两块相同的竖直木板A、B间有质量均为m的四块相同的木块，用两个大小均为F的水平力压木板，使木板均处于平衡，如图所示．设所有接触面间的动摩擦因数均为μ．则第2块对第3块的摩擦力大小为______．
先把四块砖看作一个整体：四块砖的总重力是4mg，两边的压力F相同，两边受到的摩擦力也相同，根据平衡力知识，向下的重力4mg与两边的摩擦力之和相等；故两边的砖块1和4各受到2mg的摩擦力作用，且方向竖直向上；
对于第一块砖（1号）：受到木板A向上的摩擦力作用，大小为2mg；自身向下的重力作用，大小为mg；根据平衡力知识，它（1号）还受到2号砖对它向下的摩擦力作用，大小为mg；根据力的作用是相互的，1号砖对2号砖的摩擦力大小为mg方向向上；
对于第二块砖（2号）：已经受到了两个力的作用，分别是：自身向下的重力mg、1号砖对它（2号）的向上的摩擦力mg，这两个力正好是一对平衡力；故2号砖不再受其它力，即2号砖和3号砖之间没有力的作用；第二块砖对第三块砖的摩擦力大小也就为0；
故答案为：0．
| 完形填空。 | ||||
| I grew up in a community called Estepona. I was 16 when one morning, Dad told me I could drive him into a 1 village called Mijas, on condition that I took the car in to be 2 at a nearby garage. I readily accepted. I drove Dad into Mijas, and 3 to pick him up at 4 pm, and then dropped off the car at the 4 . With several hours to spare, I went to a theater. 5 when the last movie finished, it was six. I was two hours late! I knew Dad would be angry if he 6 I'd been watching movies. So I decided not to tell him the truth. When I 7 there I apologized for being late, and told him I'd 8 as quickly as I could, but that the car had needed a major repair. I'll never forget the 9 he gave me. "I'm disappointed you 10 you have to lie to me, Jason." Dad looked at me again. "When you didn't 11 , I called the garage to ask if there were any 12 , and they told me you hadn't yet picked up the car." I felt 13 as I weakly told him the real reason. A 14 passed through Dad as he listened attentively. "I'm angry with 15 . I realize I've failed as a father. I'm going to walk home now and think seriously about 16 I've gone wrong all these years." "But Dad, it's 18 miles!" My protests and apologies were 17 . Dad walked home that day. I drove behind him, 18 him all the way, but he walked silently. Seeing Dad in so much 19 and emotional pain was my most painful experience. However, it was 20 the most successful lesson. I have never lied since. | ||||
|