Non-uniform motion — common mistakes

Always draw a free-body diagram when analysing forces on an object. Clearly label all forces and their directions. Remember that Newton's Third Law forces act on different objects, so don't include them in the free-body diagram for a single object to calculate its resultant force.

Non-uniform motion is always caused by a resultant (net) force acting on an object. This unbalanced force leads to a change in the object's velocity, either in magnitude (speed) or direction, or both.

Newton's First Law states that a resultant force is necessary for any change in motion. His Second Law, $F_{resultant} = ma$, quantifies this, showing that the resultant force directly determines the object's acceleration (change in velocity) inversely with its mass. Newton's Third Law describes the interactive forces that create these resultant forces on objects.

Terminal velocity represents a crucial transition in non-uniform motion. While an object initially accelerates due to a resultant force, the increasing resistive forces eventually balance the driving forces. This results in zero net force, zero acceleration, and the object transitioning from non-uniform to uniform motion at its maximum constant speed.