when wheels or helicopter blades spin around fast enough, why do they look like they're going backwards?

Answer 1

Because our eyes don't see as constantly as it seems. They send very frequent images to our brains, but there is dead space between the updates, so to speak, like a strobe light. If you were looking at a spinning wheel that rotated 355 degrees clockwise between each strobe, our brains would interpret that as spinning 5 degrees counter-clockwise., as we always assume the slower of the two options to be correct.

Answer 2

That only happens on film or tv because the picture is snapped every 1/60th of a second. You'll never see it in person.