Study guides › C/P › Physics
Momentum & Collisions
Momentum tracks how hard an object is to stop, and it is conserved when nothing external pushes on a system. You will use impulse, solve elastic and inelastic collisions and explosions, and find centers of mass and two-dimensional results.
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What this chapter covers
- Momentum & Impulse. Impulse ties force and time to a change in momentum, which explains airbags, crumple zones, and follow-through.
- Conservation of Momentum & Inelastic Collisions. If no net external force acts on a system, total momentum stays the same, whatever happens inside.
- Elastic Collisions. Elastic collisions conserve both momentum and kinetic energy, which gives two equations for two unknown speeds.
- Center of Mass & Two-Dimensional Collisions. The center of mass moves as if all mass and all outside force were concentrated there, and 2-D collisions split into components.
Key points to know
- p = mv is a vector; impulse J = FΔt = Δp
- Area under an F–t graph is the impulse
- Isolated system: total momentum is conserved in every collision
- Perfectly inelastic: objects stick, maximum KE loss, v_f = Σp/Σm
- Elastic: momentum and KE both conserved
- Relative speed of approach = relative speed of separation
- x_cm = Σmx/Σm; the CM lies nearer the heavier mass
- Internal forces cannot change the velocity of the CM
Try a practice question
Which unit is equivalent to the unit of momentum?
- J/s
- N/s
- N·s
- kg·m/s²
Show the answer
C. Impulse equals the change in momentum, so N·s = (kg·m/s²)·s = kg·m/s. J/s is a watt and kg·m/s² is a newton.
Common questions
What does this chapter cover?
Momentum & Collisions covers Momentum & Impulse, Conservation of Momentum & Inelastic Collisions, Elastic Collisions, Center of Mass & Two-Dimensional Collisions.
What should I know first?
p = mv is a vector; impulse J = FΔt = Δp Area under an F–t graph is the impulse Isolated system: total momentum is conserved in every collision
21 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.