Here we assume that the vertical up direction is $+$y$+$y and the block is moving in the $+$x$+$x direction when it is at the bottom.

Part A $-$ Gravitational work

What is the work done by gravity while the block is moving from bottom to the top of the circle?

$2$mgR$2$mgR$$mgRmgR$00$mgR$$mgR$2$mgR$2$mgR$$

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Part B $-$ momentum?

What is the momentum of the block while it is at the bottom of the circle in terms of the centripetal force at that point?

p$=$mFc$/$R$$i$^$p$=$mFc$/$Ri$^$p$=$mRFc$$i$^$p$=$mRFci$^$p$=$mRFc$$p$=$mRFc$$p$=$RFc$/$m$$i$^$p$=$RFc$/$mi$^$p$=$mRFci$^$p$=$mRFci$^$