Physics 30: Momentum, Forces, and Fields - Core Concepts and Exam Traps
What the exam expects you to control
Physics 30 rewards clean vector bookkeeping and confident use of conservation laws. The candidate must fluently handle: momentum as a vector (p = mv), impulse as change in momentum (FΔt = Δp), and conservation of momentum in one and two dimensions. For fields, you need electric force and field (F = kq₁q₂/r², E = F/q, E = kQ/r²), gravitational field and potential energy, and magnetic force on moving charges and current-carrying wires (F = qvB sinθ, F = BIL sinθ). Electromagnetic induction (Faraday's law, Lenz's law) and the motor/generator distinction also appear regularly, as does the conservation of energy and charge in field problems.
Traps that cost marks
Direction is the most common failure. Momentum and impulse are vectors, so a reversal of sign changes the answer - a ball rebounding off a wall has Δp equal to the sum of magnitudes, not the difference. In two-dimensional collisions, resolve into components and conserve each axis separately. For charged particles in magnetic fields, remember the force is perpendicular to velocity, so the particle moves in a circle with radius r = mv/(qB) and the magnetic force does no work. Students frequently forget to convert microcoulombs and centimetres to SI units before substitution. In induction questions, apply Lenz's law to find the direction of the induced current by opposing the change in flux, not the flux itself.
Rule of thumb
Always draw a before/after diagram with a coordinate axis and write down knowns in SI units before touching a formula. If the question asks for a direction, state it relative to a named axis.
Worked example
A 0.40 kg ball strikes a wall at 6.0 m/s and rebounds at 4.0 m/s. Taking the initial direction as positive: Δp = m(v_f − v_i) = 0.40(−4.0 − 6.0) = −4.0 kg·m/s. The impulse is 4.0 N·s directed away from the wall.
Study checklist
- Momentum and impulse as vectors
- Conservation in 1D and 2D
- Electric and gravitational fields and potential energy
- Magnetic force on charges and wires, circular motion
- Faraday's and Lenz's laws, motor and generator principles