You Asked:

When a weapon impacts a target, does it carry a measurable kinetic energy? — Jason, Home, Wash.

Marshall Brain Answers:

When a gun shoots a bullet, the only thing the bullet has going for it is kinetic energy. The kinetic energy is what allows the bullet to penetrate and damage the target. The bullet’s kinetic energy is expressed by this equation:

Kinetic energy = 1/2 mass * velocity ^ 2

So if a bullet weighs 20 grams (0.02 kg) and is traveling at 670 MPH (which equals 300 meters/sec), its kinetic energy is:

1/2 * 0.02 kg * 300 m/s * 300 m/s = 900 joules

For comparison, when a pitcher hits a batter with a baseball in a major league game, the baseball weighs 145 grams (0.145 kilograms) and is traveling at 100 MPH (45 m/s). The ball’s kinetic energy is:

1/2 * 0.145 * 45 * 45 = 147 joules

So a bullet has a lot more kinetic energy than a baseball, and being so much smaller, the bullet does a better job at penetration.

A typical bomb dropped out of an airplane might weigh 1,000 pounds (450 kg) and have a velocity of 600 MPH (268 m/s). Even if the bomb doesn’t explode, it has a good bit of kinetic energy:

1/2 * 450 * 268 * 268 = 16,160,400 joules

For comparison, a stick of dynamite carries approximately 2 million joules of energy, so the bomb’s kinetic energy alone is equivalent to about 8 sticks of dynamite.

As speeds get higher, the amount of kinetic energy becomes so great that “kinetic weapons” become a possibility. Simply their kinetic energy provides the destructive force. For example, there has been discussion of dropping very heavy objects sized and shaped like a telephone pole from orbit:

An aerodynamically shaped phone pole dropped from that altitude might be traveling 7,000 MPH (3,130 m/s) when it hits the ground. A phone pole (one foot diameter, 50 feet long) made of tungsten would weigh 21,000 kilograms. So the kinetic energy is:

1/2 * 21,000 * 3,130 * 3,130 ~ 100 billion joules

That’s the equivalent of 50,000 sticks of dynamite. That’s the energy equivalent of something like 25 tons of TNT. Not bad considering that there’s nothing but gravity providing the power.

More info: How Bunker Busters Work