Well in all Sci-fi shows defy Physics, but Power Rangers do have their own rules, that breaks all rules of actual logic and I would like to shed some light on that.
Moon and Space
The moon does have Earth-like gravity and has breathable Oxygen. In the first episode "Day of the Dumpster," we see spacemen run without problems with gravity. We see Rita breathe air. We later see in "In Space," the Rangers breathing air on the moon and later in "Forever Red." Most planets have oxygen in the Power Rangers Universe. Only "In Space" showed some planets that they put on gas masks and they had no oxygen. In real life, the moon has almost no gravity and you need an oxygen tank, as far as we know now in 2010, we have yet to find substantial proof that there is a terraform planet similar to ours.
Time Travel was first used with second season when Tommy lost his Green Ranger powers and sent himself a message. Then in "Wild West Rangers," Kimberly goes to the wild west and effected the past, by making the first humans to have powers. In "Return of the Green Ranger," Tommy's clone came to colonial times and probably became Tommy's ancestor, it is debatable, we never find out. And of course, there is a bit more time travel in Time Force.
Accelerating the Earth's Rotation
Speaking about Time Travel, just like Superman reversed time by turning the Earth's rotation, Master Vile did the same thing and made the teenagers into kids. He reverse time, but because of the kid's ninja coins, they remembered everything but funny enough, everything remained the same and the Rangers were still in Angel Grove--at least Bulk and Skull only knew Billy.
Billy can defy any physics
Billy invents many science-bending things like body swapper machine and others things I didn't understand.
Speaking of Billy's inventions, cars can fly in the Power Rangers' world. In the real world, cars are heavy and can't defy gravity.
I wish scientists would figure teleportation out already (because it saves a lot with fuel!) but they say that the official word on modern scientists is that they are not looking into teleporting. Anyhoo, it is a important part of the Power Rangers World.
Aliens speak English
The majority of aliens speak English in the Power Rangers world. Only in SPD, there were some that spoke other languages. This isn't really a physics thing. I'm talking mostly technology here, in the first seasons, it was technology vs. magic and later in Mystic Force, it was magic vs evil demons.
Since the first episode, we saw A.I. in Alpha and later with more sophisticated A.I. as the Machine Empire and Mac from Operation Overdrive and Sophie from SPD.
Breathing in Outer Space
Oh yeah I forgot! In the Power Rangers world, you can magically float in the vastness of space without the need of an oxygen tank!
Also, we have yet to see a planet that is made completely of water.