如图甲所示，一弹簧测力计下挂一圆柱体，从盛有水的烧杯上方离水

问题选择题

如图甲所示，一弹簧测力计下挂一圆柱体，从盛有水的烧杯上方离水面某一高度处开始缓慢下降，下降过程中弹簧测力计示数F随圆柱体下降高度几变化的实验图线如图乙所示，由图可知下列结论正确的是（g=10N/kg）（　　）

A．圆柱体受到的重力是10N

B．圆柱体的体积为4×10^-3m³

C．圆柱体的密度为1.5×10³kg/m³

D．圆柱体所受的最大浮力为4N

答案

由于物体是从盛有水的烧杯上方离水面某一高度处开始缓慢下降，所以开始并没有浸入水中，此时测力计的示数即为圆柱体的重力G，由图象知为12N；故A错误．

当物体下降的高度为3cm时，物体开始浸入水中；此时的物体受到三个力的作用：测力计的拉力为F_拉、物体的重力G、液体对它的浮力F_浮，三个力彼此平衡即：G=F_拉+F_浮．

当物体下降的高度为7厘米时，圆柱体全部浸入水中．此时物体排开的液体的体积等于物体的体积，受到的浮力最大．

此时物体受到的最大浮力可以利用公式：F_浮=G-F_拉=12M-4N=8N．故D错误．

因为水的密度ρ=1×10³kg/m³，g=10N/kg，F_浮=8N；

利用阿基米德原理可以求得圆柱体的体积：V=

F浮

ρ水g

1×103kg/m3×10N/kg

=8×10^-3m³，故B错误．

圆柱体的密度：ρ_物=

12N

8×10-3m3×10N/kg

=1.5×10³kg/m³．故C正确．

故选C．

完形填空。
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