If no forces acts on an object or the forces on it are balanced then it will either remain at rest, or move with a constant velocity.
One example is that of a book resting on a table. The weight of the book, due to the force of gravity, is equal to the reaction of the table, therefore the book is at rest.
Another example is that of a parachutist. The weight of the person, due to the force of gravity, is equal to the opposing force of air resistance. This means the man is falling at a constant velocity.
Inertia is another part of Newton's First Law. All objects have inertia, which is the tendency to resist change i.e. to remain at rest, or if moving, to continue moving at a constant velocity.
If an unbalanced force acts on an object then it will either accelerate or decelerate.
Experiments show that F = ma
where F = unbalanced force in Newtons (N)
and m = mass in kilograms (kg)
and a = acceleration in metres per second squared (m/s2)
To every action there is an equal but opposite reaction.
For example, when a cannon fires out a cannon ball, a force is exerted on the ball but an equal but opposite force is exerted on the cannon as well. That is why it is jolted backwards.