Physics 16 Simple Harmonic Motion (12 of 19) x(t=4s)=?, A=?, f=?, T=?

In this video I will find x(t=4s)=?, A=?, f=?, and T=?, given x(t)=(10cm)cos[(pi)t/3)].

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[holmes]

To find (f), i put (w=1.05) directly but i got a different answer unlike when the value (w=pi/3) is used directly and (F) is then made the subject of formular

[rahultiwari]

Sir if there is a horizontal spring(of spring constant k) block system we apply a force on the block of mass m then will its amplitude be F/k.If so they why sir.I mean at the extreme position of SHM there is a net acceleration inwards they why F will be taken equal to 'kx' to find the amplitude

[mhuc]

why we did't use phase angle on this equation ?

[Peter_]

T = 6 seconds?
That seems a pretty long period!