When a gun is fired straight up into the air, the bullet will eventually reach a maximum height and then fall back down to the ground due to the force of gravity. The bullet's maximum height will depend on factors such as the initial velocity, mass of the bullet, and air resistance.
Assuming there is no air resistance, the bullet will experience a constant downward acceleration due to gravity. As it reaches the highest point in its trajectory, its vertical velocity will momentarily become zero. From that point onward, the bullet will begin to accelerate downward due to gravity.
The bullet's downward acceleration will be the same as its upward acceleration when it was initially fired. However, since the bullet is now moving in the opposite direction, its velocity will have reversed sign. In other words, the bullet will be returning to the ground with the same magnitude of speed, but in the opposite direction.
It's important to note that the bullet's maximum velocity will not be slower than its initial velocity when it was fired. The magnitude of its velocity will be the same when it returns to the ground, but the direction will be opposite.