Two pulleys are connected by a belt as shown in the drawing above. If pulley Q makes 300 evolutions per minute, how many revolutions per minute does pulley P make ?()
(1)The length of the belt is 12π.
(2)The ratio of the radius of pulley P to the radius of pulley Q is 2 to 1.
A.条件(1)充分,但条件(2)不充分.
B.条件(2)充分,但条件(1)不充分.
C.条件(1)和(2)单独都不充分,但条件(1)和条件(2)联合起来充分.
D.条件(1)充分,条件(2)也充分.
E.条件(1)和条件(2)单独都不充分,条件(1)和条件(2)联合起来也不充分.
参考答案:B
解析:
因为两个滑轮由一根皮带相连,所以两个滑轮上任一点的线速度都是一样的,且滑轮的转速与皮带的长短无关,因此(1)不充分;根据(2)可设滑轮P的半径为2r,滑轮Q的半径为r,两个滑轮上任一点的线速度都一样,也即在相同时间内两个滑轮转过的路程是一样的,设滑轮P每分钟转x圈,则根据题意可得:
300×πr=27πx→x=150圈/分钟所以(2)充分。