/sci/ - Science & Math » Thread #11340245

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Anonymous Tue 28 Jan 22:48:13 2020 No.11340245 View Reply Original Report

Quoted By: >>11340256 >>11340265 >>11340279 >>11340811 >>11343015

If you can lift a 5 pound weight with 2 pounds of force using pulleys, why cant you make a perpetual motion machine?

The 2 pounds lifts the 5 pounds, then you use the 5 pounds to lift the 2 pounds.

Anonymous

Anonymous Tue 28 Jan 2020 22:52:37 No.11340256 Report

Quoted By: >>11340262

>>11340245
you apply 2 pounds over 2.5 feet to lift the 5 pounds one foot.

how do you plan on pulling 2 pounds a foot gives you 5 pound force over 0.2 feet, so you're gonna pull the next 2 pounds 0.2 feet to pull the next 5 feet 0.04 feet..

Anonymous

Anonymous Tue 28 Jan 2020 22:54:11 No.11340262 Report

Quoted By: >>11340268 >>11340276 >>11342976

>>11340256
How can that be if I have a cable without any flex? How could the pull end move further than the lift end?

Anonymous

Anonymous Tue 28 Jan 2020 22:54:16 No.11340264 Report

Quoted By:

Because there is no such thing as "conservation of force," but there is a such thing as conservation of energy. Mechanical advantage is not free energy. When you use a pulley to multiple your force by a factor X, you do so at the cost of slowing down the rate at which you can lift the weight by X. Same thing with gear reduction/multiplication.

Anonymous

Anonymous Tue 28 Jan 2020 22:54:32 No.11340265 Report

Quoted By:

>>11340245
ignoring friction, both a one pulley and two pulley system do the same amount of work lifting a mass.

work (amount of energy added to the system) = force * displacement

for the one pulley problem, one unit of force is applied through one unit of displacement equaling one unit of work.

for the two pulley system, half a unit of force is applied through two units of displacement equaling one unit of work.

Anonymous

Anonymous Tue 28 Jan 2020 22:55:18 No.11340268 Report

Quoted By: >>11340275

>>11340262
Have you never used a compound pulley in your life

Anonymous

Anonymous Tue 28 Jan 2020 22:56:18 No.11340275 Report

Quoted By: >>11340283 >>11340807

>>11340268
Would I make this thread if I had?

Anonymous

Anonymous Tue 28 Jan 2020 22:56:18 No.11340276 Report

Quoted By: >>11340291 >>11343532

>>11340262

what? it keeps getting shorter. read it again.

Anonymous

Anonymous Tue 28 Jan 2020 22:56:34 No.11340279 Report

Quoted By:

>>11340245
you'd run out of rope, stupid

Anonymous

Anonymous Tue 28 Jan 2020 22:57:21 No.11340283 Report

Quoted By:

>>11340275
compound bullies are literal perpetual motion machines.

Anonymous

Anonymous Tue 28 Jan 2020 22:59:33 No.11340291 Report

Quoted By: >>11340386 >>11343445

>>11340276
Doesnt matter which end, if there is no flex, how could a cable of say 10 feet move anything more than it moves itself?

Anonymous

Anonymous Tue 28 Jan 2020 23:34:05 No.11340386 Report

Quoted By:

>>11340291
so is english your third or fourth language

Anonymous

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Anonymous Wed 29 Jan 2020 00:20:00 No.11340536 Report

Quoted By: >>11341962

Anonymous

Anonymous Wed 29 Jan 2020 00:27:49 No.11340564 Report

Quoted By:

Classic

Anonymous

Anonymous Wed 29 Jan 2020 01:40:34 No.11340807 Report

Quoted By:

>>11340275
well congratulations, you got yourself caught.

Anonymous

Anonymous Wed 29 Jan 2020 01:41:38 No.11340811 Report

Quoted By:

>>11340245
Fucking Pulleys!
how do they work!?
>_<

Anonymous

Anonymous Wed 29 Jan 2020 11:06:26 No.11341962 Report

Quoted By:

>>11340536
moar

Anonymous

Anonymous Wed 29 Jan 2020 18:32:34 No.11342976 Report

Quoted By: >>11343532

>>11340262
Look at your image, OP. The pulled end (far left) is moving at X speed. What about the rest of the rope? For the bottom two visible segments, we know the left one is anchored to the top pulley, so it has zero motion (except maybe some angular displacement to direct itself into the lower pulley), but not linear translation. The right segment is traveling at the same rate as the end your pulling, it has to be, so moving at X speed upward. The movement of the bottom pulley has to be in relation to the two segments of string either going into it or out of it, and we know that those velocities are zero and X upward. Therefore, the average speed is X/2, and that is how fast the bottom pulley (and load mass) must be moving

Anonymous

Anonymous Wed 29 Jan 2020 18:50:28 No.11343015 Report

Quoted By:

>>11340245
if it were possible we would be buying electricity from pepetual-machine-energy co, right?

Anonymous

Anonymous Wed 29 Jan 2020 20:22:46 No.11343316 Report

Quoted By:

force * distance.

Anonymous

Anonymous Wed 29 Jan 2020 21:19:18 No.11343445 Report

Quoted By: >>11343448

>>11340291
Total cable length is a. The distance from left end to upper pulley is b. Remaining length is c = a - b. That remaining length goes to the lower pulley where it loops back to the upper pulley on which it is anchored.

So the distance between the upper pulley and lower pulley is c / 2. When you pull the cable by one meter, you decrease b by 1, increase c by 1, and decrease the distance between pulleys by 1/2.

Anonymous

Anonymous Wed 29 Jan 2020 21:20:33 No.11343448 Report

Quoted By:

>>11343445
Fucking fuck. Increase b by 1, decrease c by 1.

Anonymous

Anonymous Wed 29 Jan 2020 21:49:53 No.11343532 Report

Quoted By:

>>11342976
>>11340276
What are you retards on about? It's like you don't even understand the simple concern. The rope doesn't magically become shorter. It has to go somewhere.

OP's pic is called mechanical advantage, and certainly doesn't require the length of rope attached to the weight be somehow shorter. The extra force is from the rigidity of the pulleys themselves, i.e. the rope being pulled and the rope attached to the weight are the same length and travel 1:1 distance.

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