Rucete ✏ AP Physics 1 In a Nutshell
3. Forces and Newton’s Laws of Motion
This chapter explores the concept of forces and how they govern motion using Newton’s three laws. Topics include types of forces, equilibrium, free-body diagrams, applications in static and dynamic systems, circular motion, and friction.
Types of Forces
• A force is an interaction (push or pull) that can cause acceleration.
• Contact forces: friction, tension, normal, push/pull, spring.
• Long-range forces: gravity, electricity, magnetism.
• Weight (Fg) = mg, where g = 9.8 N/kg on Earth.
• Mass is a scalar (inertia); weight is a vector (force).
Force Equilibrium
• Net force = 0 → object is in equilibrium (static or dynamic).
• Static equilibrium: object at rest.
• Dynamic equilibrium: object moving at constant velocity.
• All forces balance out; no acceleration.
Newton’s First Law (Law of Inertia)
• An object remains at rest or moves at constant velocity unless acted on by a net external force.
• Inertia: resistance to change in motion (depends on mass).
• Inertial frame of reference: no acceleration; constant velocity.
Newton’s Second Law
• Net force causes acceleration: Fnet = ma
• Acceleration is in the direction of the net force.
• Units: 1 N = 1 kg·m/s²
Newton’s Third Law
• For every force, there is an equal and opposite force.
• Action-reaction pairs act on different objects.
• They do not cancel because they are applied to separate bodies.
• Example: Earth pulls on you → you pull back on Earth with equal force.
Free-Body Diagrams (FBDs)
• Diagram that shows all forces acting on a single object.
• Object is represented as a point or box; vectors indicate force direction and relative magnitude.
• Only include forces acting *on* the object (not forces the object exerts on others).
• Typical forces: gravity (mg), normal force, tension, applied force, friction.
Static and Dynamic Applications
Horizontal Systems
• No net force: all forces balance (e.g., pushing box at constant speed).
• With acceleration: Fnet = ma; applied force must overcome friction.
Inclined Planes
• Break gravity into components: – Parallel: mg·sin(θ) (pulls down slope) – Perpendicular: mg·cos(θ) (balances normal force)
• Normal force ≠ mg on incline.
Tension and Pulleys
• Ideal pulley: massless and frictionless; same tension throughout the rope.
• Two-mass systems: apply Newton’s second law to each object, then solve system.
Friction
Types of Friction
• Static: resists motion before object starts moving.
• Kinetic: resists motion while object is sliding.
• fstatic ≤ μsN, fkinetic = μkN
Key Concepts
• Friction opposes relative motion.
• Coefficients (μ) depend on surface types.
• Static friction is usually greater than kinetic friction.
In a Nutshell
Forces cause changes in motion according to Newton’s laws. Understanding how to draw and interpret free-body diagrams, apply Newton’s second law, and analyze systems with friction, tension, or inclined planes allows you to predict and explain motion. Force analysis is fundamental to problem-solving in physics.