Mathematically if u wanna see then impulse of mg is mg ∆t and ∆t is very small for collisions like in the order of ms. So let's take an example of simply dropping a ball of 1kg from a height of 5m taking g as 10 velocity of impact is 10m/s. Say collision is elastic so rebound velocity is also 10m/s. ∆p is 20Ns. Let's take ∆t for collision as 1ms so the average upward force on the ball is ∆p/∆t which is 20000N. Which is 2000 times greater than the weight of the ball. Thats why we can neglect Fg in comparison to the normal force during the duration of collision.
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