Finally, draw a box and add arrows for each existing force in the appropriate direction label each force arrow according to its type. Then determine the direction in which each force is acting. If given a description of a physical situation, begin by using your understanding of the force types to identify which forces are present. Thus, to construct free-body diagrams, it is extremely important to know the various types of forces. The only rule for drawing free-body diagrams is to depict all the forces that exist for that object in the given situation. There is no hard and fast rule about the number of forces that must be drawn in a free-body diagram. There will be cases in which the number of forces depicted by a free-body diagram will be one, two, or three. Objects do not necessarily always have four forces acting upon them. T he free-body diagram above depicts four forces acting upon the object. An example of a free-body diagram is shown at the right It is generally customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction that the force is acting. Each force arrow in the diagram is labeled to indicate the exact type of force. The direction of the arrow shows the direction that the force is acting. The size of the arrow in a free-body diagram reflects the magnitude of the force. These diagrams will be used throughout our study of physics. A free-body diagram is a special example of the vector diagrams that were discussed in an earlier unit. Consider the three situations below in which the net force is determined by summing the individual force vectors that are acting upon the objects.Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. The addition of force vectors can be done in the same manner in order to determine the net force (i.e., the vector sum of all the individual forces). And a leftward vector will provide a partial or full cancellation of a rightward vector. Observe in the diagram above that a downward vector will provide a partial or full cancellation of an upward vector. Observe the following examples of summing two forces: At this point, the rules for summing vectors (such as force vectors) will be kept relatively simple. That is to say, the net force is the sum of all the forces, taking into account the fact that a force is a vector and two forces of equal magnitude and opposite direction will cancel each other out. The net force is the vector sum of all the forces that act upon an object. It is commonly said that in each situation there is a net force acting upon the object. In each of the above situations, there is an unbalanced force. Note that the actual magnitudes of the individual forces are indicated on the diagram. Free-body diagrams for three situations are shown below. The existence of an unbalanced force for a given situation can be quickly realized by looking at the free-body diagram for that situation. If either all the vertical forces (up and down) do not cancel each other and/or all horizontal forces do not cancel each other, then an unbalanced force exists. In the statement of Newton's first law, the unbalanced force refers to that force that does not become completely balanced (or canceled) by the other individual forces. 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. If you have been reading through Lessons 1 and 2, then Newton's first law of motion ought to be thoroughly understood.
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