In the simplest case, a force applied to an object at rest causes it to accelerate in the direction of the force. When a constant force acts on a massive body, it causes it to accelerate, i.e., to change its velocity, at a constant rate. The force can be a single force, or it can be the vector sum of more than one force, which is the net force after all the forces are combined.
The bold letters indicate that force and acceleration are vector quantities, which means they have both magnitude and direction. It states, 'The force acting on an object is equal to the mass of that object times its acceleration.' This is written in mathematical form as F = m a, where F is force, m is mass, and a is acceleration. The Second Law of Motion describes what happens to a massive body when it is acted upon by an external force. This property of massive bodies to resist changes in their state of motion is sometimes called inertia. It takes some force acting on them from the outside to cause such a change.
The First Law of Motion states, 'A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force.' This simply means that things cannot start, stop, or change direction all by themselves.
