Also, when I say you can hit the sun by “angling the shot to compensate” I am assuming that we can launch a rocket at over 30 km/s. The faster you can go, the less angle you need, and the faster you can get to the sun. If you can only launch at 30.1 km/s, for example, then your launch angle would have to be almost directly opposite the earth’s movement, relative to the sun.
Here’s a question: If you launch something at the exact same speed as the earth’s tangential motion in the opposite direction, how long will it take to “fall” into the sun?
What if you launch it a little too fast? I assume you’ll just get an eliptical orbit starting on the opposite side.
Also, it seems really difficult/improbable to get a circular orbit. How circular are the orbits of the planets in our solar system? I know Neptune’s is more eliptical than most, but what about the others?
Why couldn’t we just shoot it up, relative to the plane of the Solar system? It seems odd to me that all discussion is confined to the plane of orbit of planets when there are, oh, 340-350 degrees or so in which firing something off would take it out of line with our, and all other planets, orbit.
If you shoot it “up” it would still have the earth-orbit tangential speed. It would go into an orbit the same shape and size as the earth’s, but at a cockeyed angle.
dil, I realize that, but my point is that people always seem to be concerned only with the idea of debris being in the way relative to possible travel and this travel always seems to be in the direction of the plane of our System.
I’m saying, if we’re considering mucking things up, we might as well do so primarily in the Z-axis, keeping the road open to other planets if we miss Sol.
If the physics book you bought says that there’s no such thing, try to get a refund. Centrifugal force is a perfectly valid concept, provided you’re in the right reference frame to use it. If you’re in the co-rotating reference frame, then it’s just plain wrong to not use centrifugal force. scr4 is the only person in the thread thus far who is using centrifugal force, and he’s using it correctly.
Quoth Mont Furd:
Incorrect. Killing off all of the orbital speed drops you straight in. Lowering the orbital speed puts you into an elliptical orbit, which will come all the way back out to Earth’s orbit, every cycle. Which leaves open the possibility of a rather unpleasant collision some time in the future. If the only force available is Newtonian gravity (an excellent approximation in the Solar System), you simply cannot get a spiral or decaying orbit.
Now, admittedly, you don’t quite need to kill all of the orbital energy. An elliptical orbit is acceptable, if the perihelion hits the surface of the Sun. But that’s a mighty small target.
Quoth Lemur866:
No, we’re screaming towards the Sun with fairly high acceleration, but approximately no velocity (towards the Sun). The key is that “falling” refers to acceleration, not velocity: If you throw a ball in the air, it’s falling from the moment it leaves your hand.
Quoth thinksnow:
If you do this at less than escape speed, the debris will come back and cross the orbital plane, again and again. The problem with putting debris into a closed orbit isn’t everything else in the way, it’s us in the way. If this thing is always going to stay in our vicinity, or always come back to our vicinity, then I’m going to worry about it maybe eventually hitting us. And if you do want to get the stuff to escape speed, then you still don’t want to shoot it out of the plane, because you might as well take advantage of the orbital speed we’ve already got, and shoot it “forward”.
I’m sorry I tarred you in with the “spiral orbit” crowd, you weren’t one of those, and I apologize for that.
But I stand by my rocket statement. A rocket like you describe would violate the laws of physics. Rockets have a fixed amount of delta-v. A non-rocket device that could constantly accelerate a spaceship might or might not violate the laws of physics, but it wouldn’t be a rocket.
Rockets work by a simple physical principle. Let’s take an example. Suppose a rocket masses 1 ton, and you have 1 ton of reaction mass. If you throw all the reaction mass overboard at 1 meter per second, your spaceship is going to end up traveling in the opposite direction at 1 meter per second. If you increase the speed you throw the reaction mass overboard, you increase the final speed of the rocket.
But even if you had some non-rocket propulsion system, and pointed your ship at the sun and started blasting, you’d probably miss the sun unless you calculated things perfectly. Blasting towards the sun doesn’t cancel your sideways velocity, and it doesn’t cancel the acceleration you get from the sun’s gravity. You are going to be traveling faster and faster. The closer you get to the sun, the more that sideways velocity is going to show. At 30 km/sec sideways motion, you are really traveling very very fast sideways. The most likely result is that you will zoom past the sun, and enter a highly elliptical orbit, with an ahelion much higher than earth’s orbit, since you added so much velocity with your non-rocket spaceship.
The correct way to hit the sun is not to blast towards the sun, but to blast opposite the earth’s movement around the sun. The sun’s gravity will pull you in.
And I also see that Chronos is of course correct…we have almost no VELOCITY towards the sun, of course. What we really have is a huge ACCELERATION towards the sun. We are falling towards the sun, but we have enough sideways force to always miss. Adding more acceleration towards the sun doesn’t cancel that sideways force. The sun is very hard to hit, since every body in the solar system has this huge angular momentum. By definition really, since any object with less angular momentum was swallowed by the sun during the formation of the solar system, and anything with more was ejected.
