SilvrGrey330

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This is going to be hard, but ill try my best:

What are T1 and T2 in the diagram below. All objects are at rest. How do u figure this out?

------------------------------- <--- Ceiling
..................l
..................l
..................l
..................l (T1)
..................l
..................l
..................l
...............[50N] <----- A block thats 50N
..................l
..................l
..................l
..................l (T2)
..................l
..................l
..................l
.............[[100N]] <---- A block thats 100N




Task is to find the tensions in both ropes. Need work shown too. thanks
 
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SilvrGrey330

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key is T1 = 150N
T2 = 100 N

can anyone explain?
 

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SilvrGrey330 said:
key is T1 = 150N
T2 = 100 N

can anyone explain?
T2 = 100N because its the weight (mg), so T = mg. And T1 = mg of the first block but also in series there is 100N weight. So, T1 = 50+100 = 150N

S.
 
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Use F = ma. If the blocks are at rest, the forces acting on them must sum up to be 0, since the acceleration is 0 (the blocks are not moving). Weight and tension are both vector forces, that always act in opposite directions. If you choose tension upwards, and weight downwards, and the object is at rest, the forces will cancel out.

T1 - 50N + T2 - 100 N = 0.

T1 + T2 = 150 N

T1 = 50 (the weight of T1 downwards must be balanced by the tension exerted by the rope upwards). Therefore, T2 = 100 N.
 
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SilvrGrey330

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cool thx, so if in parrallel, do the weights get halfed on each tension?
 

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SilvrGrey330 said:
cool thx, so if in parrallel, do the weights get halfed on each tension?
What do you mean by parallel?
 
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SilvrGrey330

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------------------------------- <--- Ceiling
..................l
..................l
..................l
..................l (T1)
..................l
..................l
..................l
...............[50N] <----- A block thats 50N
..................l
..................l
..................l
..................l (T2)
..................l
..................l
..................l
.............[[100N]] <---- A block thats 100N
............l............l
............l............l
..(T4)....l............l (T3)
............l............l
.........[[[[[400 N]]]]]


ok so what would be the new tensions in all of these: T1,T2,T3,T4
 

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SilvrGrey330 said:
------------------------------- <--- Ceiling
..................l
..................l
..................l
..................l (T1)
..................l
..................l
..................l
...............[50N] <----- A block thats 50N
..................l
..................l
..................l
..................l (T2)
..................l
..................l
..................l
.............[[100N]] <---- A block thats 100N
...........l............l
...........l............l
...........l............l
...........l............l
........[25N].....[25N]

Do you have a pulley connecting the first rope to the second?
 
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SilvrGrey330

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sorry, heres the final edit

there are no pulleys involved, only ropes and weights

------------------------------- <--- Ceiling
..................l
..................l
..................l
..................l (T1)
..................l
..................l
..................l
...............[50N] <----- A block thats 50N
..................l
..................l
..................l
..................l (T2)
..................l
..................l
..................l
.............[[100N]] <---- A block thats 100N
............l............l
............l............l
..(T4)....l............l (T3)
............l............l
.........[[[[[400 N]]]]]
 
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SilvrGrey330

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my bad, i did an edit on the one u were working with...didnt mean to confuse anyone who is reading this thread. can you try to work on the "most" updated image. thanks. i appreciate ur work on the middle image. By parallel i means a 1 weight connected by 2 ropes...so that the tension in each rope is 1/2 of the weight thats pulling down on it.
 

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SilvrGrey330 said:
sorry, heres the final edit

there are no pulleys involved, only ropes and weights

------------------------------- <--- Ceiling
..................l
..................l
..................l
..................l (T1)
..................l
..................l
..................l
...............[50N] <----- A block thats 50N
..................l
..................l
..................l
..................l (T2)
..................l
..................l
..................l
.............[[100N]] <---- A block thats 100N
............l............l
............l............l
..(T4)....l............l (T3)
............l............l
.........[[[[[400 N]]]]]
I think T3 and T4 each = 400 N, since the ropes aren't connected.
 
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SilvrGrey330

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Really?

so then i guess if there was a pulley below 100N and wraped T3 and T4, then it would be 1/2 weight distribution between the 400N

So t3 and t4 = 200N
 

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Are T3 and T4 connected to the other blocks? Or just to the ceiling?
 
