Here's the situation. A fellow Engineer was teaching a math class for Engineers and was looking at the some ballistics equations and published ballistics tables.

There's something that neither of us can explain, so I'll ask here.

We know that the basic free-fall equation is:

drop = 1/2 a * t squared, where a = acceleration of gravity = 32.2 ft/sec squared

When you look at the trajectory tables, you get distance, time of flight and drop. When you back calculate the vertical acceleration, you compute a constant in the range of 25-26 ft/sec squared.

Why it this?

does the gyroscopic spin of the bullet make it resist the motion normal to its axis?

Is there a vertical lift due to air pressure if the bullet is yawing?

Does someone know the answer to this question?


MM

Looks like your wondering why a bullet doesn't drop as rapidly in air as it would in a vacuum. The reason for this should be obvious.

The problem stems from the face that the line of sight (through the scope) and the centerline of the bore are not parallel. The bore is angled "upward" in relation to the bore. The bullet still obeys the law of gravity.

Draw a sketch of the bullet trajectory and line of sight through the scope with a 200 yard zero. The bullet appears to climb for some distance before falling.

If the scope line of sight and barrel were aligned to be parallel, it would be more oblivious that the bullet does violate the laws of gravity.

quote:

Originally posted by sonofagun:
Looks like your wondering why a bullet doesn't drop as rapidly in air as it would in a vacuum. The reason for this should be obvious.

wouldn't a vacuum have less effect on a projectile?

A vacuum being nothing would have no effect on a bullet. Only factor concerning drop in a vacuum is gravity.

"The bullet appears to climb for some distance before falling."


Drop is generally calculated below line of the bore. Bullets NEVER climb above the line of the bore, only above line of sight when zeroed at a fixed distance.

Did you take into account that the bullet is slowing down as it makes it flight?

quote:

Originally posted by Pegleg:
Did you take into account that the bullet is slowing down as it makes it flight?

Doesn't make a difference...

AC has it right. Our standard drop tables are calculated for a "zero" at some specific range, all of it is based on the fact that the line of bore and line of sight are not the same.

The actual bullet drop from a truly horizontal bore line will follow the standard drop formula because there is no "lift" effect that isn't offset by an equal "drop" effect on the other side of the rotating bullet.

At practical distances, the vertical (drop) velocity of a bullet through air is so slow it's almost unaffected by air resistance.