Newton’s First Law and Space Travel
Published on Sep 23, 2006 at 2:47 pm.
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Filed under physics, science fiction.
A drug runner is racing towards the US in a powerboat. The Coast Guard is giving chase. The Coast Guard orders the drug runner to cut their engine. The drug runner refuses. The Coast Guard opens fire into the path of the drug runner. Seeing that they mean business, the drug runner finally cuts his throttle, and the boat coasts to a stop and is boarded by the Coast Guard.
Police are chasing desparate criminals. Here in Texas, if the chase keeps going out of the city limits, then the state Department of Public Safety trooper take up the chase. At that point, the criminals ALWAYS are caught. They usually give up, stop. Sometimes, though, the state troopers use spike strips to cut the tires, or on occasion they have bee known to shoot out the radiator, and the engine overheats and dies. Shooting out the radiator was the strategy a month or so ago on a televised chase of a big truck.
In all these cases, killing the engine stops the vehicle. That is our common experience. So, when you watch science fiction shows on TV, or scifi movies, or even read scifi novels, most people don’t catch a glaring error that shows up over and over again. I’ll pick on Star Trek, since this year is the 40th anniversary of that show, though it is definitely not the only offender. You see the Enterprise chasing some other ship, and Captain Picard (Kirk, Sisko, Janeway, whoever) orders the other ship to stop. They seldom do. So, the captain order Worf (Sulu, Scotty, Tuvak, etc) to fire phasors. “Aim at the engines,” he says. So the Enterprise fires, knocks out the other ships engines, and the pursued vessel coasts to a stop. Makes sense, right? Well it wouldn’t work that way!
The problem is Newton’s First Law of Motion. This says that any body at rest will remain at rest, and any body in motion will remain in uniform motion, unless a force acts on said body. Newton’s Second Law goes on to say that if an unballanced force act on the body, the the acceleration will be proportional to the magnitude of the force, and inversely proportional to the mass of the object. If you actually think about it, the First Law is really just a corollary to the Second Law. If there is no force, then the magnitude of unballanced force is zero, so the acceleration is zero. Acceleration is defined as the rate of change of velocity. So, if the acceleration is zero, the rate of change of velocity is zero. That does not mean that the velocity is zero, only that its change is zero. So, no force means no acceleration, and no acceleration means no change in velocity. That means that the object would continue moving in a straight line at constant speed.
On Earth, there are always resistive forces at work. For the boat, water resistance slows the boat. For the car (or truck), air resistance and rolling friction slows the vehicle. If you cut the engines in an aircraft, air resistance slows the aircraft. Water resistance is much larger, so we see the effects far more dramatically with a boat. An aircraft might glide for many miles before slowing to the point that it can no longer fly. And the car or truck can roll for quite a ways before coming to rest. But in space, there is no air or water. The interstellar medium exists, and it would provide some resistance to motion, but only a very tiny amount. It would take millions of years for the interstellar medium to slow a craft appreciably, and that would take too long for most science fiction writers to incorporate into their story!
So, in space, cutting the engines would simply stop the spacecraft from going faster. It would not slow down. In fact, shooting out the engines would prevent the craft from slowing down, as it would have to use engine thrust to slow itself! But, even though I know that the physics is all wrong, I can stil ignore it and have fun watching the show. That is called suspension of disbelief, and it is essential for enjoying science fiction. Now, I will admit, though, that the original Star Trek was far worse about this than the later versions. When Star Trek was reborn with Next Generation, they actually had sense to occasionally ask someone when they did something like that. There is far more science in the later Star Treks than there was in the first (though it is still definitely science fiction). And, sometimes they writers and producers simply ignored their science advisor, because reality messed up the story. But, at least, they didn’t flagrantly violate almost every known law of physics every episode!
So, when you are watching a scifi flick, watch to see if they keep to Newton’s Laws of Motion!
-Astroprof
(Image credit:Â Paramount)
