Newton's First, Second, and Third Laws of Motion
Newton’s First Law: This is called the “law of inertia." A body remains in a constant state of motion or at rest unless acted upon by an external force. In other words, if all forces exerted on an object add up to zero, its state of motion will remain constant; its acceleration will be zero.
Newton’s Second Law: A force applied to an object produces a time rate of change of its linear momentum.
In mathematical jargon, the force is equal to the derivative of the linear momentum with respect to time.
F = d(mv)/dt
Since by classical mechanics mass is constant, for most purposes the expression may be simplified,
F = ma
Newton’s Third Law: For every action, there is an equal and opposite reaction. This tells us that if a force, F, is applied to an object, then that object responds with an equal force of opposite direction, -F. Combined with the second law, its equivalent is,
m1a1 = -m2a2
As an example, a hunter pulls the trigger that fires a bullet from a gun. A reactive force is very real; the gun recoils. In the case of a rocket, the fuel ignites and pushes the rocket forward. The rocket pushes back and the exhausted products exit from the rear. In this instance, both the second and third laws obviously apply.
If an applied force is constant, as in the case of the rocket and not of the bullet, acceleration is constant. The Earth is always under acceleration while orbiting the Sun, the force must be continuous - in this instance the force is the gravitational force of the Sun.