$ is not perpendicular to $A$? Whereas the book says it is necessary for it to be so because $A$ is not changing in magnitude?

4. Feb 1, 2014

### Staff: Mentor

In the limit as Δt goes to zero (which is what you need to do in order to have the instantaneous derivative), ΔA becomes perpendicular to both A(t) and A(t+Δt). And of course A(t) and A(t+Δt) become equal to each other.

5. Feb 1, 2014

### Seydlitz

Ah yes I didn't think of that carefully. Thanks for the remark and the sketch. :)

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