如图所示的皮带传动装置中，轮B和C同轴，A、B、C分别是三个轮边缘的质点，且其半径RA=RC=2RB，则三质点的向心加速度之比aA：aB：aC等于（ ）
A．4：2：1
B．2：1：2
C．1：2：4
D．4：1：4
由于B轮和A轮是皮带传动，皮带传动的特点是两轮与皮带接触点的线速度的大小与皮带的线速度大小相同，
故vA=vB，
∴vB：vA=1：1
由于C轮和B轮共轴，故两轮角速度相同，
即ωC=ωB，
故ωC：ωB=1：1
由角速度和线速度的关系式v=ωR可得
vC：vB=RC：RB=2：1
∴vA：vB：vC=1：1：2
又因为RA=RC=2RB
根据a=
得：v2 R
aA：aB：aC=1：2：4
故选C．
Your parents always tell you that a good education is the key to a bright future. The promise you made to finish your schooling has been completed. Another few years in some college classroom just doesn’t sound too attractive; so what are your choices? Let’s see, Choice One—You can live with your parents and do nothing. Or Choice Two—You could get yourself a job. Of course, there are other choices, but for this little discussion, we’ll just suppose that they aren’t related (有关的). OK, so Choice Two is obviously going to be more effective. You will finally enter the real world. Well, hopefully high school helped to give you some kind of an idea of what type of job you would like to enter. If not, then it would be a good idea to speak with an expert working in the field you may be interested in before you jump into a job that you won’t like. Gaining knowledge can be good by helping you find the job that you will enjoy doing. Another reason to get some valued things before working. Internships (见习) or volunteering are usually a good place to start. If you are ready to begin a job after high school, you will save money from college cost and happily making a living in your chosen jobs. From another point of view, jumping into the workforce (劳动力) is sometimes the only way to see for yourself if you are ready to work a 40-hour week yet. If you decide that full time work is not what you are ready for, then this may advise that you should go back to a university and get a certificate. Two different
|