How to get the constraint equation of this problem

[goodboy]

A particle moves in the xy plane under the constraint that its velocity vector is always directed towards a point on the x-axis whose abscissa is some given function of time f(t). Show that for f(t) differentiable, but otherwise arbitrary, the constraint is nonholonomic.
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it have cost me 3 hours,but i cann't write out the constraint equation .
help
thanks

 

[OlderDan]

A particle moves in the xy plane under the constraint that its velocity vector is always directed towards a point on the x-axis whose abscissa is some given function of time f(t). Show that for f(t) differentiable, but otherwise arbitrary, the constraint is nonholonomic.
--------------------------------------------
it have cost me 3 hours,but i cann't write out the constraint equation .
help
thanks

https://www.physicsforums.com/showthread.php?t=138432&highlight=holonomic