I’d like to start with Newton’s second law, because Newton’s first law can actually be considered as a special case of the second law.  
In Newton’s second law, it is stated that if there is an unbalanced force acting on an object, it will cause accelerated motion. As a result
the acceleration of the object will be proportional to the resultant force, and this is summarized in this equation, F=ma, 
where m represents the mass of the object, a is acceleration and F is the force.
We can also rewrite this equation in the form that a equals to F over m.
In this form, we can not only see that acceleration is caused by the force, and more importantly, we can see that the mass of the 
object acts as a resistance to motion. In other words, the mass of the object is an inherent property of the object to resist change in its 
state of motion.
For the same force applied, the object with a bigger mass will accelerate more slowly. 
Also, as you can notice, both F and a are given in bold letters, indicating that they are vectors, with not only magnitudes but also directions. 
The acceleration will have the same direction as the resultant force.  
As a special case of Newton’s second law, when the resultant force acting on the object is zero, that is, there is no unbalanced force, 
then according to Newton’s second law, the acceleration is also 0, 
indicating no change in the velocity. 
We also call it the state of equilibrium.
The object either stays motionless, or moves with a constant velocity.
In this class, most objects that we deal with are at rest. 
Newton’s third law is the law of action and reaction. Let’s say object A exerts a force on object B. Inevitably and simultaneously, 
object B will also exert a force on object A. These two forces will be equal, collinear and opposite, which means they will have the
same magnitude, same line of action and opposite directions. 
These two forces are known as action and reaction. That is why if you punch the wall very hard, you might get hurt instead, 
because the wall is exerting the same force back to you.
Lastly, let’s look at the Newton’s law of gravitation. 
Attractive force exists between any two objects with mass. 
As you can see from this equation
this force is proportional to the masses of the objects, m_1 and m_2, and inversely 
proportional to the second order of the distance between them, r.
The reason why you can not really feel this attractive force between, say, you and a table, 
is because of the coefficient G in this equation is extremely small. 
But the gravitational attraction forces do exist between the two objects, and they are action and reaction. 
The only gravitational force that you do feel, is the one between you and the earth, normally known 
as the weight. This is the force that will cause you to fall if you are not otherwise supported. 
The weight is indeed calculated from the Newton’s law of gravitation, with m being the mass of the object on earth, M_e being the 
mass of the earth and R being the radius of the earth. 
Since both the mass and the radius of the earth can be assumed constant, we can use a
small letter g to represent the product of all the constants in this equation. 
g is calculated to be 9.81 meter per second squared in SI unit system, or 32.2 feet per second squared
in the US customary unit system. 
Therefore, the weight of an object can be simply calculated as m times g. Keep in mind 
that g could vary depending on the different locations on the earth,
so you might have different weights depending on where you stand.
