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updating a function within a struct

What I want: An object that contains a function that I can update after creation. 

I created a `struct` that contains a parameter `b` and a function(closure?)  named `Internal_Fn`. 

    struct MyThing {
        b: f32,
        Internal_Fn: Box<dyn Fn(f32) -> f32>,
    }
To allow me to update `Internal_Fn`, I add the `update_Fn` method to `MyThing`.

    impl MyThing {
        fn update_Fn(&mut self, delta_b: f32) {
            let new_b = self.b + delta_b;
            self.b = new_b;
            self.Internal_Fn = Box::new(|x: f32| ((1.0 + new_b).powf(x) - 1.0) / new_b);
        }
    }

I get a compiler error 

    9  |         self.Internal_Fn = Box::new(|x: f32| ((1.0 + new_b).powf(x) - 1.0) / new_b);
       |        --------------------------^^^^^-------------------------
       |                            |        |                |
       |                            |        |                borrowed value does not live long enough
       |                            |        value captured here
       |                            cast requires that `new_b` is borrowed for `'static`
    10 |     }
       |     - `new_b` dropped here while still borrowed


If I understand correctly, the problem is that the parameter that I set in my new `Internal_Fn` (i.e. `new_b`) is dropped once the `update_Fn` is finished but `Internal_Fn` is supposed to live on.
Is that correct?

How do I do this? I don't just want to hard code the shape of the function and only update the parameter `b`. I want to be able to also provide entirely differently shaped functions.