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 05-14-07, 04:58 PM #1 EJ123 djentleman Thread Starter     Join Date: Aug 2005 Bikes: Posts: 3,370 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 5 Post(s) What equations do I need-homework help It's actually a study review he gave us for finals next week. 1. Tarzan swings on a 30.m vine initially inclinded at a 37º angle with the vertical. What is his speed at the bottom of the swing if he: a. starts from rest b. pushes off with a speed of 4m/s (I dont have his mass, and I have no idea what to do with the 37º) 2. A window washer is on one of those scaffolds with a rope on each end. The scaffold=205N and is 3m long. What is the force each rope exerts when a 675N worker stands 1m from one end? Thanks for any help. (remember I just need equations...book seems to not want to say)
 05-14-07, 05:08 PM #2 VegaVixen Senior Member   Join Date: Nov 2006 Location: Charlotte, North Carolina, USA Bikes: '71 Raleigh Sports, '84 Schwinn LeTour on the trainer (and available for hill repeats), '06 Scott CR1 SL (Ksyrium SL), and a yet-to-be-determined TT bike. Posts: 1,643 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) No. You need understanding of the problems. Then you'll understand how to formulate the solutions mathematically. Start with problem one: Can you first define in words the concepts of kinetic energy and potential energy?
 05-14-07, 06:18 PM #3 pedex  Senior Member   Join Date: Mar 2004 Location: under bridge in cardboard box Bikes: Posts: 5,358 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 8 Post(s) with #1 mass wont matter, the acceleration is constant from gravity mitigated by the arc of the pendulum formed, thats more than enough of a hint the second one doesnt deserve any hints, just draw the picture of it and fill in what you know and what your looking for......... __________________ Decentralize or die.
 05-14-07, 06:22 PM #4 bikingshearer  Crawlin' up, flyin' down     Join Date: Jan 2006 Location: Democratic Peoples' Republic of Berkeley Bikes: 1967 Paramount, 1982-ish Ron Cooper,1978 Eisentraut "A," mid-1960s Cinelli Speciale Corsa, 1961 Bianchi Competizione (an Eroica bike), 1994 Trek 520, 199? Burley Bossa Nova, early-1970s Cinelli Speciale Corsa (also an Eroica bike) Posts: 3,486 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 79 Post(s) I don't know about the math equations, but I know the lawyer equations. In both cases, fee for representing plaintiff = 33% of recovery up to mediation, 40% thereafter. Fee for representing defendant = \$300+/hr. __________________ "I'm in shape -- round is a shape." Andy Rooney
 05-14-07, 06:29 PM #5 EJ123 djentleman Thread Starter     Join Date: Aug 2005 Bikes: Posts: 3,370 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 5 Post(s) Ohh the pendulum jumbo, that's what I was needing to think of. Thanks pedex.
05-14-07, 08:02 PM   #6
dcon
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 Originally Posted by bikingshearer I don't know about the math equations, but I know the lawyer equations. In both cases, fee for representing plaintiff = 33% of recovery up to mediation, 40% thereafter. Fee for representing defendant = \$300+/hr.

 05-14-07, 08:16 PM #7 jyossarian SERENITY NOW!!!     Join Date: Aug 2005 Location: In the 212 Bikes: Haro Vector, IRO Rob Roy, Bianchi Veloce Posts: 8,756 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) In my day, we didn't have equations. We didn't even have math. We just swung from trees and didn't worry about how fast we were going. __________________ HHCMF - Take pride in your ability to amaze lesser mortals! - MikeR We demand rigidly defined areas of doubt and uncertainty!
 05-14-07, 08:19 PM #8 junkyard Fourth Degree Legend     Join Date: Jun 2005 Location: American Gardens Building Bikes: 2005 Kona Cinder Cone & 2010 Cannondale SuperSix Posts: 3,826 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) tarzan is not real, therefore the question is irrelevant
 05-14-07, 10:22 PM #9 polara426sh Displaced Southerner     Join Date: Apr 2006 Location: Webster, NY Bikes: 1967 Peugeot UO-8, 1984 Torpado Beta, 1985 Trek 300, 1989 Dave Moulton Fuso FRX, '90s Hardrock x2, '90s Norco Pinnacle, Focus MB-350 IRO SSFGGB Posts: 254 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s)
05-15-07, 01:09 AM   #10
jschen
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 Originally Posted by EJ123 Ohh the pendulum jumbo, that's what I was needing to think of.
Oh dear... where's our new physics expert, PC2? He should be able to clarify things for you. In the meantime...

Where to start? Umm... why don't you draw a force diagram and scan it in for us, with every relevant piece of info clearly labeled? Seriously, this needs to ALWAYS be step one. And VegaVixen is right. You're not lacking equations. You're lacking understanding.