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SilvrGrey330 said:
Really?

so then i guess if there was a pulley below 100N and wraped T3 and T4, then it would be 1/2 weight distribution between the 400N

So t3 and t4 = 200N
If the pulley is frictionless, and the acceleration is 0, then the tension is the same on both sides of the rope. Don't want to comment on the weight distribution though, not sure about that. Usually they give you the weight of one block?
 
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SilvrGrey330

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t3 and t3 are connected to 100N and 400N...not all the way to the ceiling
 
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SilvrGrey330

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so when can we use pulleys to reduce force? i gues thats my point all along. when is it...1/2 or 1/3 or 1/4 the weight?
 

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SilvrGrey330 said:
t3 and t3 are connected to 100N and 400N...not all the way to the ceiling
So T3 and T4 are connected to both the 100 N block, and a 400 N block?
 

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SilvrGrey330 said:
so when can we use pulleys to reduce force? i gues thats my point all along. when is it...1/2 or 1/3 or 1/4 the weight?
Not sure about this.
 

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SilvrGrey330 said:
sorry, heres the final edit

there are no pulleys involved, only ropes and weights

------------------------------- <--- Ceiling
..................l
..................l
..................l
..................l (T1)
..................l
..................l
..................l
...............[50N] <----- A block thats 50N
..................l
..................l
..................l
..................l (T2)
..................l
..................l
..................l
.............[[100N]] <---- A block thats 100N
............l............l
............l............l
..(T4)....l............l (T3)
............l............l
.........[[[[[400 N]]]]]

Under the assumption that T3 and T4 are equal distances from the center of the 400N mass, so there's no uneven weight distribution, then T3 = T4 = 200N. If T3 and T4 were each 400N, that would mean there was a net force of 400N upwards on the block, and it would accelerate. The 'job' of holding the mass up is divided between T3 and T4 though, so they each do half the work. T2 would be 400N + 100N = 500N, and T1 would be 550N.
 
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MattD said:
Under the assumption that T3 and T4 are equal distances from the center of the 400N mass, so there's no uneven weight distribution, then T3 = T4 = 200N. If T3 and T4 were each 400N, that would mean there was a net force of 400N upwards on the block, and it would accelerate. The 'job' of holding the mass up is divided between T3 and T4 though, so they each do half the work. T2 would be 400N + 100N = 500N, and T1 would be 550N.

Great answer. For those who don't get it, you should probably consult a physics book if you have one handy. Tension in ropes with statics masses is a simple yet crucial concept.
 
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SilvrGrey330

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MattD, you totally clarified it all. I see it. it makes sense now that the 400 gets split between the two (t3 and t4) and like u said it it were both 400 then it would fly up and thats certainly not the case.

cool, i get it.

Histo, get it too? i appreciate u helping out too man.
 

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SilvrGrey330 said:
MattD, you totally clarified it all. I see it. it makes sense now that the 400 gets split between the two (t3 and t4) and like u said it it were both 400 then it would fly up and thats certainly not the case.

cool, i get it.

Histo, get it too? i appreciate u helping out too man.

NO problemo, for a more complete explanation of these types of problems, run to the library and look through a book on statics. Beer and Johnston is a good one. It'll go into a little more detail than your typical physics text.
 

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SilvrGrey330 said:
so when can we use pulleys to reduce force? i gues thats my point all along. when is it...1/2 or 1/3 or 1/4 the weight?

Please, please, please remember that machines DO NOT reduce the amount of force needed to move an object. Just the amount of work it takes.

With that said. A pully is a machine. For this system, let us take T3 and T4. We know that the combined tension in both of the ropes is 400N. If you need to lift the block, the minimum force you need is 400N.

Work=Force X displacement. If you wanted to life the object 5 meters, then the work done is going to be 2000 Joules. If you add a pully to it, the amount of work needed to be done will be reduced but the force needed will stay the same.
 
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SilvrGrey330

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i thought it was: machines dont change the amount of WORK...but does change the amount of distance one applies.

