The condition of curvilinear motion is that when the resultant force (acceleration) of an object is not on the same straight line with its velocity direction, the object does curvilinear motion. The key to judge whether an object moves in a curve is to look at the relationship between the direction of resultant force or acceleration and the direction of velocity. If the * * * line in both directions is a straight line, the non * * line (biasing force) is a curve.