relationship between work and potential energy

by lzh
Tags: energy, potential, relationship, work
 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.
 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)
 P: 111 work is kinetic energy here, right?
P: 44

relationship between work and potential energy

and what else...
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P: 5,794
 Quote by lzh work is kinetic energy here, right?
No. When non-conservative forces act, total mechanical energy is not conserved. How much energy did you start with? What did you end up with? Can you explain this?
 P: 111 well, I started with 43.66J. But i dont know how much ends up being left.
 P: 111 wait... can this possibly be right?: W=KE+PE
 P: 44 That's my line of thought...work done by non conservative forces= change in kinetic energy + change in potential energy
 P: 111 so does the answer to this part end up the same as first part of the question?
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