There are a number of preposition words that change the meaning of a phrase when used with a particular word. For example, you can use Obliged with different prepositions, and you will get a different meaning every time. You can see the examples below:
1. Obliged to
Obliged to (person)
Example: I am very much obliged to you.
2. Obliged at
Obliged at (thing)
Example: She is indeed an unquestionable authority on Physics.