Buzz
A bullet's velocity on a moving train is dependent on the frame of reference. Image by Emre Baser/Getty ImagesHow fast does a bullet move? And what occurs if you discharge a weapon aboard a train traveling at the velocity of a typical bullet? This question is particularly intriguing as it relates to the concept of reference frames.
In simple terms, the bullet will always travel at the same speed relative to you. However, in other reference frames, the situation may unfold in surprising ways!
Firing a Bullet at High Speed from a Fast-Moving Train
You may be familiar with Newton's first law:
"An object will remain in its state of rest or uniform motion in a straight line unless acted upon by an external force."
We often simplify this by saying that an object in motion stays in motion, and an object at rest remains at rest unless influenced by an external force.
Applying Newton's First Law to Physical Objects
Imagine you're aboard a perfectly smooth, fast-moving train, traveling at a constant speed (without any acceleration or turns), inside a car with no windows. You have no way of knowing how fast you're going or if you're even moving at all.
If you toss a ball straight up, it will fall straight back down regardless of whether the train is stationary or traveling at 1,000 mph (1,609 kph). Since you and the ball are already moving at the same speed as the train, the only forces affecting the ball are your hand and gravity. Therefore, the ball behaves just like it would if you were standing still on the ground.
What does this imply for our gun?
If the bullet leaves the gun at 1,000 mph, it will consistently move away from the gun at 1,000 mph (1,609 kph). If you position yourself at the front of a train traveling at 1,000 mph (1,609 kph) and fire the gun forward, the bullet moves away from both you and the train at 1,000 mph (1,609 kph), just as it would if the train were stationary.
However, relative to the ground, the bullet will be traveling at 2,000 mph (3,219 kph), which is the combined speed of the bullet and the train. So, if the bullet strikes something on the ground, it will do so at 2,000 mph.
If you fire the bullet off the back of the train, it will still move away from you and the gun at 1,000 mph, but now the speed of the train will reduce the bullet's speed. Relative to the ground, the bullet will have no motion and will fall straight to the ground.
The Physics of Sound Waves
What holds true for the typical bullet does not apply to certain other things you might "fire" from the front of the train. A prime example is sound waves.
When you turn on the stereo in your living room, sound waves "shoot out" from the speaker at the speed of sound — around 700 mph (1,127 kph). The waves travel through the air at this constant speed and cannot exceed it.
Therefore, if you place a speaker at the front of a 1,000 mph (1,609 kph) train, the sound waves won't leave the train at 1,700 mph (2,736 kph). They can't surpass the speed of sound. This is why airplanes flying faster than the speed of sound create sonic booms.
Factors Affecting Muzzle Velocity
Muzzle velocity refers to the speed at which a bullet exits the firearm's barrel. This is the initial velocity imparted to the bullet, and it varies greatly based on several factors, including:
- Barrel length: The bullet's speed is influenced, at least in part, by the length of the firearm's barrel. Longer barrels, such as those on rifles, offer more room for the propellant gases to accelerate the bullet, leading to higher speeds.
- Air resistance: Air resistance, or drag, acts as a force opposing the bullet's movement. The bullet's shape and design determine how well it can overcome this resistance.
- Bullet weight: The weight of the bullet can also impact muzzle velocity. Generally, lighter bullets travel at faster initial speeds compared to heavier ones.
- Cartridge type: The cartridge used, whether it's rimfire or centerfire, can affect the speed of the bullet.
- Gas pressure: The gas pressure produced when firing the gun influences the bullet's speed. Higher gas pressure typically results in a higher velocity.
There are additional factors that can influence how fast a bullet travels, such as the type of firearm used. For example, a bullet fired from a rifle, which has a longer barrel, can reach a higher velocity than one from a handgun.
Even firearms of the same caliber can have different barrel lengths or designs, which can affect bullet velocity. Moreover, various types of ammunition, including cartridges, propellants, and primers, can produce different bullet speeds.
