[ARCHIVED THREAD] - Physics Question: (Page 1 of 2)
Posted: 10/17/2007 6:06:11 PM EDT
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I have a physics question. Lets say you have a system that is frictionless. There is a table with a block on it that weighs 433 Newtons, its connected by a taught rope via a pulley, to a free hanging block that weighs 153 Newtons. The free hanging block is not in contact with the table. What is the tension on the rope and acceleration of the blocks? I came up with 9.8 m/s for the block hanging off the table and 3.5 m/s for the block hanging on the table. The 3.5 m/s is calculated from the Force of 153N and the mass of the 433N block. With a tension of 153 Newtons on the rope. The book says this is wrong. What am I doing wrong? |
Lemme get this straight, the blocks are attached to each other by a rope and you came up with different accelerations for the two? Try again. edited to add: OK, I'll be helpful. Total force on system = 153 Newtons (the block that's hanging) total mass of system (ignoring mass of rope) = (433 Newtons + 153 Newtons)/9.8 Newtons/kg = 59.796 kg F=MA Therefore A=F/M = 153 Newtons/59.796kg = 2.559 m/s^2 Tension on the rope = the force on the second block. You know its mass and its acceleration, so solve for the force. Another edit: here's the tension T=MA=((433 Newtons)/(9.8 Newtons/kg))*2.559m/s^2=113.1 Newtons |
Fixed! 
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How else do you get the same Tension on each side of the pulley with different masses? F=ma. mass and force are constant in this situation, right? |
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Both blocks are a single system. You need the mass of the system (which is (433+153)/9.81). You already have the force applied to the system (153N, the weight of the hanging block). Then just do f = ma for the acceleration of the system. The tension on the rope is equal to the weight of the hanging block. Edited to add: The acceleration of the system is 2.5587 m/s^2. And no, it will NOT take off 
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Does it have something to do with the gravitational force and normal force acting on the block on the table? Both y components. That is in a different direction from the force from the falling block. Which is an x component from the pulley. |
Weight is force. Weight gets confused with mass frequently, but it's actually the force of gravity acting on mass. That's why mass is constant regardless of gravity, it's an intrinsic property of matter, whereas weight obviously isn't. |
I edited my post to help you out. Think about it, if one mass is accelerating at a different rate than the other, the rope is either going to be slack or stretch (and typically, we assume its inelastic). |
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Kylaer_ told you the solution above. The force in the rope can be only one value, and the FBD will show that right off. Stop for little bit and clear your head, then come back to this in a few minutes. |
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The force on the rope is 153N. Acceleration of the hanging block is 9.8m/s^2. There is no horizontal force to offset gravity (assuming the pulley is on the same horizontal as the block on the table). Acceleration of the other block is going to be the same.... unless the rope is stretching. Try tying yourself to a car, have your buddy drive the car off a building, and see if you accelerate at a different rate....  Disclaimer: The poster of this response assumes no responsibility WHATSOEVER if anyone is actually stupid enough to tie oneself to a vehicle and have said vehicle set in motion. |
Correct except the tension on the rope is not equal to the weight of the hanging block. (if it were, the block would not fall because the force on the rope would counterbalance gravitational force). |
F= m*a Should be EASY. The force of 153 N is accelerating a mass of ~44 kg plus 15.6 kg in the hanging block. The rest is algebra. You can use integral calculus or you can use reiteration. Either way, I am not doing your homework so no solution provided. Where you are missing is the force of the 153 N is from a mass of ~15.6 kg which is also accelerating at a rate somewhat less than 9.81 m/s^2. |
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yeah, screw having to actually think about it too much. heh, i'm being too rough in this thread. gj understanding your FBD error sparky. you can get all sorts of answers just plugging an chugging. and one can draw all sorts of FBD's that show the tension in the string will hold the hanging block in mid air  . It takes a smart mind to look at the FBD's and equations and say "does that really make sense?"
