照明用的电光源按其发光原理可分为几类?举例说明?
参考答案:
两类:
热幅射电光源,如:白炽灯;
气体放电光源,如:荧光灯。
关于自由落体运动,下列说法中正确的是
A.它是v0=0,a=g、竖直向下的匀加速直线运动
B.在开始连续的三个1s内通过的位移之比是1:4:9
C.在开始连续的三个1s末的速度大小之比是1:2:3
D.若g=10m/s2,则物体在下落第3s内的平均速度为15m/s
A project manager needs to analyze the project cost to find ways to decrease costs. It would be BEST if the project manager looks at ______ .
A) variable costs and fixed costsB) fixed costs and indirect costC) direct costs and variable costsD) indirect costs and direct costs