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Tuesday, December 28, 2010

Dodging Balls and Bullets

Melodee's Home Page

I can't seem to leave some news stories alone...

If You Can Dodge A Wrench, You Can Dodge A Ball

Before we get into why this important, you need to be up to speed on kinetic energy...

Kinetic Energy of an object is defined as: "The energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body in decelerating from its current speed to a state of rest." Therefore, the kinetic energy of an object is a function of its mass (weight for our purposes) and its velocity (speed). If we increase either the mass or the velocity, the total kinetic energy goes up.

For those who are really curious, the formula is: ((m*v)*(m*v))/2. At least for classical physics.


An average person can run short distances at about 5 MPH. Go outside and run as fast as you can into the wall of your house. When you wake up and get home from the hospital, come back to reading.

Back? Good. How's the head?

Now, go out to your car and drive it at 5 MPH into the wall of your house.

See any difference in the damage to your house?

Even though the velocity was the same (5 MPH), the mass of the objects was vastly different. If you weigh, say, 150 pounds, your kinetic energy is about 11.5 kilojoules. Don't worry about what a kilojoule is. For the car, assuming its mass is 3,000 pounds, the energy is about 4,626.3 kilojoules. For an increase in mass of about a factor of 20, the kinetic energy went up by a factor of more than 400.

Just for fun, here are the kinetic energies of a few objects you might recognize...

22 caliber LR bullet: 0.2 kilojoules
357 magnum bullet: 1 kilojoule
Car at 29 MPH: 156,521 kilojoules
Car at 70 MPH: 307,350 kilojoules
Semi Truck at 70 MPH: 645,001,625 kilojoules
Fright train at 70 MPH: 23,220,485,106 kilojoules

Now, back to the article...

Let's assume a piece of space junk that has a mass of only 5 pounds. That's not much. The problem is that orbital velocity is about 17,000 MPH at an altitude of 150 miles. Lots of satellites live in that area, too. So what? The kinetic energy of a hunk of junk with a mass of 5 pounds at 17,000 MPH is about 152,775 kilojoules. See the table above.

Now imagine what would happen to the typical satellite if you hit it with your car at about 30 MPH. Yeah, it ain't gonna be pretty.

Now imagine ramming your car into the International Space Station. So much for the crew.

Interestingly enough, the airbags would probably let you walk away from the collision.

Now here's another twist...

Some armor-piercing rounds fired from a gun can blow clear through a steel plate up to several inches thick, but even though a car at 29 MPH has more kinetic energy, it won't plow through that same plate wall. Why not? Because the bullet offers a smaller area of impact, so the energy it carries is concentrated more than that of the car...

Assume a bullet of 0.357 inches diameter. If we ignore the fact that the bullet comes to a point, that's an impact area of about 0.4 square inches. If the front of the car (again assumed to be flat and a perfect rectangle) is 8 feet by 3 feet, that's an area of 3,456 square inches. The 357 slug carries about 2.5 kilojoules per square inch of energy (1Kj/0.4), while the car at 29 MPH imparts about 45 kilojoules per square inch. Then things start to spread out from the impact. The fact is that the car spreads out more and faster than does the bullet. According to calculations, 1 millisecond after impact, the bullet still has about 2.4 kilojoules per square inch of energy, but the car has fallen off to about 11. After 4 milliseconds, the bullet is at 2.1 and the car is below 1.

The bullet is denser and stays together better, keeping its energy concentrated.

Most space junk is fairly dense. Remember our 5 pound space junk from above? That would be a hunk of steel with a volume of about 18 cubic inches. That's a block of steel about 2.5 inches on a side. Not very big.

Again, think about the damage such an object would cause to a satellite. To use NASA's tidy little term, loss of vehicle.

What if the vehicle lost is a weather satellite watching hurricanes to provide warnings to people? How about a navigation satellite that the airlines use to get around? Maybe it's just the satellite that is feeding the Super Bowl game to your house?

Yeah, this is serious stuff, and we need to address the issue, and sooner is better.

The longer we wait, the worse it will get.

Keep Loving!

Melodee Aaron, Erotica Romance Author
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Melodee's Books at BookStrand


  1. Oops -- the point of your article isn't changed, but you goofed on the equation.

    Ke = 0.5*m*v^2 not what you wrote which squares the mass term as well. So, if you multiply the mass by 20, you get 20 times the kinetic energy. Multiply the velocity by 20, and you get 20*20 = 400 times as much energy.

    It doesn't alter your argument, of course.

  2. Oops!

    You are right, Jeff, and I did mis-type the equation!

    That's why I have editors...but they don't edit my blog postings!