# Foo - What equations do I need-homework help

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EJ123
05-14-07, 04:58 PM
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)

VegaVixen
05-14-07, 05:08 PM
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?

pedex
05-14-07, 06:18 PM
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.........

bikingshearer
05-14-07, 06:22 PM
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.

EJ123
05-14-07, 06:29 PM
Ohh the pendulum jumbo, that's what I was needing to think of. Thanks pedex.

dcon
05-14-07, 08:02 PM
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.

:lol:

jyossarian
05-14-07, 08:16 PM
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.

junkyard
05-14-07, 08:19 PM
tarzan is not real, therefore the question is irrelevant

polara426sh
05-14-07, 10:22 PM
http://img.photobucket.com/albums/v650/polara426sh/u-do-mah-homework.jpg

jschen
05-15-07, 01:09 AM
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

Ken B.
05-15-07, 07:06 AM
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).

Falkon
05-15-07, 07:19 AM
0, there's an elephant in the way

Nicodemus
05-15-07, 09:11 AM
Just use Lagrange

gbcb
05-15-07, 09:22 AM
http://img.photobucket.com/albums/v650/polara426sh/u-do-mah-homework.jpg

:roflmao:

Awesome.

CdCf
05-15-07, 09:32 AM
#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.

phantomcow2
05-15-07, 10:51 AM
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.

phantomcow2
05-15-07, 10:53 AM
Gravity is -9.81m/s^2
But you can round to 10 :D

Voidbringer
05-15-07, 10:56 AM
Gravity is -9.81m/s^2
But you can round to 10 :D
Double posting liar! :P
Use the 9.81, accuracy is your friend.
I'll give you a tip, Phantom measures with his thumbs. :D

bikingshearer
05-15-07, 01:03 PM
9.8 m/s^2 - it's not just a good idea, it's the law!

jschen
05-15-07, 01:46 PM
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.

SirScott
05-15-07, 02:06 PM
e =mc^2 solves everything.

EJ123
05-15-07, 03:07 PM
Got there this morning and did the conservation equation, and all is good now.

phantomcow2
05-15-07, 04:14 PM

jschen
05-15-07, 04:24 PM
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.

phantomcow2
05-15-07, 04:33 PM
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'

jschen
05-15-07, 04:41 PM
PC2, thanks for the clarification. EJ123, please note how PC2's explanation ultimately appeals to a concept (energy is conserved) rather than to an equation. As much as possible, appeal to concepts (potentially expressed mathematically). If you learn to apply concepts broadly, you won't have to go around memorizing very many equations. For example, in this case, the equation is quite obvious and not requiring memorization if the concept is understood.