30 JANUARY 2026 14:30- 16:00 PHYSICAL SCIENCES GRADE 12

Momentum in Grade 12 Physical Sciences is treated as the bridge between motion, forces, and collisions—then extended to impulse, safety design, and conservation laws. The lesson centers on the core definition that momentum (p) equals mass times velocity (p = m v). Because velocity includes both magnitude and direction, linear momentum is a vector quantity, meaning answers must include direction (often handled with a chosen positive direction in one-dimensional problems). Momentum only exists when an object is moving; if velocity is zero (object at rest), momentum is zero.

From there, the session drills the “change” idea that appears throughout exam questions. Change in momentum is always final minus initial, so Δp = p_f − p_i. With constant mass, this becomes m(v_f − v_i) = mΔv. The lesson then connects momentum to Newton’s Second Law in momentum form: the net force equals the rate of change of momentum, written as F_net = Δp/Δt. This relationship is used to interpret how velocity changes under a net force—velocity increasing in the direction of motion increases momentum, while braking or reversing direction decreases momentum or flips its sign depending on the chosen positive direction.

Impulse is introduced as the force–time counterpart to momentum change. Impulse is defined as the product of net force and the time interval over which it acts: J = F_net Δt. The impulse–momentum theorem links the two quantities directly: J = Δp. That means exam problems can ask for net force, time, velocity, or change in momentum using the same relationship. The lesson emphasizes that impulse and momentum change are the same “story” told in different units and forms.

A major application ties these physics ideas to real-world safety. Airbags, seat belts, and arrest beds reduce injury by increasing contact time during a collision. Since net force is tied to the rate of change of momentum, increasing the time interval lowers the average net force for the same momentum change—so the impact is less severe even though the collision still changes the vehicle occupant’s momentum.

The session also sets up collision modeling using isolated systems. An isolated system has no net external force (F_net = 0), so momentum conservation applies: the total linear momentum of an isolated system remains constant. Internal forces act during collisions, but external forces are excluded. Finally, the lesson distinguishes elastic versus inelastic collisions: elastic collisions conserve both momentum and total kinetic energy, while inelastic collisions conserve momentum but not kinetic energy.

To prepare for problem-solving, the lesson repeatedly stresses exam technique: write down given data and unknowns, use correct SI units (mass in kg, velocity in m/s), choose a consistent positive direction, and substitute into formulas without rearranging them incorrectly. Worked examples include calculating momentum of a cricket ball, solving for mass or velocity using p = m v, applying conservation of momentum to trolley collisions (including stationary objects and coupled motion), using velocity–time graphs to extract changes in velocity and compute net force, and multi-object momentum problems involving Peter, John, and a trolley. The takeaway is that momentum, impulse, and conservation laws form a single toolkit for both calculations and interpreting collisions and safety engineering.