Unit 4 · Linear Momentum

Linear momentum ● (core concept) — Momentum: p = m·v, a vector pointing in the same direction as the velocity. It is the quantity used to analyze collisions and explosions.

Collision model ● (core concept) — A model for brief interactions in which the forces between the objects far exceed any net external force acting during the event, so analysis needs only the initial and final states.

Explosion model ● (core concept) — A model in which internal forces push parts of a system apart, converting stored energy into the kinetic energy of the fragments.

Impulse ● (core concept) — The product of the average net external force and the time it acts: J = F_avg·Δt, a vector pointing in the direction of the force. Impulse equals the area under a force–time graph.

Impulse-momentum theorem ● (core concept) — The impulse exerted on a system equals the system's change in momentum: J = Δp. Newton's second law of motion is a direct result of the impulse–momentum theorem applied to systems with constant mass.

Force, momentum, and time graphs ● (core concept) — On a graph of net external force versus time, the area under the curve is the impulse. On a graph of momentum versus time, the slope is the net external force.

Conservation of linear momentum ● (core concept) — If the net external force on a system is zero, the system's total momentum is constant: changes for individual parts must balance, and any change in the system's momentum comes from momentum transfer with the surroundings. AP Physics 1 uses it quantitatively in one dimension and semiquantitatively in two.

Center-of-mass velocity ● (core concept) — The velocity of a system's center of mass remains constant when no net external force acts; the system's total momentum equals its total mass times v_cm.

Elastic collision ● (core concept) — A collision in which the system's total kinetic energy is the same before and after, even though individual objects' kinetic energies may change.

Inelastic collision ● (core concept) — A collision in which the system's total kinetic energy decreases; the lost energy is transformed into other forms by nonconservative forces acting during the collision.

Perfectly inelastic collision ● (core concept) — A collision in which the colliding objects stick together and move with one common velocity afterward.