Forces

 A force is a push or pull upon an object resulting from the object's interaction with another object.  Whenever there is an interaction between two objects, there is a force upon each of the objects.  When the interaction ceases, the two objects no longer experience the force.  Forces only exist as a result of an interaction.

Two Main Forces  For simplicity sake, all forces (interactions) between objects can be placed into two broad categories:  contact forces, and  forces resulting from action-at-a-distance

 Contact forces are types of forces in which the two interacting objects are physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces.

 Action-at-a-distance forces are types of forces in which the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite a physical separation.  Examples of action-at-a-distance forces include gravitational forces, electric forces and magnetic forces.

Unit of Force  Force is a quantity which is measured using the standard metric unit known as the Newton.  One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 ms -2.  A Newton is abbreviated by a "N." To say "10.0 N" means 10.0 Newtons of force. Thus, the following unit equivalency can be stated:

 A force is a vector quantity.  Because a force is a vector which has magnitude and direction, it is common to represent forces using diagrams in which a force is represented by an arrow. Balanced Unbalanced

Determining the Net Force  An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Free Body Diagrams  Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

Construct free-body diagrams for the various situations described.  A book is at rest on a table top. Diagram the forces acting on the book.

 A girl is suspended motionless from a bar which hangs from the ceiling by two ropes.

 An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it is falling.

 A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel.

 A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.

 A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance.

 A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder.

 A skydiver is descending with a constant velocity. Consider air resistance.

 A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration.

Free-body diagrams for four situations are shown below. For each situation, determine the net force acting upon the object.

Now Try This!  Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known.  Analyze each situation individually and determine the magnitude of the unknown forces.