A 20 kg block slides to the left along a horizontal frictionless surface and hits an uncompressed spring of spring constant 810 N/m, which sends the mass back to the right with the same speed. For how long is the block in contact with the spring?

Two resistors r1 and r2 may be connected either in series or parallel across an ideal battery with emf ε. we desire the rate of energy dissipation of the parallel combination to be 8.75 times that of the series combination. if r1 = 105 ω, what are the (a) smaller and (b) larger of the two values of r2 that result in that dissipation rate?