Who want's to help me with a physics problem?

Quackzilla

Quackzilla

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I'm in a haze tonight, I can't think straight at all.

Here is what I have so far, what did I do wrong?

energy is conserved, no slipping or air resistance or anything.

There is a pulley, on one side is a mass resting on the ground, and on the other side is a larger mass above the ground. If the larger mass is released, what is it's speed when it hits.

okay, here is what I have:

PgM = Pgm + KM + KR + Km
Mgh = mgh + v[ srqt(.5M) + sqrt(.5I)/r + sqrt(.5m) ]

I=
.5mr^2 = .5(7.50kg)(.260m)^2 = 0.2535kg*m^2
h=3
m=18
M=26.5
so
PgM = 779.1 J
Pgm = 529.2 J

My answer is just onder 50m/s, completely illogical, and the book answer is 3.22m/s. What did I do wrong? All the numbers look right, and it appears to make sense, but the answer is way off...
 
[quote name='Quackzilla']I'm in a haze tonight, I can't think straight at all.

Here is what I have so far, what did I do wrong?

energy is conserved, no slipping or air resistance or anything.

There is a pulley, on one side is a mass resting on the ground, and on the other side is a larger mass above the ground. If the larger mass is released, what is it's speed when it hits.

okay, here is what I have:

PgM = Pgm + KM + KR + Km
Mgh = mgh + v[ srqt(.5M) + sqrt(.5I)/r + sqrt(.5m) ]

I=
.5mr^2 = .5(7.50kg)(.260m)^2 = 0.2535kg*m^2
h=3
m=18
M=26.5
so
PgM = 779.1 J
Pgm = 529.2 J

My answer is just onder 50m/s, completely illogical, and the book answer is 3.22m/s. What did I do wrong? All the numbers look right, and it appears to make sense, but the answer is way off...[/quote]

When do you need this by?
 
7.5kg is the mass of the pulley, and it's due tomorrow.

I did all the rest of the problems, then realized tonight I have to do this one also.

This is really embarassing because I am one of those bastards who knows everything, and lets everyone know it, and I got hung up on this simple ALGEBRAIC physics problem. :whistle2:x
 
[quote name='Quackzilla']7.5kg is the mass of the pulley, and it's due tomorrow.

I did all the rest of the problems, then realized tonight I have to do this one also.

This is really embarassing because I am one of those bastards who knows everything, and lets everyone know it, and I got hung up on this simple ALGEBRAIC physics problem. :whistle2:x[/quote]

I'd help you out as I love anything math, computer, or physics oriented, but I've got my own things happening tonight. Earliest I could get it thoroughly done/worked on is tomorrow by around 11:00 A.M. PST
 
ah, my class is at 11, but thanks anyway!

[quote name='dopa345']I keep getting around 6.3 m/s. It's definitely 3.22 m/s?[/QUOTE]
yeah, it's def 3.22 m/s, I checked the answer key.
 
It would have helped if you more thoroughly wrote out the problem itself and stated your units better...

Ok, the way I see it, you have a conservation of energy problem. The total potential and kinetic energies of the beginning case (large mass hanging, small mass on ground) and the ending case (large mass on ground, small mass flying upwards), so energy should be conserved. However, you mentioned that the pulley has mass, but is also frictionless... so I think that some energy should be lost due to the mass of the pulley. I'm not completely sure about that, and I coudn't find any quick formulas for it from my old physics textbook, but I think that is what you have to account for.

Anyway, initailly, you have only the gravitational potential energy of the large mass (call this PM), since the large mass starts at rest.

At the end, you have the grav. pot. energy of the small mass (say Pm) and the kinetic energy of the small mass (say Km). And this total should be the same as the inital PGM minus whatever is lost due to the pulley.

SO...

PM - (loss from pulley) = Pm + Km, where PM = M*g*h
Pm = m*g*h

And since Km = .5 * m * v^2, you should be able to solve this for v.


Or maybe it has something to do with the tension in the string in the pulley set up...

Ah, hell, I tried. I'm going to sleep now. BTW, assuming there is no energy lost to friction and treating the pully as massless/frictionless, with M = 26.5 kg, m = 18 kg, and h = 3 m, I came up with 5.27 m/s for the upward velocity of the smaller mass at the end.
 
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