Compel vs. Impel
Both compel and impel contain the idea of using physical or other force to cause something to be done.
Compel means to constrain someone in some way to yield or do what one wishes.
Example: to compel a debtor to pay
Example: Fate compels us to face danger and trouble.
Impel means to provide a strong motive or incentive toward a certain end.
Example: The wind impelled the ship.
Example: Curiosity impels her to ask questions.
It might help, in some instances, to think of impel as the carrot and compel as the stick.