[題目] Drag Force

看板Physics作者 (偽積分)時間15年前 (2009/07/06 20:46), 編輯推噓2(2推 0噓 1→)
留言3則, 2人參與, 最新討論串1/1
[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? -- ※ 發信站: 批踢踢實業坊(ptt.cc) ◆ From: 140.112.115.224
推
07/06 21:25, , 1F
阻力要加負號 F=ma=-f
07/06 21:25, 1F
推
07/06 23:30, , 2F
Drag force decreases the speed, so you should add
07/06 23:30, 2F
→
07/06 23:31, , 3F
minus sign before the mass.
07/06 23:31, 3F
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