More on Newton's Laws, from Scott Anderson, Astronomy Open Course, http://www.opencourse.info/astronomy/introduction/06.motion_gravity_laws/

• Newton's First Law of Motion states:

In other words, an object will never change its speed or direction unless something comes along and forces it to do so.

This is an example of the principle of Conservation of Linear Momentum.

An object's resistance to changes in motion is known as inertia

 

• The First Law was actually stated a century earlier by Galileo (and even many centuries earlier than that by several others).
  
  However, the First Law was in direct contradiction to the still-dominant teachings of Aristotle, who thought that all objects in motion will eventually come to rest of their own accord.
  
  Newton therefore felt it necessary to make a forceful contradiction.

• Newton's Second Law of Motion describes how a force changes the motion of an object:
  
  

  F = ma

Here, F describes the force acting, m is the mass of the object, and a is its acceleration, the change in its motion.


• Like motion, force has both a value or "strength", and a direction in which it acts.
  
  We commonly think of "acceleration" as a speeding up, but in physics it can also include a slowing down ("deceleration") as well as a change in direction.

• We can deduce several things about moving objects by considering Newton's Second Law, F = ma:
   
  
  • For example, the larger the force is, the larger the acceleration:

  

  • Second, the Second Law actually includes the First Law within it.
    
    Question: why is this?
    
    
  • Third, for an equal force, a larger mass must have a smaller acceleration, and vice versa:

  

  In other words, a larger mass has greater inertia than a smaller mass.
  

• Newton's Third Law of Motion describes two additional characteristics of forces:
  
  

The force on an object is always due to another object, and that other object always feels an equal and opposite force.

You are familiar with this law from striking an object with your hand: the object moves as a result, but your hand also feels a force, and bounces back:

Question: if the forces are equal, why does the smaller object bounce back faster?