【題目】如圖所示,勁度系數(shù)為k的輕質(zhì)彈簧,一端系在豎直放置、半徑為R的光滑圓環(huán)頂點(diǎn)P,另一端連接一套在圓環(huán)上且質(zhì)量為m的小球。開始時(shí)小球位于A點(diǎn),此時(shí)彈簧處于原長(zhǎng)且與豎直方向的夾角為45°,之后小球由靜止沿圓環(huán)下滑,小球運(yùn)動(dòng)到最低點(diǎn)B時(shí)的速率為v,此時(shí)小球與圓環(huán)之間的壓力恰好為零,已知重力加速度為g。下列分析正確的是( )
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A. 輕質(zhì)彈簧的原長(zhǎng)為R
B. 小球過B點(diǎn)時(shí),所受的合力為
C. 小球從A到B的過程中,重力勢(shì)能轉(zhuǎn)化為彈簧的彈性勢(shì)能
D. 小球運(yùn)動(dòng)到B點(diǎn)時(shí),彈簧的彈性勢(shì)能為