Re: [題目] 定力作功的功率(高中物理)
首先我先做一般推導,
由 W =∫F.ds 開始,又 F = dp/dt = m(dv/dt) + v(dm/dt)
因此 W =∫m(dv/dt).ds + v(dm/dt).ds
=∫mv dv +∫v^2 dm
P = dW/dt = ∫m(dv/dt) dv +∫v(dm/dt) dv + ∫d(v^2)/dt dm
若在質量不變的情況下,dm/dt = 0,且∫dm = 0,
可得到 P = ∫ma dv = Fv
由於在這一題v不變,故 F = v(dm/dt),又 dm/dt = μv
可以得到 F = μv^2,為定值,
代入定義 W = ∫F.ds = F∫ds,
再由 P = dW/dt = F∫d(ds/dt) = F∫dv = Fv = μv^3
大致上是這樣,但不曉得問題出在哪裡,
理想上(這題也是理想化)作功應該全部轉變成繩子的動能,
計算式應該沒問題,但很明顯的兩者不同且找不出原因,
有請各位版友指點迷津,感謝!
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