Conservation of Momentum in Two Dimensions

The law of conservation of momentum states that in a closed system, the total momentum before and after a collision remains the same, provided no external forces act. Most students first encounter this concept in one dimension, but momentum becomes much more interesting when collisions occur at angles. Using an air hockey table or an air puck system, students can observe momentum conservation in two dimensions and see the theory unfold frame by frame.

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The Experiment

An air hockey table or an air track with gliders provides a near-frictionless surface. Two pucks are set on a collision path, either head-on or at an angle. A top-down video camera records the collision.

Using software such as PASCO Capstone, Tracker, or Logger Pro, students can:

1. Track each puck’s motion before and after impact.

2. Draw velocity vectors showing direction and magnitude.

3. Split momentum into x- and y-components and calculate totals before and after the collision.

Total momentum before = Total momentum after

$$m_1u_{1x} + m_2u_{2x} = m_1v_{1x} + m_2v_{2x}$$ $$m_1u_{1y} + m_2u_{2y} = m_1v_{1y} + m_2v_{2y}$$

The results show that even when the pucks scatter in different directions, the total momentum in both axes remains constant.

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The Science

Collisions can be elastic (kinetic energy conserved) or inelastic (some energy lost as heat or deformation). However, momentum is always conserved.
The vector approach shows that momentum is not just about speed but direction — making it essential for understanding real-world physics such as vehicle collisions, snooker impacts, or atomic interactions.

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Skills Highlight

• Recording and analysing motion using video tracking

• Decomposing vectors into x- and y-components

• Verifying conservation laws experimentally

• Linking abstract vector mathematics to physical evidence

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Why It Works in Teaching

The combination of air hockey, sensors, and video analysis makes an abstract law tangible. Students can see how momentum balances in both directions, not through equations alone but through real motion, geometry, and evidence.