四川物理真题21

21因为物体移动1m，移动滑轮有三段绳子连接，绳子两端需要移动3m。陈骁做的功W=FS=250*3=750J。

功率P=W/t=750/5=150W。

W has = 600 * 1 = 600j W total = 250 * 3 = 750j机械效率=W has /W total =600/750=80%。

22上推1m，斜面上推2m。

w total = fs = 500 * 2 = 1000j p = w/t = 1000/5 = 200 w。

W has = mgh = 70 * 10 * 1 = 700J机械效率W has /W total =700/1000=70%。

因为物体在斜面上匀速直线运动，f推=f摩擦力+mgsin 30。

F摩擦力=F推力-MGS in 30 = 500-70 * 10 * 1/2 = 500-350 = 150n。