Conservation of Momentum Flashcards

All 6 cards in this deck

State the law of conservation of momentum.

In a closed system (no external forces), the total momentum before an event equals the total momentum after it.

What are the steps to find the velocity of two objects that stick together in a collision?

e.g. a $2\ \mathrm{kg}$ trolley at $3\ \mathrm{m/s}$ hits a stationary $1\ \mathrm{kg}$ trolley and they join

1. Total momentum before: $2 \times 3 = 6\ \mathrm{kg\ m/s}$
2. Total mass after: $2 + 1 = 3\ \mathrm{kg}$
3. Velocity after $= 6 \div 3 = 2\ \mathrm{m/s}$

What are the steps to find a velocity after an explosion (two objects pushed apart from rest)?

e.g. a stationary $1\ \mathrm{kg}$ trolley and $2\ \mathrm{kg}$ trolley spring apart, the $1\ \mathrm{kg}$ one at $4\ \mathrm{m/s}$

1. Total momentum before $= 0$
2. So the two momenta after are equal and opposite: $1 \times 4 = 4\ \mathrm{kg\ m/s}$ one way, $4\ \mathrm{kg\ m/s}$ the other
3. Velocity of $2\ \mathrm{kg}$ trolley $= 4 \div 2 = 2\ \mathrm{m/s}$ in the opposite direction

In a trolley collision investigation, why is the runway set at a slight slope?

To compensate for friction, so the trolley travels at a constant speed.

What are the steps to investigate conservation of momentum using trolleys?

e.g. trolley A runs into stationary trolley B and they stick together

1. Measure the mass of each trolley on a balance.
2. Use light gates to measure A's velocity before and the combined velocity after.
3. Compare momentum before ($m_A u_A$) with momentum after ($(m_A + m_B)v$) — they should be equal.

True or false? When two objects collide and stick together, some momentum is lost.

False. Total momentum is conserved; it is kinetic energy that can be lost in the collision.