A bullet fired from a rifle begins to fall
(a) as soon as it leaves the barrel.
(b) after air friction reduces its speed.
(c) not at all if air resistance is ignored.
The correct answer and explanation is:
The correct answer is:
(a) as soon as it leaves the barrel.
Explanation:
When a bullet is fired from a rifle, it is subjected to two primary forces: gravity and air resistance. For the purpose of this question, air resistance is ignored, so the motion of the bullet can be analyzed using basic principles of physics.
- Gravity Acts Immediately: Gravity is a constant force acting on the bullet as soon as it leaves the barrel. This force pulls the bullet downward at an acceleration of approximately 9.8 m/s29.8 \, \text{m/s}^2 (on Earth). This means the bullet begins to fall immediately upon exiting the rifle. The vertical motion of the bullet is independent of its horizontal velocity, as described by Galileo’s principle of independence of motion.
- Horizontal and Vertical Motion: Upon being fired, the bullet has a high horizontal velocity imparted by the rifle. However, this horizontal motion does not affect the downward pull of gravity. The bullet’s motion can be divided into two components:
- Horizontal Motion: The bullet moves forward at a constant velocity (ignoring air resistance).
- Vertical Motion: The bullet simultaneously begins to fall under the influence of gravity.
- Common Misconceptions: Some may think the bullet “falls” only when it slows down due to air resistance or that it might not fall at all if it’s moving fast enough. These are incorrect. Even if the bullet travels at very high speeds, gravity acts immediately and continuously. The trajectory of the bullet becomes a curved path known as a parabola, as gravity constantly pulls it downward.
- Practical Implications: This concept is important in ballistics and explains why marksmen must account for the effect of gravity on the bullet over long distances by aiming slightly above the target.
In summary, gravity acts on the bullet from the instant it exits the barrel, causing it to begin falling immediately.