A block lying on a smooth table is hit by a bullet flying horizontally. Why can the law of conservation of momentum

A block lying on a smooth table is hit by a bullet flying horizontally. Why can the law of conservation of momentum be applied to find the speed of a bar with a bullet, although external forces act on the bar and the bullet: gravity, the normal reaction force of the table?

To find the speed of a bar with a bullet, you can use the momentum conservation law, since the resultant of the vertically acting forces of gravity and the reaction force of the support is equal to zero.

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