[題目] Drag Force
※ [本文轉錄自 Physics 看板]
作者: Integra1 (偽積分) 看板: Physics
標題: [題目] Drag Force
時間: Mon Jul 6 20:46:38 2009
[領域] 物理 微方 (題目相關領域)
[來源] Halliday 8E CH6 35題 (課本習題、考古題、參考書...)
[題目]
有艘船(mass=1000kg)以25m/s的速度在水上行進 阻力f=70v
問下降到12.5m/s需要多少時間
[瓶頸] (寫寫自己的想法，方便大家為你解答)
I wrote the differential equation f=m*dv/dt
thus, 70v=1000 dv/dt
dv/dt-0.07v=0
Solve it with initial condition v(0)=25, v(t)=25exp(0.07t)
then the answer is -9.9s, but the right answer doesn't have the minus sign.
But I think the velocity should be decreasing,
the correct v(t) should be 25exp(-0.07t)
Where is my problem solving the DE?
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