TBR physics example 2.6b

[2010premed]

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First of all, does anyone know if they give the answers anywhere in the book for examples b in the book?

Here's the question: If the astronaut is in a fixed orbit around a shrinking star, how would the force on him change as the star shrinks? (Assume that the star sheds none of its mass.)

A. The force would decrease
B. The force would remain the same
C. The force would increase.
D. The change in force would depend upon the radius of the orbit

Thanks

 

[RickyJamesMD]

D

Because the force acting on astronaut is all due to gravity of the two objects. The universal gravitational law states that there's a 1/r^2 dependence on distance between two objects if masses were to remain the same.

Hope it helps!

http://upload.wikimedia.org/wikipedia/commons/0/0e/NewtonsLawOfUniversalGravitation.svg

 

[2010premed]

Thanks that helps. I guess I was thinking B because I was looking too closely on the statement that the star would shrink, which would not cause a difference in r.

 

[RickyJamesMD]

Now that you've mentioned it, I think you're right. If it's at "fixed" orbit, I'm assuming R doesn't change. Then maybe B is the right answer.

Tricky.

 

[SoftwareKevin]

Now that you've mentioned it, I think you're right. If it's at "fixed" orbit, I'm assuming R doesn't change. Then maybe B is the right answer.

Tricky.

If the star is shrinking though, isn't its mass decreasing?

F = G(m1)(m2)/r^2

so assuming fixed orbit means r isn't changing, then one of the masses is constant while one is decreasing so F should go down.

 

[2010premed]

the problem says to assume the mass is not decreasing

 

[fizzgig]

ok so based on the F=Gm1m2/r^2 equation, I would agree that F should not change (Kevin it says assume the star sheds none of its mass, so it's getting denser).

With that in mind though, if the sun began to collapse into a denser and denser object, with the end of say, a black hole in mind, uhh, that would not change the gravity we'd see? Really? That confuses me..

 

[SoftwareKevin]

the problem says to assume the mass is not decreasing

I have a hard time with reading comprehension apparently 🙂

 

[startswithb]

I have the book, and the answer is B if you're wondering. 🙂 M and r are fixed so nothing changes.

 

[2010premed]

Startswih, where are the answers??

 

[Danlee07]

B should be the answer

Just thinking...Force = m*a. If m's the same...then force would be too

 

[fizzgig]

so if the sun turns into a black hole, we won't fall into it because its mass is the same, just crammed into a tiny point?

 

[startswithb]

Answers are on the second to last page, on the bottom. Very tiny.

 

[BerkReviewTeach]

so if the sun turns into a black hole, we won't fall into it because its mass is the same, just crammed into a tiny point?

The gravitational force won't change, because it's the same distance from the center of our mass to the center of the Sun's mass, and neither mass is changing. That's what the question is focused on.

BUT!!!, whatever force is causing the collapsing of the Sun (i.e., causing the formation of the black hole) is a force in addition to the gravitational force, so if we do in fact get sucked in, then it's due to the additional force, and not the gravitational force. So if science fiction movies are in fact true, then the Star Trek force will overpower all other forces and suck us into the collapsing sun's mass vortex until Spock does whatever science voodoo he does to generate a new and better force so that we can survive to ensure a sequel.

 

[fizzgig]

black holes and changing timelines are indeed the key to sequels 🙂

i think i'm on board now... after being tricked my my curiosity into getting waist deep in wiki black hole articles. fascinating, these things i will never understand, simply fascinating. it does make sense to me intuitively now as well as mathematically, though.