fingers in front of your mouth when saying those phrases, you should feel more air hit your fingers on "my pack," with a p sound, than "my back" with a b sound.
my + pack: i fell on my pack.
my + back: i fell on my back.
and let's do one final comparison to show differences in aspiration. let's play with the phrase "keep off." the word "keep" has two stops, a k sound and a p sound. which one will have a bigger aspiration? i hope you said the /k/. why? because the /k/ is the first sound of the word and stops at the beginning of a word have more 