This is sometimes called the Law of Inertia, or just inertia. Essentially, it makes the following two points:Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

- Newton's First Law of Motion, translated from thePrincipia's Latin

- An object that is not moving will not move until a force acts upon it.
- An object that is in motion will not change velocity (including stopping) until a force acts upon it.

**Here is another way of stating Newton's First Law:**

A body that is acted on by no net force moves at a constant velocity (which may be zero) and zero acceleration.So with no net force, the object just keeps doing what it is doing. It is important to note the words

*net force*. This means the total forces upon the object must add up to zero. An object sitting on my floor has a gravitational force pulling it downward, but there is also a

*normal force*pushing upward from the floor, so the net force is zero - therefore it doesn’t move.

To return to the hockey puck example, consider two people hitting the hockey puck on *exactly* opposite sides at *exactly* the same time and with *exactly* identical force. In this rare case, the puck would not move.

Since both velocity and force are vector quantities, the directions are important to this process. If a force (such as gravity) acts downward on an object, and there's no upward force, the object will gain a vertical acceleration downward. The horizontal velocity will not change, however.

If I throw a ball off my balcony at a horizontal speed of 3 m/s, it will hit the ground with a horizontal speed of 3 m/s (ignoring the force of air resistance), even though gravity exerted a force (and therefore acceleration) in the vertical direction. If it weren't for gravity, though, the ball would have kept going in a straight line . . .

. . . at least until it hit my neighbor's house.

## Newton's Three Laws of Motion