Dynamics:       The physics of interactions between forces.

Force:     A simple definition for force is a push or a pull on an object.   There four fundamental forces of nature:

1. Strong nuclear force:  forces which hold together the nuclei of atoms.
2. Weak nuclear force:  forces encountered during the decay of atoms.
3. Electromagnetic force:  forces of attraction between charges and interaction between charges and magnetic fields.
4. Gravitational force:  forces between masses caused by a gravitational field (acceleration due to gravity).  It is the weakest of the four fundamental forces of nature.

Consider a classmate with many other classmates around him/her all pushing on him/her with a large force yet preventing him from moving anywhere.  Why does the classmate not move anywhere? 

The answer is that all of the forces combined result in a leftover/unbalanced/net force of zero.  In other words all of the forces acting on the person cancel each other out. 

Isaac Newton's Laws:

1. An object which is moving at a constant velocity, will continue to move at that velocity unless acted on by a net force (unbalanced force).
2. If an object is acted on by a no-zero net force, then the object will accelerate in the direction of that net force according to the equation:                              

F_net = ma 

3. For every applied force, there is an equal force in the opposite direction.   

Note: do not confuse net force with applied force; net force is the result after adding all applied forces together.       

Force diagrams: In order to make solving problems easier, force diagrams should be made of every problem.  Force diagrams should consist of all applied forces involved in the problem:

Example:      A 20 kg box lies on a table.  A small boy wishes to accelerate the box toward the edge of the table at a rate of 1.5 m/s².  If the force of friction acting against this motion is 50 N, how much force must the boy apply to the box?   Solution:                               

m = 20 kg

g = 9.8 m/s²

a = 1.5 m/s²

F_f = -50 N                                                         

F_N = -F_g  so they cancel and don't affect the motion 

***Remember:  F_N is ONLY the same as F_g for an object that is on a level surface with NO OTHER vertical forces acting on it. 

F_net = ma
F_net = F_f + F_app    (ie.  Fnet = sum of all forces on the object)

F_app = F_net - F_f        (but F_f is a negative number)

F_app = (20 kg)(1.5 m/s²) - - 50 N

F_app = (30) + 50 N

F_app = 80 N

Normal Force The reason we do not fall into the ground when standing on it is that we are experiencing a force opposite to gravity applied by the ground to our bodies.  We refer to this force as a normal force (F_N) ("normal" meaning "perpendicular to").  

***Note:  The normal force is ONLY equal to the force of gravity for an object sitting on a level surface with NO OTHER vertical forces acting on it.  For a counter example, the normal force on a book pressed against the wall has NOTHING to do with gravity.