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AP Physics 1 · Unit 2: Force & Translational Dynamics

2.2 Forces and Free-Body Diagrams

What a force is

A force is a push or pull interaction between two objects. Forces are vectors — they have magnitude and direction — measured in newtons (N). Common forces include gravity (weight), normal force, tension, friction, and applied force. Forces arise from an interaction, so each force needs an agent (what's causing it) and an object it acts on.

Free-body diagrams

A free-body diagram (FBD) represents an object as a single point with arrows showing every force acting ON it, drawn from that point in the correct direction, with length roughly proportional to magnitude. Only forces acting on the object are included — never forces the object exerts on something else. FBDs help set up Newton's second law equations.

Contact vs. long-range forces

Forces are categorized as contact forces (normal, friction, tension, applied — require touching) or long-range (field) forces (gravity, electric, magnetic — act at a distance). Normal force is always perpendicular to the contact surface; tension pulls along a string/rope, always away from the object; friction acts parallel to the surface, opposing relative motion or impending motion.

Key terms

Force
A push or pull interaction between two objects; a vector quantity measured in newtons (N).
Free-body diagram (FBD)
A diagram representing an object as a point with arrows showing all forces acting on it, in correct direction and relative magnitude.
Normal force
A contact force perpendicular to the surface of contact, preventing objects from passing through each other.
Contact forces vs. long-range forces
Contact forces (normal, friction, tension, applied) require touching; long-range forces (gravity, electric, magnetic) act at a distance.
Friction
A force parallel to a contact surface that opposes relative or impending sliding motion between surfaces.

Try an AP-style question

A book rests on a table. Which forces belong on its free-body diagram?

  1. Gravity down and normal force up
  2. Gravity down and the book's push on the table
  3. Only gravity down
  4. Normal force up and friction sideways
Show the answer

A. Only forces on the book.

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