Assuming that we wish to one day colonize the moon and don’t want the waste there, Jupiter seems like the best place to send it based on the other information in this post.
Giancoli’s textbook (Physics for Scientists & Engineers, 3rd Edition) discusses centrifugal force twice. On page 115, it states: “There is a common misconception that an object moving in a circle has an outward force acting on it, a so-called centrifugal (“center-fleeing”) force.” In the margin on this page, it warns: "Beware the misconception for a “centrifugal force.”
However, on page 291, in an optional section, the book discusses inertial and noninertial reference frames, and describes how to properly use fictitious forces (pseudoforces) in a rotating noninertial reference frame. One of these fictitious forces is the centrifugal force, of course, and another is the Coriolis force.
That being said, I tend to agree with //\etalhea|)‘s point of view. I believe that the use of such fictitious forces for the vast majority of students simply leads to misconceptions. I certainly drummed it into my students’ heads that centrifugal force simply does not exist, which is quite true–in an inertial reference frame. And for most beginning physics students, it is best, IMHO, to stick with inertial reference frames.
BTW, I’ve also got a copy of Serway & Beichner, Halliday & Resnick, and Tipler, if anyone wants me to check another physics text out.
I don’t agree with //\etalhea|)'s point of view at all, in this case. One of the most famous fictitious forces of all is gravity, in some theories–as in our current best guess at the state of the universe, General Relativity.
For most beginning physics students, the use of physics leads to misconceptions.
In my experience, teaching physics goes most smoothly if you can let the students use whatever physical intuition they might have, and, in fact, encourage it. A student who has a gut feeling about the right answer is a lot better off than one who goes through two pages of correct math which doesn’t correspond to the real world (of course, the student who does correct math which does match the real world is best off, but that’s hard to find). And centrifugal force is something which the students are already familiar with, and have a gut feeling for. If you try to break down that intuition, you’re almost guaranteed to make things worse. I once saw a poster right here on the SDMB swear to me that folks walking around in a rotating space station would have their feet closer to the center than their heads, because centripetal force points in. More intuition and less “learning” would have corrected that notion.
Yes, and I’ve had students who were firmly convinced that if you swung a ball on a string around in a circle, and the string broke, the ball’s initial motion would be outward, because the direction of centrifugal force is outward, of course.
After emphasizing that centrifugal force does not exist (in an inertial frame), they were able to see that the true initial motion of the ball is tangent to the circle at the moment of release.
As far as students’ intuition is concerned, their intuition also erroneously tells them that a continuous force is necessary for constant velocity motion. Very few people’s intuition is Newtonian, in my experience.
Robby beat me to it, but since he was nice enough to back my point, I want to support his.
Chronos wrote:
In my experience, teaching physics goes most smoothly if you can let the students use whatever physical intuition they might have, and, in fact, encourage it. A student who has a gut feeling about the right answer is a lot better off than one who goes through two pages of correct math which doesn’t correspond to the real world (of course, the student who does correct math which does match the real world is best off, but that’s hard to find). And centrifugal force is something which the students are already familiar with, and have a gut feeling for. If you try to break down that intuition, you’re almost guaranteed to make things worse. I once saw a poster right here on the SDMB swear to me that folks walking around in a rotating space station would have their feet closer to the center than their heads, because centripetal force points in. More intuition and less “learning” would have corrected that notion.
Well, I remember performing the weight-on-a-string experiment at an early age (no scientific purpose, just goofing in my yard), and then seeing it again in high school physics. The teacher let go, it flew off on a tangent, I went “oh, yeah, he’s right it, does do that” Walla! A real-life intuitive experience. Centrifugal force is incorrect. It doesn’t support what my eyes and hands experience.
And I apologize sincerely to the OP for the hijack.
I don’t think a blanket statement like that is warranted. It’s not centrifugal force that is incorrect, it is that interpretation of centrifugal force.
In the rotating frame, the object does move out radially.
As much as it pains me to agree with RM Mentock (;)), centrifugal force does support what your eyes and hands experience.
If you are standing still, whirling the weight-on-a-string over your head, you are in an inertial reference frame wrt the weight. The weight will act as if there was no centrifugal force. When you let it go, it will fly off at a tangent. (Much like this thread.)
If you are using the weight-on-a-string like an Olympic hammer thrower, i.e., you are rotating your body around at the same angular velocity as the weight, then you’ll see something completely different. When you let go, you’ll see the weight accelerate in the +r direction away from you with a centrifugal acceleration of ω[sup]2[/sup]r, and also accelerate away from you in the -θ direction with a Coriolis acceleration of 2ωv[sub]r[/sub].
Spend ten minutes in the backyard, revising your childhood experiences with weights and string.
My colleague in OA reckons that you could use the geometry of the earth-moon system to slingshot a spacecraft around the earth and or moon to get in to the inner solar system…
it is a desirable place to be, as the solar energy increases rapidly as you get closer to the sun (and there are more heavy metals as well apparently)
I’m not sure if this slingslot effect could be used in this way, but if so it might avoid having to go to Jupiter or Saturn in order to get to Mercury…
