A hockey puck slides across the ice at a constant velocity. It is in mechanical equilibrium? why or why not?

 

Answer

It is in mechanical equilibrium because the net force on it is zero. Constant velocity means that there is no acceleration, and no acceleration means no force: F = ma -----> F = m(0) ----> F = 0. A system is in mechanical equilibrium when the sum of the forces, and torque, on each particle of the system is zero. Note: Just because it's in mechanical equilibrium does not mean that it's in static equilibrium.