题目

Yesterday I took my boys through the drive thru at McDonald’s. It was a hot day and they wanted a nice cool    ___21___. So I said yes. As I ordered I remembered a____22____I made to myself a year ago.. The promise is that every time I drive through the drive thru, I pay for the car____23____ me. I made this promise ____24____ I go through the drive thru about 4 times a year. So it’s an easy one to keep and up until yesterday, I have ___25___that promise. As I drove around to pay the cashier the $ 6 for our ____26____I really did not want to pay for the car___27___ behind me. I如图所示，一倾角为θ=30°的斜面体置于粗糙水平地面上．质量m=4kg的物体以的加速度沿斜面下滑，而斜面体M保持静止．斜面体与水平面间的摩擦力是多大？(g取)