The displacement time graph of a particle executing $S.H.M.$ is given in figure: (sketch is schematic and not to scale) Which of the following statements is are true for this motion?

$(A)$ The force is zero $t=\frac{3 T}{4}$

$(B)$ The acceleration is maximum at $t=T$

$(C)$ The speed is maximum at $t =\frac{ T }{4}$

$(D)$ The $P.E.$ is equal to $K.E.$ of the oscillation at $t=\frac{T}{2}$

JEE MAIN 2020, Diffcult
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Sol. $(A)$ $F =$ ma $\quad a =-\omega^{2} x$

at $\frac{3 T }{4}$ displacement zero $( x =0),$ so $a =0$

$F=0$

$(B)$ at $t=T \quad$ displacement $(x)=A$ $x$ maximum, So acceleration is maximum.

$(C)$ $V =\omega \sqrt{ A ^{2}- x ^{2}}$

$V _{\max }$ at $x =0$

$V _{\max }= A \omega$

at $t =\frac{ T }{4}, x =0, \quad$ So $V _{ max }$

$(D)$ $KE = PE$

$\therefore$ at $x=\frac{A}{\sqrt{2}}$

at $t=\frac{T}{2} \quad x=-A \quad$ (So not possible)

art

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