force acting for a length of time. This is called impulse. From this we can see that
time * force = impulse. This is, in turn, equal to momentum. In other words,
we could say that momentum = mass * velocity = force * time. It was just shown in Newton’s 3rd.
Law that F = M* a. In acceleration it was shown that a = V / t. From this we can
see that F = M* a = M* V / t or F * t = M* V. This may be just a bunch of confusing letters and symbols to readers
who are not interested in math. Read- ers who are math inclined will see that
this shows why the two equations, one momentum and the other impulse, are actually the same. Torque, often called moment, is
the twisting or rotating tendency created by a force or a combination of forces applied to an article. (Called torsion.) Torque is calculated by multiplication of the force and the shortest perpendicular distance of the force. Energy from rotary motion is equal to torque times angular displacement. Torque is different at vari- ous points on the object and it is zero for all points exactly on the line of action of
the force. Evaluating a torque requires only
that we measure a distance and a force and multiply them together. Engineers put the force before the distance to avoid confusion with energy units, which is also force times distance. In other words, torque is lb.-ft. and energy is ft.-lb. Torque has a relationship to the
twisting of both the firearm and the projectile created by the rifling. Now let us have a brief discus-
sion of mathematics. Why is the math included? There are two main reasons. First, readers who can follow the math will gain a deeper understanding of bal- listics. Second, there may be occasions when the reader will want information that can be obtained by working out a problem. Wind drift and rifling twist are two common problems that are easy to solve. If the reader studies and under-
stands the math, not only can he or she obtain answers to questions, it also will give a deeper grasp of the subject. In many instances, the math confirms that a statement or comment is a proven truth rather than a theory. (Note the math that was shown in the earlier dis- cussion of physics.) Most mathematics is easy if you
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use a scientific calculator. Even if you flunked math in school, you still can work the problems. If you don’t have a scientific calculator, it would be advis- able to purchase one. It is not necessary to have an expensive model. A few dollars spent at a discount store will do. A regular calculator is a great help, but the scientific models will do things with a keystroke that the others will require tables and tedious calculations. A scientific model will enable someone who doesn’t understand trigonometry to work trigonometry formulas. The manual for the calculator is valuable. Refer to it as needed. To convert from pounds to grains
or from grains to pounds is common in calculations involving ballistics. The for- mulas will not work if the wrong num- bers are used and yet this is a frequent mistake. If you have pounds, multiply by 7,000 to obtain grains. If you have grains, divide by 7,000 to obtain pounds. With even the cheapest calculator it is very simple. The hard part is remember- ing to do it. Example: 200 grains divided by 7,000 = 0.0285 pounds.
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