问题 选择题
如图所示,AB是一质量为m的均匀细直杆,A端靠在光滑的竖直墙壁上,B端置于水平地面上,杆身与竖直方向夹角为θ,杆与地面的摩擦系数为μ,保持平衡,则此时杆与地面的摩擦力为(  )

A.mgtgθ

B.mgsinθ

C.μmg

D.μmg

答案

答案:A

单项选择题
问答题

You probably won’t be shocked to find out that the inventor of tire rubber is Charles Goodyear, as he’s the only guy on the list to actually get his name attached to the end product. (46) It wasn’t easy coming up with a form of rubber tough enough to withstand the drag racing and car chases everyone envisioned the day the automobile was invented. In fact, if there was one man who should have given up his life dream, it was Goodyear. (47)The man spent time in and out of prison, lost every friend he had and starved his children in his tireless pursuit of a per form of rubber.

(48) After his first two years of tinkering and failing with primitive rubber in the 1830s, Goodyear and his family were camping out in an abandoned factory and fishing for sustenance. This is when he made a huge breakthrough: He’d use acid to smooth out and toughen rubber! The government bought 150 mailbags made of the stuff and the rest is...

Oh, wait. They were all defective. The process didn’t work and Goodyear was ruined. Again.

Finally in 1839, Goodyear wandered into a general store with another failure of a formula. The crowd watched and laughed at him. (49)In a rage, he began to shake his fist, flinging a piece of his rubber onto the hot stove top.

After inspecting the charred remains, he realized that he had just found a way to make durable, weatherproof rubber. (50) Despite what we’re sure were numerous failed "now let’s try setting this on fire to see if it improves it!" experiments, an empire was born.

(49)In a rage, he began to shake his fist, flinging a piece of his rubber onto the hot stove top.