# Derivative of Cosine with unit vector

Blue Kangaroo

## Homework Statement

Take ∂2E/∂t2 E(r,t)=E0cos((k(u^·r−ct)+φ) in which u^ is a unit vector.

d/dx(cosx)=-sinx

## The Attempt at a Solution

I had calc 3 four years ago and can't for the life of me remember how to differentiate the unit vector. I came up with -c2u2^cos(k(u^r-ct)+φ), but I feel like I'm not doing that right.

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Homework Helper
I think in this equation ##\hat u## is a constant in time. It's the direction of motion of this wave. And ##r## is held constant when doing the partial with respect to ##t##. So you don't need to differentiate it, just treat ##\hat u \cdot r## as a constant, which it is.

Blue Kangaroo
I think in this equation ##\hat u## is a constant in time. It's the direction of motion of this wave. And ##r## is held constant when doing the partial with respect to ##t##. So you don't need to differentiate it, just treat ##\hat u \cdot r## as a constant, which it is.
Ah I made a mistake. I meant to write ∂2E/∂r2