Okay, so this show never has been noted for its scientific accuracy. But there’s a question I have about a particular scene.
In one episode The Doctor is floating in space between a spaceship and his TARDIS. He takes a cricket ball out of his pocket, throws it. It bounces off the spaceship’s hull, The Doctor catches it, and is carried to The TARDIS.
Is this remotely possible?
The Doctor gains velocity opposite the vector of the ball as he throws it, that vector is imposed onto the space ship when the ball bounces off. The ball gains a reverse vector somewhat smaller than it’s original vector, but opposite in sign. When the Doctor catches it, he gains an additional vector, also somewhat smaller, but with the same sign as his original change of vector.
So, yeah, kinda.
Tris
I think errors in physics are the least of the problems with Dr. Who; those alien monsters are pretty problematic. (How come all alien physiology is based upon foam rubber?). But in this specific case, the basic physics is sound (conservation of momentum), though I wouldn’t care to be floating through vacuum with nothing more than a helmet to keep me from freezing.
Stranger
I bet the show made it seemed as if catching the ball is what propelled him, when in fact, he gained a lot more velocity from throwing the ball. In fact, the spaceship didn’t even have to be there.
Assuming that the collision is effectively elastic (and the TARDIS outmassing the ball by a few orders of magnitude), he gained twice as much momentum from the catching the ball as just throwing it never to return.
Stranger
You mean, he gained twice the momentum from throwing it and catching it than he wouldhave from just throwing it - and even then, only if no energy was lost in the bounce & catch. Alll that says is that without the spaceship it would have taken him twice as long to reach the Tardis. He would still have made it.
The result is equivalent to if he had simply pushed against the hull of the spaceship with the same force with which he threw the ball. I suppose the reason he threw the ball is that he wasn’t close enough to touch the hull.
Yes, but then he’d have been out a cricket ball. What if he’d needed it for a match later?
Besides, if he caught the ball, he could have thrown it again to gain more momentum if necessary.
And of course it makes a better Whoish special effect! It requires that the Doctor master the skill of throwing, bouncing and catching a ball exactly along the line from the center of gravity of his body, and the point he wishes to reach. That needs to be in line with a part of the spaceship that will provide a bounce directly back along that line.
However, you just cannot arrive at the Tardis spinning head over heels! Unseemly! Not at all the thing, you know. One must maintain at least the barest minimum of decorum.
Tris
Am I misreading what is going on? The doctor throws a ball toward the Tardis. This will result in him moving away from the Tardis. Then the ball hits the Tardis. This will result in the Tardis moving awya from the doctor. I agree that the Tardis being a massive object will not change its velocity much by this. Then the doctor catches the ball again giving him even more velocity away from the tardis. It does not seem to work at all in any way.
Yes, you’re misreading. He throws the ball at the spaceship, away from the TARDIS. This moves him towards the TARDIS. You have the physics right, just swap the locations of the TARDIS and the spaceship in your head.
I read it that way at first, too.
The Doctor is throwing the ball at object X to propel himself TOWARDS the Tardis. It bounces off object X and The Doctor is now moving toward the Tardis at twice the speed he was when he threw the ball.
-Joe
That’s also assuming that the Doctor is significantly more massive than the ball (which is probably also a good approximation, though not as good as the spaceship one). Remember, when he catches the ball, the ball ends up moving at the same speed as the Doctor, so it doesn’t lose all of its momentum.
According to the Wikipedia article on cricket the modern cricket ball weighs between 5.5 and 5.75 oz (~.16 kilo) and assuming that The Doctor is a carbon-based lifeform roughly approximating Homo sapiens in density, he’ll weight (in Incarnation 5, which IIRC is the specific Doctor being referenced) roughly 80 kilograms. So the mass of the cricket ball (assuming that it’s not actually a Krikkit ball) will be about 2% of the mass of The Doctor. So the total momentum transfer can be considered complete for a first approximation.
A better question is why isn’t The Doctor better prepared for this sort of thing? After all, if he can transcend time and space, shouldn’t be be capable of managing a little bit of orbital maneuvering and EVA?
Stranger
Oh a related note, does the vacuum in space make “swimming” useless?
Yes. Swimming requires interaction with a surrounding gas or liquid. In a space, no one can cheer your swim.
Better plan: throw the ball at der tardis, propelling himself towards the space ship. When he reaches the ship, use the full leverage of his legs to push off. That oughta get him there much more quickly.
Or, offer the janitor a shiny new barometer if he will tell you the height of the building.
(BTW, what the hell is a TARDIS?)
Time And Relative Dimension In Space.
Tris
In other words, it’s the Doctor’s space/time ship.
It’s also bigger on the inside than on the outside.