Answer:
Forward Iām heavy, but backwards Iām not.
Large as a mountain, small as a pea, Endlessly swimming in a water-less sea.
Until I am measured I am not known, Yet how you miss me when I have flown.
Power enough to smash ships and crush roofs. Yet it still must fear the sun.
We are five little objects of an everyday sort, You will find us all in a tennis court.
I have two eyes in the front and a lot of eyes on my tail
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?