题目

I’ve always known that dogs are clever, but even I was surprised when I heard about a dog called Red, at the Battersea Dog’s Home in London. The home has been caring for lost and unwanted animals for over 140 years. Recently, the workers there got amazed when they came to work in the morning. They found that some of the dogs had got out of their kennels(狗窝) during the night, opened cupboards and taken out of food and toys. And it happened again, and again. The manager described how as many as nine dogs were escaping every night. “When we came in, dogs were running around havi均匀材料制成的正方体木块，边长为a cm，质量为m kg．（1）把它放在水平地面上，如图甲所示．为了能使木块绕CC′边转动，在AA′边的中点上施加的力F至少是2mg 42mg 4．（2）若木块密度是水的密度的一半，将木块放在足够深的水面上漂浮，如图乙所示．现在木块正上方施加一竖直向下的压力，使木块缓慢下移，当木块上表面刚好与水面相平时，施加的压力F为mgmg，在此过程中施加的压力做的功为12mga12mga．（结果只能用m、a、g表示）