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All forces share certain common characteristics when considered by observers in inertial reference frames.
3.A.2: Forces are described by vectors. a. Forces are detected by their influence on the motion of an object. b. Forces have magnitude and direction.
3.A.2.1 - Represent forces in diagrams or mathematically using appropriately labeled vectors with magnitude, direction, and units during the analysis of a situation. [SP 1.1]
3.2.3: A force exerted on an object is always due to the interaction of that object with another object. a. An object cannot exert a force on itself. b. Even though an object is at rest, there may be forces exerted on that object by other objects. c. The acceleration of an object, but not necessarily its velocity, is always in the direction of the net force exerted on the object by other objects.
3.A.3.1 Analyze a scenario and make claims (develop arguments, justify assertions) about the forces exerted on an object by other objects for different types of forces or components of forces. [SP 6.4, 7.2]
3.A.3.2 Challenge a claim that an object can exert a force on itself. [SP 6.1]
3.A.3.3 Describe a force as an interaction between two objects, and identify both objects for any force. [SP 1.4]
3.A.4 If one object exerts a force on a second object, the second object always exerts a force of equal magnitude on the first object in the opposite direction.
3.A.4.1 Construct explanations of physical situations involving the interaction of bodies using Newton’s third law and the representation of actionreaction pairs of forces. [SP 1.4, 6.2]
3.A.4.2 Use Newton’s third law to make claims and predictions about the action-reaction pairs of forces when two objects interact. [SP 6.4, 7.2]
3.A.4.3 Analyze situations involving interactions among several objects by using free-body diagrams that include the application of Newton’s third law to identify forces. [SP 1.4]
Free-body diagrams are useful tools for visualizing forces being exerted on a single object and writing the equations that represent a physical situation.
a. An object can be drawn as if it were extracted from its environment and the interactions with the environment were identified.
b. A force exerted on an object can be represented as an arrow whose length represents the magnitude of the force and whose direction shows the direction of the force.
c. A coordinate system with one axis parallel to the direction of the acceleration simplifies the translation from the free-body diagram to the algebraic representation.
d. Free-body or force diagrams may be depicted in one of two ways—one in which the forces exerted on an object are represented as arrows pointing outward from a dot, and the other in which the forces are specifically drawn at the point on the object at which each force is exerted.
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