Register to reply 
Force from power at rest = infinity? 
Share this thread: 
#19
Apr1814, 02:00 PM

P: 965

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: 4,078




#21
Apr1814, 04:40 PM

Mentor
P: 17,344

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

Mentor
P: 17,344

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


#23
Apr1814, 04:45 PM

Mentor
P: 17,344




#24
Apr1814, 08:59 PM

P: 965




#25
Apr1814, 08:59 PM

P: 965

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: 83




#27
Apr2014, 11:41 PM

Mentor
P: 17,344




#28
Apr2014, 11:43 PM

Mentor
P: 17,344




#29
Apr2114, 05:26 AM

P: 965




Register to reply 
Related Discussions  
Input Power against time graph  Flywheel rest to 300rpm  Engineering, Comp Sci, & Technology Homework  9  
What is infinity to the power of zero?  Calculus & Beyond Homework  4  
One to the power infinity  Calculus  3  
One raised to the infinity power help please  Calculus  5  
Set in power infinity  Set Theory, Logic, Probability, Statistics  6 