Newton First Law

Newton’s First Law says that if the total external force on an object is zero, its velocity does not change. That means an object either remains at rest or continues moving in a straight line at constant speed, because there is nothing unbalanced to accelerate it. In real life, objects often slow down or change direction because friction, air resistance, or another force is present; the law describes what happens when those effects are absent or exactly cancel out.

Use this law when analyzing motion with no net force, such as a puck gliding on ice, a book resting on a table, or an object moving at constant speed in space. It is also useful for checking whether forces are balanced before applying more advanced motion equations.

• thinking zero net force means the object must be stopped
• confusing constant velocity with constant speed in any direction, even if direction changes
• forgetting that friction or air resistance can make the net force nonzero