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Relationship between work and potential energy 
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#1
Jan2007, 07:23 PM

P: 111

1. The problem statement, all variables and given/known data
At time ti, the kinetic energy of a particle is 35.5 J and its potential energy is 8.16 J.At some later time tf , its kinetic energy is 9.38 J. If only conservative forces act on the parti cle, what is its potential energy tf ? Answer in units of J. If the potential energy at time tf is 6.95 J, what is the work done by the nonconservative forces acting on the particle? Answer in units of J. 2. Relevant equations W=F*displacement*cos(theta) 3. The attempt at a solution For the first part of the question, because there were only conservative forces, the sum of kinetic and potential must be same for both ti and tf. And i know that i have the right answer for that. But for the second part, where force isn't conserved, that would not be the case: I CANNOT take the sum of 35.5 and 8.16 and subtract 6.95 like i did for the first part. I dont think i completely understand the concept here, so i'd appreciate any help. 


#2
Jan2007, 07:41 PM

P: 44

You're missing a relationship between work and energy. W = ______
Fill in the blank and you should have your answer. Hint (you're not looking for W = Fd) 


#3
Jan2007, 07:45 PM

P: 111

work is kinetic energy here, right?



#4
Jan2007, 09:37 PM

P: 44

Relationship between work and potential energy
and what else...



#5
Jan2007, 09:38 PM

Sci Advisor
HW Helper
PF Gold
P: 6,035




#6
Jan2007, 09:54 PM

P: 111

well, I started with 43.66J. But i dont know how much ends up being left.



#7
Jan2007, 10:03 PM

P: 111

wait... can this possibly be right?:
W=KE+PE 


#8
Jan2007, 10:11 PM

P: 44

That's my line of thought...work done by non conservative forces= change in kinetic energy + change in potential energy



#9
Jan2007, 10:42 PM

P: 111

so does the answer to this part end up the same as first part of the question?



#10
Jan2007, 11:42 PM

Sci Advisor
HW Helper
PF Gold
P: 6,035

(KE + PE)_initial, is 43.66J. The final mechanical energy, (KE + PE)_final, is 16.33J. The difference between the two represents the change in the total mechanical energy of the system , which is, as stated above, equal to the work done by the nonconservative forces. 


#11
Jan2107, 09:59 AM

P: 111

would the change in mechanical energy be negative?
27.33J in this case. because +27.33J is wrong, according to my hw service 


#12
Jan2107, 02:03 PM

P: 44

Assuming you did the calculation right, the work should be negative.



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