Q9 - Measurement of the motion

[XEVIAN_ZONG]

Hello, can you please explain to me why the answer B is the correct one? Also, I don't really get the logic relationship between the information provided in the stem of the questions. Thanks!

[giladedelman]

Thanks for posting!

Measurements of Uranus's orbit seem to show it being tugged away from the sun and the other planets by some other force -- and neither Pluto nor Neptune, the only planets on the far side of Uranus, are big enough to exert that kind of gravitational force. From these facts, the argument concludes that there must be some other planet we don't know about.

Well, okay, that could explain the measurements, but does it have to be true? No! The argument is making a big assumption: only planets can exert the kind of force that explains Uranus's motion. What if something else were responsible -- like a meteor, or debris from an alien spaceship? (Just sayin'...)

That's why (B) is correct. A belt of comets with a strong gravitational pull is exactly the kind of alternative explanation that would weaken this argument.

(A) is out of scope -- who cares when Pluto was discovered?

(C ) doesn't matter: we already know those planets aren't big enough.

(D) is tempting, but this doesn't explain why there seems to be a force pulling Uranus away.

(E) is totally out of scope.

Does that answer your question?

[XEVIAN_ZONG]

Yes! Now it is much clearer to me. Thank you!

[roflcoptersoisoi]

Thanks for posting!

Measurements of Uranus's orbit seem to show it being tugged away from the sun and the other planets by some other force -- and neither Pluto nor Neptune, the only planets on the far side of Uranus, are big enough to exert that kind of gravitational force. From these facts, the argument concludes that there must be some other planet we don't know about.

Well, okay, that could explain the measurements, but does it have to be true? No! The argument is making a big assumption: only planets can exert the kind of force that explains Uranus's motion. What if something else were responsible -- like a meteor, or debris from an alien spaceship? (Just sayin'...)

That's why (B) is correct. A belt of comets with a strong gravitational pull is exactly the kind of alternative explanation that would weaken this argument.

(A) is out of scope -- who cares when Pluto was discovered?

(C ) doesn't matter: we already know those planets aren't big enough.

(D) is tempting, but this doesn't explain why there seems to be a force pulling Uranus away.

(E) is totally out of scope.

Does that answer your question?

Great explanation, although I disagree a little bit with your reasoning for eliminating (D), if the Sun exerts less force on Uranus relative to other planets then could that not also conceivably explain why Uranus may start to gravitate away from the sun and the inner planets, towards pluto and Neptune? I myself am trying to come up with a more compelling reason to eliminate (D), if anyone has one, let me know.

EDIT: Nevermind, you're exactly right, stimulus explicitly says that a force is tugging Uranus away from inner planets, it isn't just gravitating there on it's own volition, in this case (D) is irrelevant.