The first law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant. Velocity is a vector quantity which expresses both the object's speed
and the direction of its motion; therefore, the statement that the
object's velocity is constant is a statement that both its speed and the
direction of its motion are constant.
The first law can be stated mathematically as
An object that is at rest will stay at rest unless an external force acts upon it.An object that is in motion will not change its velocity unless an external force acts upon it.