becker@uiucdcs.UUCP (08/04/83)
#N:uiucdcs:24400033:000:501 uiucdcs!becker Aug 3 05:19:00 1983 this is perhaps a silly question, but here goes.... i have a lever that is ten light-years long, hinged at one end, free at the other. for the sake of argument, Mr. A is at the free end, and Mr. B is at the other. the two have agreed that Mr. A will swing his end of the lever, and as soon as Mr. B detects the motion, he will signal Mr. A with a laser. my question is, how long will Mr. A have to wait after pushing the lever before he sees Mr. B's laser? thanks in advance, craig becker uiucdcs
krueger@uiuccsb.UUCP (08/04/83)
#R:uiucdcs:24400033:uiuccsb:10800001:000:925 uiuccsb!krueger Aug 3 15:13:00 1983 20 years. My line of reasoning: Assume Mr. A and Mr. B can see one another (using telescopes or something). Let's also assume that if Mr. A moves his end of the lever at all, Mr. B can see exactly how far Mr. A's end moved. Here's the crucial assumption: The forces holding the lever together travel no faster that the speed of light. Then, when Mr. A moves his end of the lever, both the image of the motion and the lever motion proceed towards Mr. B., with the image of Mr. A's end movement probably beating out the force propogation. It takes 10 yrs for Mr. B to see Mr. A move his end, and he sees some rotation at the pivot some time later. If Mr. B sends his laser signal when he sees Mr. A move, Mr. A will see the laser 10 yrs later. 10 + 10 = 20 (<- difficult calculation). I hope I'm not making a fool of myself, Physics was a long time ago for me. Jon Krueger, ...!pur-ee!uiucdcs!krueger
ecn-ec:ecn-pc:ecn-ed:vu@pur-ee.UUCP (08/08/83)
The sln by uiuccsb!krueger sounds good to me, but only that `forces holding the lever travel no faster than ...' does not make sense. The molecular forces inside the bar do not travel from A to B but in an arc about B. Instead, what is traveling here is the impulse made by A to swing the lever: it is then a form of wave propagation. And mechanical wave (A's impulse) on a solid (lever) propagates much slower than E-M waves. Thus, if B relies *only* on movement of lever to signal the laser, then A will have to wait much more than 20 years before receiving it. Hao-Nhien Vu. pur-ee!vu or(better) pur-ee!norris
kwmc@hou5d.UUCP (08/08/83)
I would think that the impulse would travel along the lever at the speed of sound (in whatever material the lever is made of) Ken Cochran hou5d!kwmc
jvenner@wateng.UUCP (Jason P. Venner) (08/09/83)
You did not make a fool of yourself by assuming that the forces holding the rod together travel at no faster than the speed of light. for all out there in net.physics land: that forces holding the atoms/molecules (ie atom to atom etc) of the rod together are electromagnetic and therefore travel are only propagated at the speed of light. yours sincerly, Jason P. Venner (jvenner@wateng) (watmath!wateng!jvenner)
ecn-ec:ecn-pc:ecn-ed:vu@pur-ee.UUCP (08/09/83)
Intuitively, the impulse might travel at speed of sound. But (correct me if I am wrong) sound is a longitudal wave (direction of vibration same as direction of propagation) while the impulse is not. So there is no reason why they should have the same speed. Hao-Nhien Vu (pur-ee!vu or better: pur-ee!norris)
ecn-ec:ecn-pc:ecn-ed:vu@pur-ee.UUCP (08/10/83)
I still don't see how forces holding the rod together should travel from A to B. And also, they are E-M forces all right, but they don't have to travel like an E-M wave. Rather, those forces don't play any role in this problem. They are stationary with respect to the rod. Hao-Nhien Vu (pur-ee!vu or: pur-ee!norris)
ecn-ec:ecn-pc:ecn-ed:vu@pur-ee.UUCP (08/10/83)
By the way, *IF* there is some wave that travels from A to B with speed of light, then that wave will (what do you call it? reflects? anyway) reflects at B (since B's end is hinged) back to A with speed of light. Thus: B does not need to hang around. Hao-Nhien Vu (pur-ee!vu or pur-ee!norris)