MECHANICS · LESSON 04
Conservation of Momentum
Learn to choose the system before you write the conservation equation.
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Predict before calculating
Two carts move toward one another and stick together. Before doing algebra, predict the direction of the combined cart.
Cart A · 2 kgvA = +4 m/s →
⇄
Cart B · 3 kg← vB = −1 m/s
Choose the physical system
Momentum conservation is a statement about a system. Choose the system and the condition that makes the model valid.
System model
pi = pf
mAvA + mBvB = (mA+mB)vfSolve the collision
Use the momentum model to determine the final velocity after the carts stick together.
(2)(+4) + (3)(−1) = (2+3)vf
m/s
Reflect on the strategy
Which statement best explains why the final velocity is positive?
✓ Lesson complete
You completed the full Predict → Model → Solve → Reflect cycle.