Conservation of Momentum
Aligned to HS-PS2-2 — Next Generation Science Standards.
What this lesson teaches
Momentum is mass times velocity (p = mv). When no outside force pushes on a system, the total momentum before an interaction equals the total momentum after. That is why a fired rifle kicks backward while the bullet flies forward.
Worked example
A 60 kg skater at rest throws a 3 kg ball at 4 m/s. Before: total p = 0. After, the ball carries p = 3 x 4 = 12 kg·m/s forward, so the skater must carry 12 kg·m/s backward. Her speed is v = p/m = 12/60 = 0.2 m/s backward.
Practice questions
- Write the equation for the momentum of a single object and define each symbol.
- A 1000 kg car at 20 m/s rear-ends a stopped 1000 kg car and they lock together. Find their shared velocity after impact.
- Explain, using conservation of momentum, why a cannon recoils far slower than the cannonball it fires.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Conservación del Momento Lineal
El momento lineal (cantidad de movimiento) es la masa por la velocidad (p = mv). Cuando ninguna fuerza externa actúa sobre un sistema, el momento total antes de una interacción es igual al momento total después. Por eso un rifle retrocede hacia atrás mientras la bala sale disparada hacia adelante.
Ejemplo: Una patinadora de 60 kg en reposo lanza una pelota de 3 kg a 4 m/s. Antes: p total = 0. Después, la pelota lleva p = 3 x 4 = 12 kg·m/s hacia adelante, así que la patinadora debe llevar 12 kg·m/s hacia atrás. Su rapidez es v = p/m = 12/60 = 0.2 m/s hacia atrás.
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