作者Integra1 (伪积分)
看板Physics
标题[题目] Drag Force
时间Mon Jul 6 20:46:38 2009
[领域] 物理 微方 (题目相关领域)
[来源] Halliday 8E CH6 35题 (课本习题、考古题、参考书...)
[题目]
有艘船(mass=1000kg)以25m/s的速度在水上行进 阻力f=70v
问下降到12.5m/s需要多少时间
[瓶颈] (写写自己的想法，方便大家为你解答)
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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◆ From: 140.112.115.224
1F：推 youmehim:阻力要加负号 F=ma=-f 07/06 21:25
2F：推 joe70136:Drag force decreases the speed, so you should add 07/06 23:30
3F：→ joe70136:minus sign before the mass. 07/06 23:31