# Violation of conservation of angular momentum

ZapperZ
Staff Emeritus
eosphorus said:
all right ill change my question:

i have a tetherball that can unwind to the infinity

i give a initial angular momentum to the thetherball of 1

seems agreed by everybody here that the linear speed of the tetherball will keep to infinitum

the initial angular momentum of the earth is 0

the theter unwinds clockwise to infinitum at a constant linear speed

the thickness of the pole exerces a torque which causes earth to acquire an angular momentun conterclockwise

being the torque kept ad infinitum the earth aquires infinitum angular momentum counterclockwise

now we have a counterclockwise infinite torque on earth and infinitum angular momentum on the tetherball with respect to the earth on the other sense

of course they together add an angular momentum of 0 but well lets just throw the tetherball to meet the universe and let us keep the infinite torque aquired by earth

i just dont get where this energy has come from

Huh?

Do you realize that you KEEP CHANGING the situation almost on every posts that you have made? First it was just a simple tetherball, then it became a tetherball in space, then there's TWO tetherball in space, and now a tetherball at infinity....

Here's a question for you. When you pull your arms in while spinning on ice, your angular momentum remains constant, but your ROTATIONAL KINETIC ENERGY does not! Where do you think this energy comes from?

Zz.

all right ill change my question
The conservation of angular momentum doesn't care how you change the question. The conservation will always hold.

Danger
Gold Member
Is it premature of me to say "Hi, Aviator?":uhh:

Last edited by a moderator:
ZapperZ
Staff Emeritus
Danger said:
Is it premature of me to say "Hi, Aviator?":uhh:

Nope.. because I felt it early last night already.

:)

Zz.

i change the example because somebody proves it wrong like in the rebounding tetherball where it wouldnt rebound

but what happens in my last example

everybody agrees that the linear speed remains constant however the radius of the tetherball and that a torque is exerted on earth because of the thickness of the pole

i just reach the obvious conclusion that theres a creation of kinetic energy without apporting energy because the earth gets infinite torque from an initial angular momentum in an expanding tetherball

saying that conservation of energy and angular momentum cant be wrong is not an argument, discuss my point

as for the increase of kinetic energy in the case of the ice skater it comes from food

Doc Al
Mentor
It's difficult to follow your example. Please restate it without any wildly unphysical "infinities". Where do you see energy being created? (Note that in a real tetherball case there is gravity pulling the ball down.)

Haelfix
Conservation of angular momentum is a mathematical theorem, its simply unavoidable. Its proved in any number of ways in most elementary books on mechanics. Try as you like to find a counter example, and they can get arbitrarily complicated, you won't succeed without making a mistake (and sometimes the mistake can be made arbitrarily subtle).

I have read this thread with great interest, as well as the link to another thread given at the beginning. I have a question:

If the tether ball is released from the string just before it's spiral motion impacts the pole, would it travel farther than is accounted for from the initial impact force of hitting the ball it the first place?
Hope my question makes any sense.

If I am understanding all of this correctly, there is a net gain in energy, but that the energy source is the torque resistance of planet earth within that frame of reference.

"If the tether ball is released from the string just before it's spiral motion impacts the pole, would it travel farther than is accounted for from the initial impact force of hitting the ball it the first place?""

the trick of the tetherball is that the linear speed remains constant however the radius so when it hits or is relesed from the pole the energy is the same either kinetic when released or transformed in heat and noise when hitting

take a tehterball 1kg mass 1 m initial radius 1 million kilometers final radius

take an earth 1000 tons mass so gravity gets really weak

create amomentum on the tetherball of 1 and the earth has o momentum of 0

lets suppose a thickness of the pole of 1 m and the thickness of the cable of 0

the tetherball starts to unwind, its been agreed that the tetherball keeps a constant linear velocity however the radius and that a torque is exerced on the pole and transferred to earth because of its thickness

the linear speed of the ball is the same and theres a constant torque on the earth

the ball gains angular momentum from the earth because the radius increases from 1 m to 1 million km and the linear speed remains at 1 m/s

the earth gains torque from 0 to a big number because the radius of the thickness of the pole and the tension of the string have created this torque

both the torque of the earth and the angular momentum of the earth will add a total momentum of one the initial one

ZapperZ
Staff Emeritus
You still have not shown ANY understanding of the fact that the tetherball in your example undergoes coupling with an external source.

If you still think you have found a violation of angular momentum, you are welcome to submit to the IR section. This thread has gone nowhere but round and round and round like the tetherball. It is now done.

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