Newtons Third Law

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The hammer strikes the nail exerting a force on the nail.  Newtons third law states that if a force is exerted on an object, the object exerts a force back. So the nail exerts a force back on the hammer. But it is critical that you understand this.

The Force Exerted back is the same TYPE and MAGNITUDE just in the opposite direction

A force is a push or a pull upon an object which results from its interaction with another object. Forces result from interactions! According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion. Formally stated, Newton's third law is:

For every action, there is an equal and opposite reaction.

Definiation provided by Physics Classroom

Why I Really like Newtons laws of Motion

I feel of all of the "laws" of physics Newton's third is one of the most philosophical, it has a sort of Zen Buddhist.  If you Attempt to exact a change in an object, (applying a force) the attempt exacts a change in you.  If you apply Newton's second law to this philosophical argument, as one person attempts to exact a change on a large group of people or system,  that one person would experience a greater personal change in the "opposite direction,"  in a sense attempting to change a system will more then likely result in your own corruption.