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。