AP Physics 1 / Topic
Linear Momentum
Unit 4 covers momentum and impulse, conservation of momentum in collisions and explosions, elastic versus inelastic collisions, and center of mass.
What the exam asks
- Relate impulse to momentum change and to force-time graphs
- Apply conservation of momentum to 1D and simple 2D collisions
- Classify collisions: elastic (KE conserved) versus perfectly inelastic (objects stick)
- Explain why KE is lost in inelastic collisions while momentum is not
- Locate and track the center of mass of a system
Key formulas and rules
p = mv; J = FΔt = Δp; area under F-t graph
m1v1 + m2v2 = m1v1' + m2v2' (isolated system)
m1v1 + m2v2 = (m1 + m2)v'
elastic collisions also conserve ½mv² totals
x(cm) = Σmixi / Σmi; no external force ⇒ v(cm) constant
Question bank breakdown
41
3
10 easy · 22 medium · 9 hard
Skills covered
Every question in the bank comes with a per-choice explanation, so you learn why each wrong answer is wrong — not just the key.
Sample question
From the free tier of the ScoreMint AP Physics 1 bank — try it, then check the answer.
A $0.50 \text{ kg}$ ball traveling at $8.0 \text{ m/s}$ to the right strikes a wall and bounces back at $6.0 \text{ m/s}$ to the left. The ball is in contact with the wall for $0.020 \text{ s}$. What is the magnitude of the average force exerted by the wall on the ball?
- A. $50 \text{ N}$
- B. $150 \text{ N}$
- C. $350 \text{ N}$
- D. $200 \text{ N}$
Show answer and explanation
Answer: C. Impulse equals the change in momentum: $J = \Delta p = m(v_f - v_i)$. Since momentum is a vector, direction matters critically. With rightward as positive: $v_i = +8.0$ m/s and $v_f = -6.0$ m/s. Thus $\Delta v = -6.0 - (+8.0) = -14$ m/s. The change in momentum is $0.50 \times (-14) = -7.0$ kg m/s (the negative sign means the impulse is to the left, which makes sense). The average force magnitude is $|F| = 7.0/0.020 = 350$ N. The most common error is subtracting speeds ($8 - 6 = 2$) instead of accounting for the reversal of direction.
Study tip
Momentum is a vector: pick a positive direction before plugging in, and keep the sign on every velocity. Most collision errors are sign errors, not physics errors.