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Force from power at rest = infinity? 
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#19
Apr1814, 02:00 PM

P: 907

If we are given that KE=0 at t=0 then it is the (improper) definite integral from 0 to t that matters. If we are only interested in the motion for t>0 then the values for P(t) for t<0 simply do not enter in. If one insists on extending positive constant power into the past, we would admittedly have the problem of an imaginary KE at t<0. That would not seem to be very physical. Edit: The point about power being possibly undefined at t=0 is irrelevant. The trajectory you get if you crank the numbers will have a well defined P at t=0 even if the force or acceleration at t=0 are undefined. It will be a onesided limit, but that's OK as long as we are only considering a trajectory from the starting point forward and not from the starting point going into the past. 


#20
Apr1814, 03:36 PM

P: 3,927




#21
Apr1814, 04:40 PM

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I don't know what you and dauto are going on about. Even if you can mathematically work around the infinite force, and even if other quantities do not diverge, the fact remains that an infinite force is not physical. The objections based on that fact, which point out that F is infinite or undefined at v=0 for a finite P, are entirely correct objections. 


#22
Apr1814, 04:41 PM

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P: 16,989

http://en.wikipedia.org/wiki/Electro...eracy_pressure 


#23
Apr1814, 04:45 PM

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#24
Apr1814, 08:59 PM

P: 907




#25
Apr1814, 08:59 PM

P: 907

It is valid to define P as the derivitive of KE with respect to time. That formulation does not suffer from the problem of an undefined force multiplied by a zero velocity. 


#26
Apr1914, 05:14 AM

P: 77




#27
Apr2014, 11:41 PM

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#28
Apr2014, 11:43 PM

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#29
Apr2114, 05:26 AM

P: 907




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