If you want one equation, here it is: F = ma
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 05-15-07, 07:06 AM #11 Ken B. Grand Vizier   Join Date: Feb 2006 Bikes: Posts: 21 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Here's what you need for #1: (potential energy from the starting point to the bottom of the arc) PLUS (added kinetic energy when he pushes off (assuming he pushes tangential to the arc (and downward))) EQUALs (kinetic energy at the bottom of the arc (assuming negligible loss of energy due to friction and wind resistance)) Here's what you need for #2: All translational forces cancel out, and all rotational forces cancel out (assuming the scaffold is not accelerating either translationally or rotationally).
05-15-07, 07:19 AM   #12
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0, there's an elephant in the way
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05-15-07, 09:11 AM   #13
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Just use Lagrange
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05-15-07, 09:22 AM   #14
gbcb
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 Originally Posted by polara426sh

Awesome.

 05-15-07, 09:32 AM #15 CdCf Videre non videri   Join Date: Sep 2004 Location: Gothenburg, Sweden Bikes: 1 road bike (simple, light), 1 TT bike (could be more aero, could be lighter), 1 all-weather commuter and winter bike, 1 Monark 828E ergometer indoor bike Posts: 3,208 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) #1 Get some squared paper and make a scale drawing of Tarzan and the vine, with the angle as accurate as you can make it (and in getting it as accurate as possible, you'll hopefully realise what to do with the 37° angle, and why it is important). Then think about what Ken B. wrote in his post earlier.
 05-15-07, 10:51 AM #16 phantomcow2 la vache fantôme     Join Date: Aug 2004 Location: NH Bikes: Posts: 6,266 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) Have you taken trig yet? Remember that lovely energy balance? PE+KE = PE' + KE' Where PE is potential energy (m*g*height) and KE is Kinetic Energy (1/2mv^2), and ' means after the event has occurred. Use your trig knowledge to determine the height of the guy. You've got a side, and an angle. As with all of these types of projectile motion problems, you don't need mass. Not including mass always throws off a portion of students, always. __________________ C://dos C://dos.run run.dos.run
 05-15-07, 10:53 AM #17 phantomcow2 la vache fantôme     Join Date: Aug 2004 Location: NH Bikes: Posts: 6,266 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) Gravity is -9.81m/s^2 But you can round to 10 __________________ C://dos C://dos.run run.dos.run
05-15-07, 10:56 AM   #18
Voidbringer
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 Originally Posted by phantomcow2 Gravity is -9.81m/s^2 But you can round to 10
Double posting liar!
Use the 9.81, accuracy is your friend.
I'll give you a tip, Phantom measures with his thumbs.

 05-15-07, 01:03 PM #19 bikingshearer  Crawlin' up, flyin' down     Join Date: Jan 2006 Location: Democratic Peoples' Republic of Berkeley Bikes: 1967 Paramount, 1982-ish Ron Cooper,1978 Eisentraut "A," mid-1960s Cinelli Speciale Corsa, 1961 Bianchi Competizione (an Eroica bike), 1994 Trek 520, 199? Burley Bossa Nova, early-1970s Cinelli Speciale Corsa (also an Eroica bike) Posts: 3,486 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 79 Post(s) 9.8 m/s^2 - it's not just a good idea, it's the law! __________________ "I'm in shape -- round is a shape." Andy Rooney
 05-15-07, 01:46 PM #20 jschen riding once again     Join Date: Oct 2005 Location: San Diego, CA Bikes: '06 Cervelo R3, '05 Specialized Allez Posts: 7,359 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Carry symbolic values as long as possible. Acceleration due to gravity is +g or -g, depending on how you orient your axis. Don't worry about the value of g until you have solved the problem symbolically. __________________ If you notice this notice then you will notice that this notice is not worth noticing.
 05-15-07, 02:06 PM #21 SirScott Senior Member     Join Date: Jul 2005 Location: Littleton, CO Bikes: Posts: 206 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) e =mc^2 solves everything.
 05-15-07, 03:07 PM #22 EJ123 djentleman Thread Starter     Join Date: Aug 2005 Bikes: Posts: 3,370 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 5 Post(s) Got there this morning and did the conservation equation, and all is good now.
 05-15-07, 04:14 PM #23 phantomcow2 la vache fantôme     Join Date: Aug 2004 Location: NH Bikes: Posts: 6,266 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) So, whats the answer? __________________ C://dos C://dos.run run.dos.run
05-15-07, 04:24 PM   #24
jschen
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 Originally Posted by EJ123 did the conservation equation
Either my physics is no good or they sure don't teach it the way they used to. I have no clue what you just said.
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05-15-07, 04:33 PM   #25
phantomcow2
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 Originally Posted by jschen Either my physics is no good or they sure don't teach it the way they used to. I have no clue what you just said.
I think he is talking about conservation of energy. We call it the energy balance in my class. It's
PE+KE = PE' + KE'
Where PE' and KE' is the PE and KE after the event has occured. And of course, he is just solving for v'
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