When a player runs into a goalpost and bounces back, momentum is conserved because:

1. Momentum conservation applies to the whole interacting system, not just the visible part. 2. The Earth also recoils, conserving momentum, because of the force applied to it through the goalpost. 3. Because Earth is many orders of magnitude more massive than the player, its recoil is immeasurably small. 4. It can be neglected in any practical sense, but it is real nevertheless. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 8 'Linear Momentum and Collisions', sections 8.1-8.7_