Kinetic friction of gold on gold

[mariacurlej]

A bit of a strange request:

My 13 y/o son is presenting his research project on the difference in spin time of a yoyo with a stainless steel 10-ball bearing versus a gold-plated 10-ball bearing. I have found the coefficient of friction for the static gold on gold (0.49), but could not find the kinetic COF.

Any guidance to where we can find this?

Many thanks,
Maria

[PinkertonD]

Hi Maria, and welcome to this forum.

Question:
Could this be a trick question set by the teacher?

Gold is a very soft metal and totally inappropriate for a bearing or coating of a ball for a bearing. If push came to shove, it may be usable as a sleeve bearing, but even then it galls easily and may not perform well.

Finally, when it asks "gold-plated ball bearing," do they mean the outer and inner rings are plated or the balls and rings are gold plated? If the former then there will be no difference, if the latter then my statement above still applies.

I know you are a Mother, and Mothers love to help, but I would hesitate in helping too much with this project. Homework assignments are set to discover how much the student has learned during a teaching period. With your assistance in this you are NOT helping your son in any way. He may get a passing grade for the test, but will not have understood the engineering behind the question or how to find the answers for himself.

Homework assignments are NEVER set on a topic that has not been covered in some sense applicable to the question.

[Kelly_Bramble]

Hi Maria, and welcome to this forum.

Question:
Could this be a trick question set by the teacher?

Gold is a very soft metal and totally inappropriate for a bearing or coating of a ball for a bearing. If push came to shove, it may be usable as a sleeve bearing, but even then it galls easily and may not perform well.

Finally, when it asks "gold-plated ball bearing," do they mean the outer and inner rings are plated or the balls and rings are gold plated? If the former then there will be no difference, if the latter then my statement above still applies.

I know you are a Mother, and Mothers love to help, but I would hesitate in helping too much with this project. Homework assignments are set to discover how much the student has learned during a teaching period. With your assistance in this you are NOT helping your son in any way. He may get a passing grade for the test, but will not have understood the engineering behind the question or how to find the answers for himself.

Homework assignments are NEVER set on a topic that has not been covered in some sense applicable to the question.

x2 .... Don't see the value less electrical contacts, maybe?

[jboggs]

Also - in the case of ball bearing, the elements have rolling contact, not sliding contact. Regular coefficients of friction don't apply. You would be after the rolling coefficient of friction, sometimes called "rolling resistance".

[dshandling]

Pretty puzzling for a 13 year old I'd say!