At time ti, the kinetic energy of a particle is 30.0 J and the potential energy of the system to which it belongs is 10.0 J. At some later time tf, the kinetic energy of the particle is 18.0 J. (a) If only conservative forces act on the particle, what are the potential energy and the total energy at time tf? (b) If the potential energy of the system at time tf is 5.00 J, are there any nonconservative forces acting on the particle? Explain.