Ok, I give in....ATOT please help me help a friend

[PurdueRy]

Ok, I have been mulling over this problem for about a half hour now and I don't want to spend all night on hw that isn't mine. However, I would really like to help a friend out.


We have a pulley that is a solid cylinder and a bucket attached via a "weightless" string. We know the radius r of the cylinder is .27m and the mass of the bucket is 1.47kg. The bucket starts falling 4.85m above the ground and falls for 2.53 seconds.

They want to know a ton of different information in the end...like linear acc. of the bucket, angular acc., Mass of the cylinder...

I am confident I could solve for all one I get one....however everything equation I come up with ends up with two unknowns.

Obviously...this is a conservation of momentum problem so I tried setting up those equations but they were a dead end to...anyone want to offer a little insight...I am sure its something easy I am overlooking.

 

[jhayx7]

Just remember:

.999999 = 1

and

42

 

[AgaBoogaBoo]

how dare you call yourself a purdue student, you fail

 

[PurdueRy]

Originally posted by: AgaBoogaBoo
how dare you call yourself a purdue student, you fail

Damn Freshman

 

[hypn0tik]

Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

 

[PurdueRy]

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

 

[hypn0tik]

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

 

[TheoPetro]

Originally posted by: hypn0tik

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

Listen to this man, he speaks the truth.

 

[PurdueRy]

Originally posted by: TheoPetro

Originally posted by: hypn0tik

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

Listen to this man, he speaks the truth.

That doesn't work due to the pulley...(tried it btw)

 

[TheoPetro]

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: hypn0tik

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

Listen to this man, he speaks the truth.

That doesn't work due to the pulley...(tried it btw)

the angular acc of the pully will eb the the same acc as the bucket falling. Do you need to use the inirtia sh!t with a thin cylinder?

 

[PurdueRy]

Originally posted by: TheoPetro

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: hypn0tik

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

Listen to this man, he speaks the truth.

That doesn't work due to the pulley...(tried it btw)

the angular acc of the pully will eb the the same acc as the bucket falling. Do you need to use the inirtia sh!t with a thin cylinder?

ya and I tried that conversion(I realized it after I posted that) and it didn't work either. I am seriously pulling my hair out here...not being able to solve this pisses me off. And ya, he has to use intertia

 

[TheoPetro]

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: hypn0tik

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

Listen to this man, he speaks the truth.

That doesn't work due to the pulley...(tried it btw)

the angular acc of the pully will eb the the same acc as the bucket falling. Do you need to use the inirtia sh!t with a thin cylinder?

ya and I tried that conversion(I realized it after I posted that) and it didn't work either. I am seriously pulling my hair out here...not being able to solve this pisses me off. And ya, he has to use intertia

well I know who NOT to PM about physics help

If my phsycis book was here I could findout in about 2 secs but its lost somewhere in my room at home. Sorry.

 

[PurdueRy]

Originally posted by: TheoPetro

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: hypn0tik

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

Listen to this man, he speaks the truth.

That doesn't work due to the pulley...(tried it btw)

the angular acc of the pully will eb the the same acc as the bucket falling. Do you need to use the inirtia sh!t with a thin cylinder?

ya and I tried that conversion(I realized it after I posted that) and it didn't work either. I am seriously pulling my hair out here...not being able to solve this pisses me off. And ya, he has to use intertia

well I know who NOT to PM about physics help

If my phsycis book was here I could findout in about 2 secs but its lost somewhere in my room at home. Sorry.

Ouch dude. I actually just solved it...turns out my only problem was that my book gave the wrong equation for the momentum of a solid cylinder(It gave 1/2MR^2 when I needed to use 1/3MR^2)

 

[TheoPetro]

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: PurdueRy

Originally posted by: TheoPetro

Originally posted by: hypn0tik

Originally posted by: PurdueRy

Originally posted by: hypn0tik
Maybe try

Conservation of Energy + Conservation of Momentum

mgh = 1/2mv^2 + 1/2 Iw^2
mv = Iw

I = Moment of intertia of cylinder
w = Angular velocity

Two equations and two unknowns (Mass of Cylinder and Angular velocity of cylinder)

unless my eyes deceive me there are three unknowns(velocity of the bucket)

Vi = 0
a = 9.8 (or -9.8 depending on your reference)
t = 2.53

Vf = Vi + at

Listen to this man, he speaks the truth.

That doesn't work due to the pulley...(tried it btw)

the angular acc of the pully will eb the the same acc as the bucket falling. Do you need to use the inirtia sh!t with a thin cylinder?

ya and I tried that conversion(I realized it after I posted that) and it didn't work either. I am seriously pulling my hair out here...not being able to solve this pisses me off. And ya, he has to use intertia

well I know who NOT to PM about physics help

If my phsycis book was here I could findout in about 2 secs but its lost somewhere in my room at home. Sorry.

Ouch dude. I actually just solved it...turns out my only problem was that my book gave the wrong equation for the momentum of a solid cylinder(It gave 1/2MR^2 when I needed to use 1/3MR^2)

psssh noob. I had that one tatooed on the inside of my eyelid!

 

[jagec]

Originally posted by: PurdueRy
Ok, I have been mulling over this problem for about a half hour now and I don't want to spend all night on hw that isn't mine. However, I would really like to help a friend out.


We have a pulley that is a solid cylinder and a bucket attached via a "weightless" string. We know the radius r of the cylinder is .27m and the mass of the bucket is 1.47kg. The bucket starts falling 4.85m above the ground and falls for 2.53 seconds.

They want to know a ton of different information in the end...like linear acc. of the bucket, angular acc., Mass of the cylinder...

I am confident I could solve for all one I get one....however everything equation I come up with ends up with two unknowns.

Obviously...this is a conservation of momentum problem so I tried setting up those equations but they were a dead end to...anyone want to offer a little insight...I am sure its something easy I am overlooking.

You need to clarify the problem, since as stated it could be interpreted any number of ways. I assume you mean the middle example.

Also, is it assumed that the bucket hits the ground at 2.53 seconds?

If so, just calculate the total energy of the system (ie, the change in potential energy). That will be equal to the angular momentum of the cylinder at the end. From there, you should be able to work backwards.