AP Physics 1 — Unit 4: Linear Momentum
Practice questions, answers, and key terms for Unit 4, aligned to the College Board CED.
AP Physics 1 Unit 4 practice questions
A 0.5 kg ball and a 2 kg ball both move at 4 m/s. You want a single number that captures their 'quantity of motion.' Which equation gives it, and which ball has more?
p = mv, so the 2 kg ball (8 kg·m/s) beats the 0.5 kg ball (2). Momentum scales with both mass and velocity.
Two carts have momenta of equal size, 6 kg·m/s, but one rolls east and one rolls west. Treating them as a system, your total isn't 12. Why not?
Momentum is a vector, so direction matters: +6 and −6 cancel out. The system total is 0 kg·m/s, not 12.
During a 0.02 s car crash, the force between the cars is enormous compared to friction or gravity. What does that imbalance let you model the event as?
A collision — internal forces far exceed external ones. So the system's momentum is conserved through the interaction.
A firecracker at rest blows into two chunks that fly apart in opposite directions. Total momentum before was zero. What had to happen to total momentum after, and what kind of interaction is this?
Total momentum stays zero after — the chunks carry equal and opposite momenta. It's an explosion: internal forces drive parts of the system apart.
When analyzing a collision you only write equations for the instant before and the instant after — never the messy middle. What assumption lets you skip the deformation phase entirely?
Only the before and after states matter, so each object is a point. The messy contact forces never need solving.
A glove applies an average 200 N to a ball for 0.05 s. Your friend says the 'impulse' is just 200 N. What are they missing, and what's the actual impulse?
Impulse = force × time: J = FΔt = (200 N)(0.05) = 10 N·s. Force alone isn't impulse; time matters.
A 0.15 kg baseball comes in at 40 m/s and leaves the bat at 50 m/s the other way. Predict the impulse the bat delivered.
J = Δp = m(v_f − v_i) = 0.15(−50 − 40) = −13.5 N·s total. Magnitude 13.5 N·s, directed back where the ball came.
A skater speeds up from 2 m/s to 6 m/s. To find how much her motion changed, do you add her two momenta or subtract them?
Subtract: change in momentum Δp = p_f − p_i = final minus initial. It's a difference, not a sum.
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Key terms in Unit 4
These 12 terms show up in the Unit 4 cards. Each one links to its definition in the AP Physics 1 key-term reference.
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