6. In which of the following cases forces may not be required to keep the (a) Particle going in a circle (b)Particle going along a straight line (c) The momentum of the particle constant (d) Acceleration of the particle constant

1.        In which of the following cases forces may not be required to keep the                                  

         (a) Particle going in a circle

         (b)Particle going along a

          straight line

         (c)  The momentum of the particle constant

         (d) Acceleration of the particle constant