Newtonian mechanics

• Can Conservative Forces Change a System’s Mechanical Energy?
  A conservative force can change kinetic energy, but its work is offset by an equal change in potential energy when no nonconservative work acts.
• Can Internal Forces Change a System’s Total Momentum?
  Internal forces can redistribute momentum among a system’s parts, but they cannot change its total momentum when the system is isolated.
• Can Magnitude Be Negative?
  Magnitude is never negative: it measures size. Signed quantities such as potential energy or a vector component can be negative, depending on direction or reference.