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AP Course

AP Physics 1

Updated for the AP Physics 1 framework effective fall 2024, including mechanics, rotation, oscillations, and fluids.

Study every current unit through original concept lessons, formula derivations, experiments, quizzes, and practice sets.

Lessons
Scalars and Vectors in One DimensionDisplacement, Velocity, and AccelerationRepresenting MotionReference Frames and Relative MotionVectors and Motion in Two DimensionsSystems and Center of MassForces and Free-Body DiagramsNewton's Third LawNewton's First LawNewton's Second LawGravitational ForceKinetic and Static FrictionSpring ForcesCircular MotionTranslational Kinetic EnergyWorkPotential EnergyConservation of EnergyPowerLinear MomentumChange in Momentum and ImpulseConservation of Linear MomentumElastic and Inelastic CollisionsRotational KinematicsConnecting Linear and Rotational MotionTorqueRotational InertiaRotational Equilibrium and Newton’s First Law in Rotational FormNewton’s Second Law in Rotational FormRotational Kinetic EnergyTorque and WorkAngular Momentum and Angular ImpulseConservation of Angular MomentumRollingMotion of Orbiting SatellitesDefining Simple Harmonic Motion (SHM)Frequency and Period of SHMRepresenting and Analyzing SHMEnergy of Simple Harmonic OscillatorsInternal Structure and DensityPressureFluids and Newton’s LawsFluids and Conservation Laws
Quizzes
Practice Problems Formula notes, diagrams, and practice sets will be added here.

AP Physics 1 · Unit 2 · Reviewed representative lesson

Forces and Free-Body Diagrams

A free-body diagram represents one selected system and every external force acting on that system. It is useful because the vector sum of those forces determines the system's acceleration.

Definition

An external force is an interaction exerted on the chosen system by something outside it. Weight, normal force, tension, friction, and applied forces belong on the diagram only when the corresponding interaction acts on that system.

Key Formula

\[\sum \vec F_{\mathrm{external}}=m\vec a\]

The equation is vector-valued. After choosing axes, write a separate component equation for each direction. A zero component of acceleration means the force components in that direction sum to zero; it does not require every force to vanish.

Worked Example

Question: A \(5.0\,\mathrm{kg}\) block is pulled horizontally with \(20\,\mathrm N\). Kinetic friction is \(5.0\,\mathrm N\) opposite the motion. Find the acceleration.

\[\sum F_x=20-5.0=15\,\mathrm N\]
\[a_x=\frac{15\,\mathrm N}{5.0\,\mathrm{kg}}=3.0\,\mathrm{m\,s^{-2}}\]

Answer: The block accelerates at \(3.0\,\mathrm{m\,s^{-2}}\) in the pulling direction.

Why This Method Works

Drawing the interactions before writing equations prevents a force from being invented to match the motion. The acceleration direction follows the net force, while velocity describes how the object is already moving.

Common Mistakes

  • Drawing velocity or acceleration as a force.
  • Putting both an angled force and its components on the final diagram.
  • Placing a Newton's-third-law partner force on the same object's diagram.
  • Assuming normal force always equals weight.

Key Takeaways

  • Name the system before naming forces.
  • Draw one arrow for each external interaction.
  • Use component sums and signs from the chosen axes.
Practice connection

Use the quiz and practice tabs to analyze inclines, friction, tension, and multi-object systems.

Official curriculum reference: College Board AP Physics 1 course page. The explanation and worked example are independently written for this study site.