Manipulate does not work if an equation with a variable parameter is not explicitly written

Question

I have a little problem using the Manipulate command.

I am trying to write down the parameter control of a function that is entered into this command not explicitly. But in this case, the variable parameter "does not cling" to the function and the command does not work.

I will show what I mean.

In this case, the command works successfully.

Manipulate[Plot[Sin[2*(x + x^Sin[1*a])], {x, 0, 30}], {a, 0, 10}]

In this case, nothing happens.

f = Sin[2*(x + x^Sin[1*a])]

Manipulate[Plot[f, {x, 0, 30}], {a, 0, 10}]

At the same time, RegionPlot3D, SurfacePlot3D, etc. successfully work as I need.

What am I doing wrong?

Answer 1

Try this:

Clear[f];
f[x_, a_] := Sin[2*(x + x^Sin[1*a])]

Manipulate[Plot[f[x, a], {x, 0, 30}], {a, 0, 10}]

Have fun!