What is the proper force in relativity?

[merrypark3]

Homework Statement

Please check the pics.

Homework Equations

The Attempt at a Solution

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Homework Statement

#8

Homework Equations

My answer is inverse of tanh

What's wrong with me?

The Attempt at a Solution

 

[tiny-tim]

hi merrypark3!

Please check the pics.

??

wot pics? ​