I'm trying to find the magnitude of a vector dot{vec{r}}(t) by two different methods, but get different results. Let vec{r}(t)=x(t)hat{x}+y(t)hat{y}. The magnitude of this vector is: norm(vec{r}(t)) = sqrt{x(t)^2 + y(t)^2} The time derivative of vec{r}(t) must then be dot{vec{r}}(t) = dot{x}(t)hat{dot{x}}+dot{y}(t)hat{dot{y}} Now, using the same formula for the magnitude as before yields: ||dot{vec{r}}(t)|| = sqrt{dot{x}(t)^2 + dot{y}(t)^2}

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