Ammunition makers traditionally express bullet weight in units of grains.
Any object in motion possesses kinetic energy, which technically expresses the amount of work the object can do on any object it strikes. Mathematically, physicists express kinetic energy, KE, as KE = 0.5 * m * v^2, where m represents mass and v^2 represents velocity raised to the power of two. Physicists further define the velocity, v, of an object as the distance, d, it travels per unit time, t, or v = d / t.
Instructions
1. Calculate the bullet's velocity by dividing the distance traveled, preferably in units of meters, by the time required, in seconds, to travel that distance. If you are working a problem from a textbook, the data for this problem may be stated explicitly in a statement such as "a bullet leaves a rifle and strikes a target 400 meters away 0.65 seconds later." In this case, v = d / t = 400 m / 0.65 s = 615 meters per second, or 615 m/s. If the units require conversion to meters and seconds, then use an online conversion tool, such as those provided in the Resources section.
2. Determine the mass of the bullet. Again, this information may be provided for you if you are working a problem from a textbook. Otherwise, the bullet can be weighed on a scale. Ammunition manufacturers also provide this information on the outside of boxes of ammunition, usually stated in units of grains. If necessary, convert the mass of the bullets to the recommended unit of kilograms using an online calculator. A 135-grain bullet, for example, weighs 0.00875 kilograms, kg.
3. Calculate the bullet's kinetic energy according to KE = 0.5 * m * v^2 using the previous results. Continuing with the examples, KE = 0.5 * (0.00875 kg) * (615 m/s)^2 = 1655 kg.m^2/s^2. Note that 1 kilogram meter squared per second squared, or 1 kg.m^2/s^2, is equivalent to 1 Newton meter, or 1 N.m, and also equivalent to the SI unit of 1 joule, or 1 J. The kinetic energy can therefore be expressed in any of these units. Use one of the online converters if you desire a different unit of energy.
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