题目

大一物理题.
A uniform steel rod of length 1.20 m and mass of 6.40 kg has attached to each end a small ball of mass
1.06 kg.The rod is constrained to rotate in a horizontal plane about a vertical axis through its midpoint.
At a certain instant it is observed to be rotating with an angular speed of 39.0 rev/s.Because of axle
friction it comes to rest 32.0 s later.Assume a constant frictional torque.
(a) Find the angular acceleration.
(b) Calculate the retarding torque exerted by axle friction.
(c) Determine the total work done by the axle friction.
(d) Calculate the number of revolutions executed during the 32.0 s.
(e) Now suppose that the frictional torque is known not to be constant.which,if any ,of the quantities abcd or d can still be computed without requiring any additional information?if any exists give its value

提问时间：2020-10-26

答案

第一个
设杆的转动惯量为I1 球的为I2
杆的转动惯量为I1=m1L^2/12 球的为 m2(L/2)^2*2=m2L^2/2
因为(I1+I2)α=T
T=F*L/2*2=FL
可以解出α
第二个
冲量矩等于角动量
因此(I1+I2)ω=FLt
第三个
W=(I1+I2)ω^2/2
第四个
W=FS 解出S
revolution=S/2πr
第五个
需要知道摩擦力和时间的函数关系.