A 20-Kg Mass Is Attached to One End of a Spring

Multiple Choice

A 2.0-kg mass is attached to one end of a spring with a spring constant of 100 N/m and a 4.0-kg mass is attached to the other end. The masses are placed on a horizontal frictionless surface and the spring is compressed 10 cm. The spring is then released with the masses at rest and the masses oscillate. When the spring has its equilibrium length for the first time the 2.0-kg mass has a speed of 0.36 m/s. The mechanical energy that has been lost to this instant is:

A) zero
B) 0.31 J
C) 0.61 J
D) 0.81 J
E) 1.2 J