Differential as a Vector Field

Question

Let $M$ be a smooth manifold, and let $f :M \to \mathbb{R}$ be a $C^\infty$ function. Is there a reason that the map: $$ p \mapsto (p,df_p) $$ is not a vector field? It seems to be a smooth assignment of a $p$-tangent vector at every point. What is it I am missing, and why is a metric/inner product structure needed to define the gradient vector field?

Answer 1

$df_p$ is not a tangent vector. It is a linear map $T_pM\to\mathbb{R}$: feed it a tangent vector at $p$, and it gives you a number (the directional derivative of $f$ with respect to that vector). An inner product then lets you turn it into a tangent vector since there will be a unique vector that this linear map is given by taking the inner product with.

(What $p\mapsto (p,df_p)$ is is a section of the dual vector bundle to the tangent bundle, also known as a $1$-form.)