Clarification on Gravity

lubuntu

If we bring something entirely to a stop with respect to the sun at 1 AU from the sun, and considering only gravity from the sun, it will fall to the sun. I calculated that it would then reach the corona with a velocity of something like 619km/s. When I was reviewing this in my textbook the book says this is "impractical" because of the amount of energy it would take? But isn't the whole point that the sun is doing all the work to increase the objects kinetic energy? How is this impractical?

Thanks!

Klockan3

How would it be practical?

There is something in the question you are omitting since that sounds as if they wanted to extract this energy in some way, what was the goal in the first place?

lubuntu

No, not to extract the energy just have for it to reach the sun with that velocity. The book makes it sound like it won't happen.

Doc Al

Mentor
What textbook are you using? Can you give the entire statement, so we can see it in context?

Klockan3

Well, its far below the speed of light so I don't see anything then. Are you sure that's all you got, since impractical implies that you wanted to use it for something.

lubuntu

The question simply has us dropping a container of waste into the sun.

Klockan3

Then it probably refers to that it would be impractical to drop waste into the sun considering the huge amount of energy you need to exert on the waste to make it stand exactly still in relation to the sun, considering how fast the earth moves.

Doc Al

Mentor
The question simply has us dropping a container of waste into the sun.
OK. It sounds like you are taking a container of waste from the earth's surface and then dropping it into the sun? If so, you first have to give it enough energy for it to escape earth's gravity--that's why it's impractical. They weren't commenting on the energy of the object as it hits the sun, but the energy it would take to get it off the earth.

Klockan3

OK. It sounds like you are taking a container of waste from the earth's surface and then dropping it into the sun? If so, you first have to give it enough energy for it to escape earth's gravity--that's why it's impractical. They weren't commenting on the energy of the object as it hits the sun, but the energy it would take to get it off the earth.
Well, considering how the problem was stated it probably have more to do to get it to stand still in relation to the sun, which requires a ton more energy than just getting it off the earth.

lubuntu

Ya the comment seemed just to be like an aside in the book but I found it confusing as to what they meant.

Doc Al

Mentor
Assuming that the real issue is whether dumping earth trash into the sun is practical, what matters is the energy it takes to get the trash off the earth and onto a collision course with the sun. But you're right that earth speed will play a key factor in that.

Klockan3

Assuming that the real issue is whether dumping earth trash into the sun is practical, what matters is the energy it takes to get the trash off the earth and onto a collision course with the sun. But you're right that earth speed will play a key factor in that.
Yeah, and the earths speed is ~3 times the escape velocity of earth, the least amount of energy needed to give it a collision course towards the sun is to accelerate it till it is at rest in relation to the sun aka you need to accelerate it up to the same velocity the earth have in relation to the sun.

(Of course assuming that the earth is traveling with orthogonal velocity compared to the vector towards the sun, but still that difference is negligible)

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