Question

Foucault's pendulum - why the last equation (Newton's second law) is equal to zero?

Answer 1

\[r = <x,y,z>\]
\[ak = <0,0,a>\]
\[r-ak = <x,y,z-a>\]
\[|r-ak|^{2} = x^{2}+y^{2}+(z-a)^{2}\]
\[=x^{2}+y^{2}+z^{2}+a^{2}-2az\]
im doing something wrong

Answer 2

by definition
\[|r|^{2} = a^{2}\]
thus you are saying ak = 0
?

Answer 3

I wrote picture like this. ak can't be zero since a is length of a string.