AP Physics 1 common mistakes
AP Physics 1 · Understand the traps and how to avoid them.
Using a single kinematic equation without first checking whether acceleration is constant, e.g. applying v = v0 + at to an object on a curved or variable-force path.
The four kinematic equations are memorized as a default toolkit, and students pattern-match on given variables (v0, v, a, t) without testing the constant-acceleration precondition.
What to do instead
Before choosing an equation, write down whether a is constant and along which axis. If a is not constant, switch to energy, momentum, or graphical area/slope methods; if motion is 2D, resolve into independent x and y equations.
Treating a free-body diagram as optional and writing net force as the sum of all forces in the problem statement, including forces that act on a different object.
Students conflate the system with its surroundings, so a normal force on the table or a reaction partner gets added to the object's net force.
What to do instead
Draw the boundary of the chosen system first, then draw only forces whose source is outside that boundary. For each force, name the other object exerting it; if that object is inside the system, delete the force.
Assuming a force does work whenever it is present, or assuming zero work whenever displacement is zero at a single instant, and ignoring that work depends on the component of force along displacement.
The everyday meaning of 'work' is effort, and the dot product W = F d cos(theta) is applied with the wrong angle or with the wrong displacement (e.g. the displacement of the wrong object).
What to do instead
For every work term, explicitly write the angle between the force and the displacement of the point of application. Identify forces perpendicular to motion (normal, tension in circular motion) and mark their work as zero before computing.
Applying conservation of mechanical energy to a system where non-conservative forces do external work, or forgetting to include rotational kinetic energy in the energy balance.
Energy conservation is learned first in frictionless contexts, and students default to it without auditing which forces cross the system boundary or whether an object rolls.
What to do instead
List every force doing work and classify it conservative or non-conservative. If any non-conservative external work exists, use W_nc = delta-E. When an object rolls without slipping, include (1/2) I omega^2 alongside (1/2) m v^2.
Treating momentum as conserved in every collision without checking for a net external impulse, particularly in explosions or collisions with friction or a pivot.
Momentum conservation is taught alongside energy conservation, and students transfer the habit without the impulse condition.
What to do instead
Before writing p_initial = p_final, state the direction(s) in which net external impulse is negligible over the collision interval. Use impulse-momentum (J = delta-p) for each object when an external impulse is significant.
Assuming mechanical energy is conserved in an inelastic collision, or assuming kinetic energy is lost when it is not.
Students conflate 'momentum is conserved' with 'kinetic energy is conserved' and do not classify the collision type.
What to do instead
Classify the collision first: elastic (kinetic energy conserved), inelastic (momentum only), perfectly inelastic (objects stick, maximum loss). Use momentum to find final velocities, then compute kinetic energy change separately.
Computing torque as F times the distance to the pivot rather than the perpendicular distance to the line of action, and ignoring the sign convention.
Students use the distance to the point of application because it is visually salient, and they do not consistently assign clockwise/counterclockwise signs.
What to do instead
Extend the line of action of the force and drop a perpendicular from the pivot; that perpendicular distance is the lever arm. Adopt a sign convention (counterclockwise positive) and write the sum of torques with explicit signs before solving.
Using the moment of inertia of a point mass or of a different shape instead of the shape and axis specified, and forgetting the parallel-axis theorem when the axis is not through the center of mass.
Students memorize a short list of I formulas without tracking which axis each applies to, and the parallel-axis term md^2 is easily omitted.
What to do instead
For each rotating body, write the shape, the axis, and whether the axis passes through the center of mass. If not, apply I = I_cm + md^2 with d measured from the center of mass to the actual axis.
Treating angular frequency, frequency, and period as interchangeable, or using omega = 2 pi f with degrees instead of radians.
The symbols omega, f, and T appear in similar-looking formulas, and calculator mode errors (degrees vs radians) go unnoticed.
What to do instead
Write the unit next to each quantity (rad/s, Hz, s) and convert with omega = 2 pi f = 2 pi / T. Set the calculator to radian mode and check that a full cycle corresponds to 2 pi radians.
Applying the wrong fluid relationship, e.g. using gauge pressure where absolute pressure is required, or applying Bernoulli's equation across a viscous or non-steady flow.
Gauge and absolute pressure differ by atmospheric pressure, and Bernoulli's equation is learned as a universal tool without its assumptions.
What to do instead
State whether the problem asks for gauge or absolute pressure and add or subtract P_atm explicitly. Before using Bernoulli, confirm steady, incompressible, non-viscous flow along a streamline; otherwise use continuity plus pressure-depth relations.