Partial derivative of a term should give zero

Questions
1

Hello! There is an equation. If I take the partial derivative from it, the part with ∂ appears in the output. This part should be automatically equal to 0. How can I fix this?

Ek = (m1*Dt[y, t]*Dt[y, t])/2 + (m2*(l*Dt[\[Phi], t]*Sin[\[Phi]])^2)/
  2 + (m2*(l*Dt[\[Phi], t]*Cos[\[Phi]] + Dt[y, t])^2)/2

Simplify[D[Ek, y]]

Simplify[D[Ek, \[Phi]]]

Thank you in advance!

mech.nb (5.76 KB)

2

When you take partial derivatives with respect to y,\[Phi], these must be independent variables, not themselves functions (of what? of themselves?).