PrepTest 156, Section 2, Question 25
According to the Newtonian theory of gravity, the(54 more words not shown)
The conclusion of the argument(11 more words not shown)
- AMass is dependent(3 more words not shown)
- BAccording to the(13 more words not shown)
- CThe Newtonian theory(12 more words not shown)
- DKnowing what an(10 more words not shown)
- EThe gravitational force(15 more words not shown)
Reading the argument
We started with gravitational force. Now we're calculating orbits. What connects the two? The theory, as described here, uses mass and distance to determine gravitational force. What the bodies are made of isn't an additional factor in that calculation. The calculation uses how much mass the bodies have and how far apart they are, without a separate input for whether they're made of hydrogen, nitrogen, or something else.
I have two questions. Could understanding an object's structure help us figure out its mass? And, even once we know the gravitational force, might calculating an orbit require something else? The second concern is the direct jump between the premise's topic and the conclusion's topic.
What we’re looking for
We're asked for an assumption that makes the conclusion follow. I'd look for a connection saying that the orbit calculation needs only the gravitational forces. Keep the scope in mind: the argument is about what this theory requires, not about proving that the theory is a complete description of real-world physics.
Working the answers
- A
The premise says gravitational force depends on mass. A gives a dependence in the other direction. Either way, it doesn't connect gravitational forces with the calculation of orbits, so it doesn't close the gap we identified.
- BCorrect
Within the theory, calculating planetary orbits requires considering only their gravitational forces. The premise already says those forces depend exclusively on mass and distance, independently of the bodies' composition. B therefore carries that independence into the orbit calculation.
What about our other question, whether composition could help us learn an object's mass? That concerns how we might obtain an input, rather than whether a separate theory of the object's structure and constitution is required in the calculation described here. B completes the intended chain: mass and distance determine the gravitational force; those forces are all the orbit calculation requires. We needn't claim that investigating composition could never be useful for any purpose.
- C
Claiming that Newtonian theory is mistaken doesn't help derive a conclusion about what follows according to that theory. It challenges the framework instead of connecting the force calculation to the orbit calculation. The question asks us to complete the argument on its stated terms.
- D
This offers a possible way to determine mass: know what the object is made of. It doesn't say that composition is required to determine mass, and it doesn't connect mass or gravitational force with everything needed for an orbit calculation. Showing that information could help is different from establishing whether we need it.
- E
This relates gravitational force to distance between the Sun and a planet. But we need the claim that gravitational forces are all we must consider to calculate the orbits. Saying force is a factor in determining distance doesn't establish that complete connection.
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