问题 选择题

如图所示,用细绳连接用同种材料制成的a和b两个物体.它们恰能沿斜面向下作匀速运动,且绳子刚好伸直,关于a、b的受力情况(  )

A.a受3个力,b受4个力

B.a受4个力,b受3个力

C.a、b均受3个力

D.a、b均受4个力

答案

对整体分析,根据平衡有:Mgsinθ=μMgcosθ,解得μ=tanθ.

再隔离对a分析,假设受到拉力,有:mgsinθ=T+μmgcosθ,解得,T=0.所以绳子无拉力,a、b均受三个力,重力、拉力和摩擦力.故C正确,A、B、D错误.

故选C.

选择题
填空题

We already live in an ever-communicated world that will only become mere so in the next tech era. We’ve (1) that gets us so much information. We’ve got every second (2) , we’ve get computers and laptops, we’ve get personal organizers and we’re just being (3) and every advance in technology seems to create more and more communications at us. We are sort of (4) .
Research suggests that all the multi-tasking may actually make our brains (5) , producing a world-wide increase in IQ (6) and mere in recent decades. Is there any real benefit in (7) we new have to go through
We’re net becoming a race of (8) , but many de think certain skills are enhanced and certain are net. You knew the ability to (9) , to answer a dozen e- mails in five minutes, or to fill out (10) . That’s enhanced. But when someone is out there with his kids (11) or something like that, he’s got his cell phone in his pocket. He’s always wondering, "Gee, did I get a voicemail " This might have negative effects (12) .
Creativity is something that happens slowly. It happens when your brain is just (13) , just playing, when it (14) which you hadn’t thought of, or maybe you have time to read a book. You are a businessperson but you have time to (15) ,or about a philosopher and something that happened long age or something or some idea (16) . Actually, it might occur to you that you (17) in that way, and so it’s this mixture of unrelated ideas that feeds your productivity, (18) . And if your mind is disciplined to answer every e-mail, then you don’t have time for that playful noodling. You don’t have time for (19) . So I think maybe we’re getting smarter in .some senses, but ever-communication is (20) and to our reflection.

We already live in an ever-communicated world that will only become mere so in the next tech era. We’ve (1) that gets us so much information. We’ve got every second (2) , we’ve get computers and laptops, we’ve get personal organizers and we’re just being (3) and every advance in technology seems to create more and more communications at us. We are sort of (4) .
Research suggests that all the multi-tasking may actually make our brains (5) , producing a world-wide increase in IQ (6) and mere in recent decades. Is there any real benefit in (7) we new have to go through
We’re net becoming a race of (8) , but many de think certain skills are enhanced and certain are net. You knew the ability to (9) , to answer a dozen e- mails in five minutes, or to fill out (10) . That’s enhanced. But when someone is out there with his kids (11) or something like that, he’s got his cell phone in his pocket. He’s always wondering, "Gee, did I get a voicemail " This might have negative effects (12) .
Creativity is something that happens slowly. It happens when your brain is just (13) , just playing, when it (14) which you hadn’t thought of, or maybe you have time to read a book. You are a businessperson but you have time to (15) ,or about a philosopher and something that happened long age or something or some idea (16) . Actually, it might occur to you that you (17) in that way, and so it’s this mixture of unrelated ideas that feeds your productivity, (18) . And if your mind is disciplined to answer every e-mail, then you don’t have time for that playful noodling. You don’t have time for (19) . So I think maybe we’re getting smarter in .some senses, but ever-communication is (20) and to our reflection.