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I myself am skeptical of anyone who visits a professor during office hours, or uses a study group to enhance learning. I figure anyone who doesn't immediately get it on their own is a dumbass. |
Well what helped the most was AeroE's advice. You can convince yourself that your wrong awnser is correct if you think about it too much. I had been thinking about this thing so damn much. I needed to put it down for a while. |
[snip] Edited to add: it's clear that if the tension on the rope equaled the weight of the hanging mass, the mass would not fall. If the tension was zero, it would fall at 9.8 m/s^2. We know the actual acceleration is between those two values, so the tension is somewhere between zero and the weight of the block. (I've outlined how to solve for it in my first post). |
Yeah and people who use books too. It should come to you, like it did for Newton, Faraday, Ohm, Einstein. I work 40 hrs a week at a research lab as an electronics tech and am married. I take two classes a semester and am pinched for time. I get help where I can, modesty in the quest for education is for pussies. I am about learning the concepts by any means nessecary. |
Not a dumb-ass. but I have spent hours until 4:00am figuring out homework by myself 20 years ago or with six or so study buddies. Posting the problem here or many other sites gets you 10,000 or more people weighing in with less mental effort on your part. |
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Weird that you asked this question. I just tutored a girl in this exact problem earlier today (along with other F=ma concepts). Anyways, I'm guessing you're in a 100-level physics class, I just wanted to offer help explaining anything else you run into along the way (you can IM me or email). I am no physics genius (2nd year mechanical engineering major) but I tutor 3 different students a week in Physics 111 here at Ohio State. |
Yeah I talked to my fellow students in my Calc III class about this for about 20 mins after class. I got back home home at 9:15pm. I finally ate dinner and have been working on homework. I have to get up at 6am and do it again tomorrow. Don't make assumptions about what other resources I have attempted to use and don't shit up my threads on conceptual help. If you don't want to help or have nothing constructive to say don't post in these sort of threads. I never asked for a numerical solution, just what I was doing wrong. I attempted to put up my ASCII art FBD but it looked like shit. |
ASCII art is cool, if done right. |
Then you really are learning.  I was being sarcastic, BTW. And I feel your pain completely. No marriage to juggle, but I do pull down full time work and everything else, along with school. I don't care how anyone learns, so long as they learn honestly, and understand. The internet can be a great tool for that. I know I've popped into webCT more than once and gotten help from someone outside my own study group because they were up at 3am when I was, and I wasn't going to wake someone in my group. |
Nothing wrong with asking for help. I did most of my homework in complex variables by going to the engineering library and taking every book on the subject off the shelves and looking at the examples. Virtually every homework problem was similar to an example in one of the books. (My teacher sucked). Ultimately, though, you'll need to learn to do the problems on your own because we can't help you on your tests. |
+2 You guys don't know physics. |
You're wrong. See my first post, I solved for the tension on the rope and it isn't the weight of the hanging block. Did you think the airplane wouldn't fly?
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The tension in the rope would only be equal to the weight of the hanging block if the system were in static equilibrium. Since the block is accelerating downward, the tension in the rope must be less than the force of gravity on the hanging block. EDIT: In other words, the airplane flies, like the other people in the thread said. 
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Ah...come ON! Its a very simple Newtonian Mechanics Problem! Study yer BOOK! Give us a REAL problem! |
Oh you're right. I even had grad courses in dynamics. Duhh! PS- That airplane riddle is dumb. It's worded poorly. Air moving over the wings makes the airplane fly. The venturi effect (they say). A stationary aircraft with zero airspeed over the wings and the wheels spinning on a conveyor belt won't fly. Unless you think the plane flies by spinning the wheels. If a headwind was blowing at flying speed, the airplane would fly as normal,except stationary relative to the ground, whether the wheels were spinning or not. The conveyor belt part of the problem is not needed. Unless it serves some purpose other than spinning the wheels. |
