61. A Newton horizontal force is needed to
    keep an object weighing 500 . Newtons moving
    at a constant velocity of 2.0 meters per second
    across a horizontal surface. The magnitude of
    the frictional force acting on the object is
    

To keep the object moving at a constant velocity, the applied force must balance the frictional force acting against it. According to Newton’s first law, if the velocity is constant, the net force acting on the object is zero. Therefore, the horizontal force of Newtons must be equal to the frictional force.

This leads us to conclude that the magnitude of the frictional force is Newtons. Thus, the correct answer is C) 50 N.

The key here is recognizing that constant velocity implies that all forces must be balanced, which means the applied force and frictional force are equal in magnitude but opposite in direction.