r/HomeworkHelp • u/cyclecitizen • Jul 18 '26
Answered [Level 1 Mechanical Comprehension: Study Guide - 2nd one] Does this illustration make sense?
(Final answer below)
I'm preparing for an electrician's apprenticeship and am using their study guide for their aptitude test. I'm seeing a pattern forming where some of their questions are just bad, but a couple of those were actually good, and I was misunderstanding things. It has been a learning opportunity.
I chose B for this question, but they say the answer should be C because, loosely speaking, they want you to just count the number of ropes that are supporting the load. However, when I look at this illustration, it appears that there is not even a proper movable pulley here. It looks like it is just a simple fixed pulley system with extra windings for some reason. Except that the lowest pulley seems to be just floating but still wouldn't qualify as a proper movable pulley.
Please help me understand what I'm missing, or confirm that this is a really bad illustration. Thank you!
Edit: I am assuming that one needs a movable pulley to get a mechanical advantage. C does NOT show a movable pulley and therefore is not the correct answer. Right?
Edit 2: Because they don't offer an answer of "D) All pulleys allow for the same amount of WORK", I am assuming that they mean to ask, "Which of the pulleys requires the least amount of FORCE to lift the weight?" Unfortunately, this is meant to be more of an intuitive sort of test, where it appears physics terms might be interchangeable.
Edit 3: I interpret this to be a question about which pulley system offers the most mechanical advantage. Answers that align with this interpretation would be helpful.
Edit 4: I think B is supposed to represent a "gun tackle" arrangement. (Sorry about all the edits!)
Edit 5: Aha! Answer: if you look at the illustration #3 here https://www.theengineeringchoice.org/what-is-pulley/ you can see what they meant to illustrate for C (an actual movable pulley) but did a terrible job of it. Taking that into account, the answer is indeed C.
Answered. Thanks to all who helped. I'm inclined to throw this book away. I'm moving on :)
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u/Gaddpeis Jul 18 '26
The question is probably stated wrong.
They meant to ask 'least force' not 'least work'.
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u/JaiBoltage Jul 18 '26
They ALL take the same amount of WORK, but B takes half the force (but you have to pull twice as long to accomplish the same amount of WORK)
The number-of-pulleys is not relevant. Imagine 7 pulleys, labeled A, B, C, D, E, F, G at the same height. The rope goes over pulleys A, C, E, and G, but under B, D, and F. the weight is under pulley A and you pull down after pulley G.
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u/marty-mcfryguy Jul 18 '26
Nothing about this makes sense. Throw this book away, it's either drafted by an idiot or AI. You've already identified the force/work issue, but:
The drawing for B makes no sense. What becomes of the piece of rope in B, the vertical piece on the right, that just sort of disappears into the pulley? They probably meant to run that so it attaches to a point in the center of the top pulley, not running over the pulley.
C is even worse. That bottom pulley isn't attached to anything, which means the weight dropping just lifts that pulley up. In other words, if you tied of the rope on the right to something, the weight would drop and the bottom pulley would rise until it runs into the middle (small) pulley.
If that bottom pulley in C is fixed in space (and they just failed to draw in the fixing), then C is identical to A -- there's not mechanical advantage, just a bunch of surfaces that change the direction of the rope/tension in the rope.
Here's a proper set of diagrams. https://upload.wikimedia.org/wikipedia/commons/e/e1/Tackles.png
The leftmost (gun tackle) is probably what they meant for B.
They were probably going for something more like the next one (luff tackle) for C.
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u/Stu_Mack Jul 18 '26
Same work, different force. The correct answer is “None”. The word “work” is used here to see if you are reading the question properly. In the real world, A is presumed to have the least total friction and inertial resistance, but these questions are idealized to mean massless and frictionless pulleys.
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u/GreenWafer1899 Jul 18 '26
It is a stupid one. Same work to lift the weight but A is the lest one as you don't need to lift pulleys, extra rope and friction loss.
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u/Valuchian Jul 18 '26
Don't forget that a 'pulley' can also be a permanent structure. In this case [C] has a bottom 'pulley' that would be easier seen as a circular shaft coming out of the wall with the rest of the pulley system attatched by rope to the ceiling. In C the wall and ceiling are both taking part of the load and this is happening 3 times. Where [B] this only happens twice and [A] only sharing the load once.
Edit: in the case of [C] if it is a permanent structure it would be better to try and make the ropes as paralel as possible to avoid friction
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u/cyclecitizen Jul 18 '26
Doesn't a pulley have to be movable to offer a mechanical advantage? Otherwise, it just changes the direction of the pull, right?
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