2.2 Forces and Free-Body Diagrams

Learning objectives

2.2A—Describe a force as an interaction between two objects or systemsDescribe a force as an interaction between two objects or systems.• Forces are vector quantities that describe the interactions between objects or systems.- i. A force exerted on an object or system is always due to the interaction of that object with another object or system.- ii. An object or system cannot exert a net force on itself.• Contact forces describe the interaction of an object or system touching another object or system and are macroscopic effects of interatomic electric forces. TOPIC 2.2 Forces and Free-Body Diagrams 2.2B—Describe the forces exerted on an object or system using a free-body diagramDescribe the forces exerted on an object or system using a free-body diagram.• Free-body diagrams 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 by the environment.• Forces exerted on an object or system are represented as vectors originating from the representation of the center of mass, such as a dot. A system is treated as though all of its mass is located at the center of mass. AP Physics 1: Algebra-Based Course and Exam Description Force and Translational Dynamics UNIT 2• A coordinate system with one axis parallel to the direction of acceleration of the object or system simplifies the translation from freebody 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. BOUNDARY STATEMENT AP Physics 1 only expects 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 force. Individual forces that are in the same direction must be drawn side by side, not overlapping. AP Physics 1: Algebra-Based Course and Exam Description Force and Translational Dynamics UNIT 2 TOPIC 2.3 Newton’s Third Law

Name source and target

A force is a vector that describes an interaction: one object or system exerts the force on another. Write $\vec F_{A\,on\,B}$ to name both the source $A$ and the target $B$ ; its direction tells how $A$ pushes or pulls $B$ .

Classify the interaction

Interaction	Objects involved	What the force means
Contact	Book and table touching	The table exerts an upward contact force on the book
Noncontact	Earth and book	Earth exerts a downward gravitational force on the book
Internal to a chosen system	Two parts inside the boundary	Parts exert forces on each other, but the system does not exert a net force on itself

Understand contact forces

At the macroscopic scale, normal force, friction and tension are contact interactions. Their microscopic origin is electromagnetic interaction between atoms, even when surfaces appear rigid.

Reject force-like inventions

A force is not something an isolated object 'has,' and motion does not require a force in the direction of motion. To validate a claimed force, identify the second interacting object and state who exerts what force on whom.

Isolate one system

A free-body diagram (FBD) isolates one object or system and shows every external force exerted on it. Replace the chosen system by a dot at its center of mass; the completed diagram is the bridge from the physical situation to force equations.

Draw only external forces

Step	AP-compliant action
1. Isolate	Name exactly one object or system
2. Inventory	For each interaction crossing its boundary, identify the force on the system
3. Draw	Use one labeled straight arrow per force, starting at the center-of-mass dot
4. Separate	Draw same-direction forces side by side, never overlapping
5. Choose axes	Align an axis with acceleration when useful; on an incline, choose an axis parallel to the surface

Choose helpful axes

Example: for a block sliding on a rough incline, the FBD contains the gravitational force by Earth, the normal force by the surface, and—when applicable—the friction force by the surface. Choose axes parallel and perpendicular to the incline, then resolve forces algebraically after the FBD is drawn.

Keep components out of the FBD

Do not draw velocity, acceleration, the path, or a force exerted by the chosen system on something else. AP Physics 1 expects individual forces—not their components—on the FBD. Components belong in the later algebra, not as extra arrows on the diagram.

Next Topic

—2.3 Newton’s Third LawRead