If it takes 1000J to life up a bowling ball thats 500N over 2 m,

vs if you had a machine do it, it would still make the machine do 1000J
 

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EMT has it backwards: pulleys, and all other simple machines, leave work unchanged while altering force and distance. In all cases, the force is multiplied by a particular factor (which can be greater or less than one) and the distance is divided by the same factor.

For MCAT-style pulley systems, output force is input force multiplied by the number of ropes that are effectively pulling on the object to be moved; equivalently, by the number of ropes that must (not just might) shorten in order for the resistance to move. Another hint: if none of the pulleys is free to move, then the factor is 1: force and distance remain the same. There is a formulaic way to know what the multiplier is by inspecting the pulley diagram, but I'm not going to write it out because it is more trouble than it's worth -- just look at the picture given with the problem.

The other simple machines:

Inclined plane: the force multipler (relative to straight lifting) is the sine of the angle of the plane with the horizontal. Distance is divided by the same factor.

Wedge: similar to an inclined plane; the force multiplier is length divided by width; distance is divided by the same factor. Unlikely on the MCAT.

Lever: the force multiplier is ratio of distance from fulcrum to object, to distance from fulcrum to applied force. Look at the picture to make sure you know whether distance goes up or down (hence force down or up). This treatment works even when the force and the object are on the same side of the fulcrum. Note that "fulcrum" just means pivot point.

Wheel and axle: force multiplier is ratio of radii. To figure whether to divide or multiply, look at the picture to see whether distance the object travels is greater or less than the distance over which the force is applied. Unlikely on the MCAT.

Gears: the force multiplier is the ratio of the numbers of teeth on the two wheels, or (equivalently) the ratio of the radii. Again, look at the picture to see which increases, force or distance. Unlikely on the MCAT.

Screw: an inclined plane wrapped around a rod. Unlikely on the MCAT. For completeness: the force multiplier is 2 x pi x radius x (turns/unit length). Don't worry about this one at all.

It is possible to combine two or more of these: for example, a cam is an inclined plane combined with a wheel and axle. But don't worry: such a problem is very unlikely to appear on the MCAT, and if it did it would necessarily be a very simple setup.

Hydraulic jack: not usually considered to be a simple machine like the aforementioned, but effectively the same. Force multiplier is the ratio of the areas of the two pistons. Recall that force changes, not pressure. Assuming that the change in height is negligible, it doesn't matter what liquid is used.​

Any time one of these devices appears, you should look to see whether the object is moving more or less than the applied force. If the object moves more than the input, then the force on it is less than the input force, and vice versa. This is important: often the easiest way to solve the problem given is to look at distance traveled.

This recap is intended for people who started out with at least some idea what these machines are. It is not intended as de novo instruction; for that, you'll need a textbook or your teacher.

If there is another question pending, I will answer it, but I've lost the point in all the clutter. If anyone is still confused, go ahead and post another question.


Shrike
TPR physics, verbal, bio
 

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Shrike said:
EMT has it backwards, of course: pulleys, and all other simple machines, leave work unchanged while altering force and distance. In all cases, the work is multiplied by a particular factor (which can be greater or less than one) and the distance is divided by the same factor.
Is that bolded term supposed to be work, or is it force?
 

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TicAL said:
Is that bolded term supposed to be work, or is it force?
Caught me in the middle of editing. Corrected above.
 

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Shrike said:
Caught me in the middle of editing. Corrected above.
Figured it was an honest mistake, just wanted to clarify ;) Thanks for the post.
 

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SilvrGrey330 said:
MattD, you totally clarified it all. I see it. it makes sense now that the 400 gets split between the two (t3 and t4) and like u said it it were both 400 then it would fly up and thats certainly not the case.

cool, i get it.

Histo, get it too? i appreciate u helping out too man.

Yeah, I get it. Never seen a problem like it in my life, though. Tks for bringing it up.
 

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SilvrGrey330 said:
machines dont change the amount of WORK...but does change the amount of distance one applies.
yes, the other guy must have made a thoughtless typo, or i hope so. you have to mind the first law of thermodynamics. machines and pulleys, and pneumatic and hydraulic pumps make it easier for weak humans like us to move big and heavy things essentially, though we have to move a lot.
 
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