It's actually very simple: just use "line breaks" (which can be input by Shift+Enter) instead of "paragraph marks/pilcrows" (which are input by Enter itself). Word will treat the equations after a "line break" or "pilcrow" as a Display equation by default. Thus, you just need to
- Press Shift+Enter after "the interaction is",
- Click "Insert an Equation"
- Type your equation
- Press Shift+Enter again.
- Continue your text input.
And Word will not treat the text before and after the equation as two sentences or two paragraphs. The final result is like this (I added a "where" before P(r) to show that the words before and after the equation belong to the same sentence, otherwise my Word 2013 will complain that the "w" is not upper-cased):
I turn on the option to display the paragraph marks (pilcrows, line breaks,...) to make my points clearer. For more information about these marks, see an old link here.