A particle of charge q and mass m moving with a velocity v enters a uniform magnetic field b in the direction of the field. The force on the particle is

A particle of charge q and mass m moving with a velocity v enters a uniform magnetic field B in the direction of the field. The force on the particle is given by the equation F = qVB sinθ, where θ is the angle between the velocity vector and the magnetic field vector. In this case, since the particle enters the field in the direction of the field, θ = 0, and sinθ = 0. Therefore, the force on the particle is zero (F = qVB sinθ = qVB(0) = 0). Option (d) is correct - the force on the particle is zero. Option (a) is incorrect because it represents the formula for the magnitude of the force on a charged particle moving perpendicular to the magnetic field. Option (b) is incorrect because it represents the formula for the acceleration of a charged particle in a magnetic field. Option (c) is incorrect because it represents the formula for the radius of the circular path followed by a charged particle moving perpendicular to the magnetic field. Therefore, the correct answer is - O.