In a zero-gravity region, a point charge +Q of mass M is attached to a spring of natural length L₀ and spring constant k. A uniform magnetic field B is directed into the plane. The charge is given initial velocity v₀ perpendicular to the spring, as shown, and B=Mv₀/(QL₀). Which quantities remain conserved during the motion?

Question

G1 Physics · Accepted Answer

The given condition makes the Lorentz force provide exactly the required centripetal force at radius L₀ while the spring remains at natural length. Hence speed and kinetic energy are constant, spring potential energy is constant, total mechanical energy is constant, and the force is central so angular momentum about O is conserved. Thus A,B,C,D.