Final answer:
A bullet begins to fall toward the ground as soon as it leaves the barrel because gravity acts on it from that moment forward. This occurs regardless of any effects from air friction, which does not need to reduce its speed for it to start falling. Adjusting gun sights for projectile motion accounts for gravity affecting the bullet's path.
Step-by-step explanation:
A bullet fired from a rifle begins to fall toward the ground as soon as it leaves the barrel. This is because the gravitational force acts on the bullet from the moment it is no longer in contact with the rifle, pulling it down towards the Earth. Air friction does not have to reduce the bullet's speed for it to start falling; gravity acts on it continuously. Therefore, the correct answer is as soon as it leaves the barrel.
If a bullet is fired with a velocity we can call va, the gravitational force draws it downwards, making it hit the ground at point a. If the bullet has a higher muzzle velocity, referred to as V, it would travel a greater horizontal distance before striking the ground at point b. The higher muzzle velocity does not prevent the bullet from falling due to gravity; it simply allows the bullet to cover more distance before hitting the ground.
In projectile motion, the horizontal velocity of a bullet (or any other object) does not affect the rate at which it falls. All objects in projectile motion fall at the same rate (ignoring air resistance), which is due to the acceleration of gravity. The adjustment of gun sights takes into account the projectile motion, aiming higher to compensate for this gravitational pull and to be accurate over specific ranges.