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Finding velocity function from a model of force 
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#1
Feb912, 09:12 PM

P: 9

1. The problem statement, all variables and given/known data
Hi, I've got an assignment which involves finding the function of velocity against time, given the information that the particle is moving with intial velocity of v_{0} and that it experience a resistive force of βv^{2} 2. Relevant equations From the problem I've derived model of: ma = βv^{2} 3. The attempt at a solution rearranging the model, I have equation of: dv/dt = β/m * v^{2} then, using techniques from seperable equation, dv/v^{2} = β/m * dt 1/v = β/m * t + C_{1} pluging in v(0) = v_{0} 1/v_{0} = β/m * 0 + C_{1} Or C_{1} = 1/v_{0} Now, plugging in C and rearraging the equation, I obtained 1/v = β/m * t  1/v_{0} v = m/(β * t) + v_{0} So as you can see, I'm able to derived the velocity function, but the problem lies to the fact that if I try to plug in t=0 for the velocity, then the function will not work as division by zero is not allowed Therefore, I'm thinking that something is wrong with my velocity function but up till now I can't still figure it out Thank you in advance! 


#2
Feb912, 09:40 PM

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P: 5,196

Welcome to PF!
Your calculus is fine, you just forgot how to do division:[tex]\frac{1}{v(t)} = \frac{\beta}{m} t + \frac{1}{v_0}[/tex][tex]\Rightarrow v(t) = \left[ \frac{\beta}{m} t +\frac{1}{v_0} \right]^{1} \neq \frac{m}{\beta t} + v_0 [/tex] 


#3
Feb1012, 12:36 AM

P: 9

Ah yes thank you very much at pointing out my mistake, guess I spend too much time thinking how to actually solve the differential equation that I forgot about that. In any case, thank you for helping me again :)



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