Newton's first law describes the motion of an object in the absence of a net applied force. It is based entirely on experimental observation, not mathematical deduction, logical reasoning, or theoretical derivation. Newton's first law says that if a massive object feels no net applied force it will be in uniform motion, and it will continue in that uniform motion until an externally applied force changes that motion. It is important to be clear on what we mean by uniform motion. Uniform motion means constant velocity. So an object that feels no net external force moves with constant velocity, that is to say that neither its speed nor the direction of its motion changes. It is also important to realize that constant velocity could mean zero velocity. If an object is not moving, and it feels no net force, it will continue to not move. If it is moving, and it feels no net force it will continue to move in a straight line with constant speed.
© 2008 Kansas State University Physics Education Research Group.