Prove that a particle beam will be collected (focused) at some certain points on axis OO′, if the velocity of each particle of the beam in section OO′′ is equal to v₀, and the force acting on the particle is F = −kr, where r is the distance from the particle to the axis of the beam. At what distance from the cross section OO′′ the particles are focused if the mass of each of them is m.