⚙️AP Physics C: Mechanics (2025) Unit 2 – Force and Motion Dynamics

Key Concepts and Definitions

• Force is any interaction that changes an object's motion or shape when unopposed (push, pull, stretch, compress)
• Mass measures the amount of matter in an object and its resistance to acceleration
• Acceleration is the rate of change of velocity over time $a = \frac{\Delta v}{\Delta t}$
• Weight is the force exerted on an object due to gravity $w = mg$
  • Varies depending on the strength of the gravitational field (Earth, Moon)
• Friction is a force that opposes the relative motion between two surfaces in contact
  • Types include static friction, kinetic friction, and rolling friction
• Tension is the force exerted by a string, cable, or rope on an object when pulled taut
• Normal force is the force exerted by a surface on an object in contact with it, perpendicular to the surface

Fundamental Laws and Principles

• Newton's First Law of Motion (Law of Inertia) states that an object at rest stays at rest, and an object in motion stays in motion with constant velocity, unless acted upon by an unbalanced force
• Newton's Second Law of Motion relates the net force acting on an object to its mass and acceleration $\vec{F}_{net} = m\vec{a}$
  • Enables the prediction of an object's motion when forces are known
• Newton's Third Law of Motion states that for every action force, there is an equal and opposite reaction force
  • Action-reaction force pairs act on different objects (book on table, car tires on road)
• Conservation of Energy principle states that energy cannot be created or destroyed, only converted from one form to another
  • Includes kinetic energy, potential energy, and other forms (thermal, chemical, electrical)
• Work-Energy Theorem relates the net work done on an object to the change in its kinetic energy $W_{net} = \Delta KE$
• Impulse-Momentum Theorem states that the impulse applied to an object equals the change in its momentum $\vec{J} = \Delta \vec{p}$

Types of Forces

• Contact forces require physical contact between objects (normal force, friction, tension)
• Field forces act through empty space without contact (gravitational, electric, magnetic)
• Gravitational force is an attractive force between any two objects with mass $F_g = G\frac{m_1m_2}{r^2}$
  • Strength depends on the masses and distance between the objects
• Elastic force is exerted by a deformed spring or elastic material, proportional to the displacement from equilibrium $F_s = -kx$
• Drag force (air resistance) opposes the motion of an object through a fluid, depends on shape and velocity
• Centripetal force is a center-seeking force that causes an object to follow a curved path (uniform circular motion)
  • Examples include gravity, tension, and friction (planets orbiting the Sun, car rounding a curve)
• Buoyant force is an upward force exerted by a fluid on an object immersed in it, equals the weight of the displaced fluid

Motion Analysis and Kinematics

• Kinematics is the study of motion without considering the forces causing it
• Displacement is the shortest distance between the initial and final positions of an object, a vector quantity
• Velocity is the rate of change of position over time, a vector quantity $\vec{v} = \frac{\Delta \vec{r}}{\Delta t}$
  • Average velocity is the displacement divided by the time interval $\vec{v}_{avg} = \frac{\Delta \vec{r}}{\Delta t}$
  • Instantaneous velocity is the velocity at a specific instant in time, found by taking the limit as $\Delta t$ approaches zero
• Acceleration is the rate of change of velocity over time, a vector quantity $\vec{a} = \frac{\Delta \vec{v}}{\Delta t}$
  • Can be caused by a change in speed, direction, or both
• Kinematic equations describe the motion of an object under constant acceleration
  • $\vec{v} = \vec{v}_0 + \vec{a}t$, $\vec{r} = \vec{r}_0 + \vec{v}_0t + \frac{1}{2}\vec{a}t^2$, $\vec{v}^2 = \vec{v}_0^2 + 2\vec{a}(\vec{r} - \vec{r}_0)$
• Projectile motion is the motion of an object launched into the air, subject only to gravity
  • Horizontal and vertical components of motion are independent (golf ball, football)

Dynamics Applications

• Inclined planes are surfaces tilted at an angle, where the gravitational force is resolved into components parallel and perpendicular to the surface
  • Affects the acceleration of objects sliding down the plane (skier, car on a hill)
• Atwood machines consist of two objects connected by a massless pulley and string, used to study Newton's Second Law
  • Tension in the string and the masses determine the acceleration of the system
• Pulleys are simple machines that change the direction of a force, provide mechanical advantage, or both
  • Types include fixed, movable, and compound pulleys (elevators, cranes)
• Pendulums are objects suspended from a pivot that swing back and forth under the influence of gravity
  • Period of a simple pendulum depends on its length and the acceleration due to gravity $T = 2\pi\sqrt{\frac{L}{g}}$
• Collisions involve two or more objects exerting forces on each other for a short time
  • Elastic collisions conserve both momentum and kinetic energy (pool balls)
  • Inelastic collisions conserve momentum but not kinetic energy (car crashes)
• Rotational dynamics studies the motion of objects rotating about an axis
  • Torque is the rotational equivalent of force, causes angular acceleration $\vec{\tau} = \vec{r} \times \vec{F}$
  • Moment of inertia is the rotational equivalent of mass, depends on the object's mass distribution $I = \sum mr^2$

Problem-Solving Strategies

• Identify the relevant concepts, principles, and equations for the problem
• Draw a clear and labeled diagram of the system, including all forces acting on the objects
• Establish a coordinate system and define positive and negative directions
• List the known and unknown quantities, and identify the target variable to solve for
• Apply the appropriate equations and solve for the target variable, checking units and signs
• Evaluate the reasonableness of the answer based on physical intuition and limiting cases
  • Consider extreme values of variables (mass approaching zero or infinity)
• Check the answer by substituting it back into the original equation or using an alternative method
• Reflect on the problem-solving process and identify any conceptual insights or generalizations

Lab Experiments and Demonstrations

• Friction block experiment measures the coefficients of static and kinetic friction between surfaces
  • Vary the normal force and observe the force required to initiate and maintain motion
• Ballistic pendulum demonstration illustrates the principles of conservation of momentum and energy in collisions
  • A projectile fired into a pendulum causes it to swing, allowing calculation of the projectile's velocity
• Centripetal force demonstration using a mass whirled on a string shows the relationship between mass, velocity, and radius
  • Observe the effect of changing each variable on the tension in the string
• Hooke's Law experiment verifies the linear relationship between the force exerted by a spring and its displacement
  • Measure the spring constant by applying known forces and measuring the displacement
• Projectile motion experiment launches a ball horizontally and measures its range for different initial velocities
  • Compare the experimental range to the predicted value based on the equations of motion
• Atwood machine experiment measures the acceleration of the system for different mass combinations
  • Verify the linear relationship between the net force and acceleration predicted by Newton's Second Law

Common Misconceptions and Pitfalls

• Confusing mass and weight, which are related but distinct concepts
  • Mass is an intrinsic property of an object, while weight depends on the gravitational field
• Neglecting to consider all forces acting on an object, especially hidden forces like friction and air resistance
• Misinterpreting Newton's Third Law and assuming that action-reaction forces cancel each other out
  • Action-reaction forces act on different objects and do not cancel (book on table, car accelerating)
• Misapplying kinematic equations by using them in situations where acceleration is not constant
• Incorrectly resolving vectors into components or failing to consider the vector nature of quantities like displacement, velocity, and force
• Confusing the signs of vectors and scalars, especially when dealing with displacement and distance
• Treating static situations as dynamic ones, or vice versa
  • Static equilibrium requires zero net force, while dynamics involves non-zero net forces
• Misunderstanding the work-energy theorem and assuming that work is always positive
  • Work can be positive, negative, or zero depending on the angle between the force and displacement vectors