The following problem is from Sears and Zemansky's textbook. A wooden rod of negligible mass and length 80.0 cm is pivoted about a horizontal axis through its center. A white rat with mass 0.500 kg clings to one end of the stick, and a mouse with mass 0.200 kg clings to the other end. The system is released from rest with the rod horizontal. If the animals can manage to hold on, what are their speeds as the rod swings through a vertical position? The problem can be done using energy conservation (K_1+U_1 = K_2+U_2). You take the total energy of the system at start when the rod is horizontal and equate it to the total energy of the system when the rod is vertical. But I have not been able to do the problem using Newton's second law (F = m d^2x/dt^2). Any guess as to how to go about it?