Timeline for Why won't a tight cable ever be fully straight?
Current License: CC BY-SA 3.0
13 events
| when toggle format | what | by | license | comment | |
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| May 12, 2013 at 14:40 | comment | added | Marcks Thomas | @Ovi: That works for solid rods that can hold shear stresses. For ropes to remain in place, not only does the grand total of the forces need to be zero, the equality needs to hold locally as well (i.e., away from the supports), or the rope will deform. | |
| May 12, 2013 at 14:12 | comment | added | Ovi | Well I was thinking the cable is attached at one end to the top of a tall pole, and the other end goes through a pulley (at the top of another tall pole), and you are pulling the string down. Theoretically, couldn't the normal force of the pulley counter the force of gravity and support the wire? | |
| May 12, 2013 at 10:29 | comment | added | Marcks Thomas | @Ovi: Then either the cable is laying flat on the ground, or slices of zipline in between the suspended points (where no normal force is applied) would have a non-zero net force on them and will accelerate downwards until tension cancels out gravity. | |
| May 12, 2013 at 10:15 | comment | added | Marcks Thomas | @Kaz: A small slice of cable need not be straight to be horizontal. Likewise, a circle may have horizontal bits, but is equally curved everywhere. | |
| May 12, 2013 at 6:33 | comment | added | Michiel | @Ovi - perhaps you could write your own answer if you think all current answers are missing something?! | |
| May 12, 2013 at 5:28 | comment | added | BlueRaja - Danny Pflughoeft | Doesn't this argument imply that no objects can be perfectly straight? | |
| May 12, 2013 at 4:29 | comment | added | Ovi | I don't believe this is a complete answer. What if the upward force needed to cancel out gravity was actually the normal force upwards from the objects supporting the wires? | |
| May 12, 2013 at 1:50 | comment | added | tom | Great answer. To make it a bit more mathematical, the vertical component of the tension force must be equal and opposite to gravity. If the angle from the horizontal is θ and the tension is f, f * sin(θ) = g and hence f = g / sin(θ). As θ approaches 0, f approaches infinity. | |
| May 11, 2013 at 23:06 | comment | added | Kaz | This is false. At the minimum height point, there is an infinitesimal slice of the cable which is horizontal. What's holding up every little infinitesimal slice of a cable which has nonzero thickness is not only tension, but sheer stress. | |
| May 11, 2013 at 22:21 | comment | added | Rob | This was one of the favorite things I learned in physics in high school. A weight attached to a string attached to a rotating platform, how fast must the platform spin for the weight and line to be parallel to the platform? Infinitely fast to overcome the (what seems insignificant at high speeds) force of gravity pulling downwards (and much less if the shape of the weight will provide lift). | |
| May 11, 2013 at 22:16 | comment | added | Alfred Centauri | @ratchetfreak, :) even massless photons gravitate. But, let's face it, a massless cable can't help but travel at light speed, right? | |
| May 11, 2013 at 22:04 | comment | added | ratchet freak | unless the cord is weightless (or massless) ;) | |
| May 11, 2013 at 18:22 | history | answered | Alfred Centauri | CC BY-SA 3.0 |