Force is an interaction between two objects or systems

Force is a vector quantity that describes the interactions between objects or systems. For example, the earth exerts a force on me (the force of gravity) that pulls me towards it. A rocket engine exerts a force on a spaceship to push it faster and faster. A force exerted on an object or system is always due to the interaction of that object or system with another object or system. An object or system cannot exert a net force on itself.

Force is measured in newton (\(\unit{\newton}\)) where \(\qty{1}{\newton}=\qty{1}{\kilo\gram\meter\per\second\squared}\). A non-SI units of force that you may see in engineering practice in the United States is pound-force (lbf); also “kips” (kilo-pound-force, 1 kip = 1000 lbf) in structural engineering; lbf also makes an appearance in pounds per square inch (psi) pressure in piping systems; foot-pounds (ft-lbf) torque in rating engines, etc.

The quantities we dealt with in Unit 1 (Kinematics) only dealt with length (\(\mathcal{L}\)) and time (\(\mathcal{T}\)); now we also see the entrance of mass m (\(\mathcal{M}\)) and quantities that depend or are affected by mass. Mass is measured in kilogram (\(\unit{\kilo\gram}\)), which was a fundamental unit based on a prototype platinum mass in Paris; however, the definition of a standard kilogram has recently been changed to depend on other physical constants. Non-SI units of mass that you may see in the United States include pounds-mass (lbm), ounces, slugs, tons...

Contact forces describe the interaction of an object or system touching another object or system and are macroscopic effects of interatomic electric forces. An example of a contact force might be the normal force of a surface holding up an object.

Forces exerted on an object or system can be described using a free body diagram

Free body diagrams (FBDs) are useful tools for visualizing forces being exerted on a single object or system and for determining the equations that represent a physical situation.

The free body diagram of an object or system shows each of the forces exerted on the object or system by the environment. Internal forces within the system are not typically shown.

⊕Boundary statement: AP Physics C Mechanics and AP Physics C Electricity and Magnetism only expect students to depict the forces exerted on objects, not the force components on free body diagrams. On the AP Physics exams, individual forces represented on a free body diagram must be drawn as individual straight arrows, originating on the dot and pointing in the direction of the ofrce. Individual forces that are in the same direction must be drawn side by side, not overlapping.

For AP Physics C Mechanics purposes, forces exerted on an object or system are represented as vectors originating from the representation of the cetner of mass, such as a dot. A system is treated as though all of its mass is located at the center of mass.

A coordinate system with one axis parallel to the directtion of acceleration of the object or system simplifies the translation from free body diagram to algebraic representation. For example, in a free body diagram of an object on an inclined plane, it is useful to set one axis parallel to the surface of the incline.