Forces… Everything you need to know about forces…for now!

What is a force? A force, by definition is: …..any push or pull upon an object resulting from the object's interaction with another object.

How are they measured? Force is a quantity which is measured using a 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 m/s 2. A Newton is abbreviated by an "N."

But that’s not all folks…. Force is a vector quantity. As you learned earlier, a vector quantity is a quantity which has both magnitude and direction. It is usually represented by an arrow (as we did with velocity in motion maps).As you learned earlier To fully describe the force acting upon an object, you must describe both its magnitude (size/amount) and its direction. 10 Newtons, downwards is a complete description of the force acting upon an object.

What kinds of forces are there? Applied Force (F a ) Gravitational Force (F g ) Tension Force (F T ) Normal Force (F n ) Frictional Force (F f )

How are they categorized? 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

Where do each of these go? Contact Forces Frictional Force Applied Force Tension Force Normal Force Action-at-a-Distance Forces Gravitational Force

Definitions…. An applied force is a force which is applied to an object by another object or by a person. Example: If a person is pushing a desk across the room, then there is an applied force acting upon the desk. The applied force is the force exerted on the desk by the person.

Definitions… Gravitational force (The force of gravity) is the force with which the earth, moon, or other massive body attracts an object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth.

Definitions… The normal force is the support force exerted upon an object which is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasion, a normal force is exerted horizontally between two objects which are in contact with each other.

Definitions… The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. The friction force opposes the motion of the object. For example, if a book moves across the surface of a desk, the desk exerts a friction force in the direction opposite to the motion of the book. Friction results when two surfaces are pressed together closely, causing attractive intermolecular forces between the molecules of the two different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together.

Definitions… Tension is the force which is transmitted through a string, rope, or wire when it is pulled tight by forces acting at each end. The tensional force is directed along the wire and pulls equally on the objects on either end of the wire.

Defining the System When you need to decide what forces are present you need to “define the system.” The system is the object and its immediate surroundings. Not everything you see in the diagram will be part of the system.

Recognizing Forces The purpose of this activity is to utilize an understanding of the various types of forces in order to analyze a given situation and to identify the presence (or absence) of a variety of forces. For each problem, list all of the possible forces and then put a ‘P’ (for present) or an ‘A’ (for absent). This activity is your exit pass for today. You must turn this in before you leave!!

Example #1. The object is pulled upward at constant speed.

Example #2 A block slides across the top of a table. Consider only the forces acting upon the block.

Example #3 A block hangs at rest from the ceiling by a piece of rope. Consider the forces acting on the block.

Exit Pass! Remember to turn in your exit pass before you leave!