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Whether it's kicks, punches, or bullets, people use ft-lbs or joules to describe kinetic energy. But how is this useful? Why use torque to describe linear motion such as a bullet? And how does one convert these units of torque into something that can tell how much acceleration the impacted object undergoes?

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ft-lbs in this case is a unit of energy, not torque, as in: energy = distance * force. 1 foot-pound = 1.356 joules. The confusion is justified, because energy and torque coincidentally use the same units of measurement, but their meaning is accordingly different.

The acceleration of the impacted object would depend on the material of the object and therefore is virtually impossible to predict. However you can calculate the resulting speed of the impacted object assuming it's momentum is equal to the momentum of the bullet. The latter you can easily find from the given energy of the bullet (although it's muzzle energy is higher due to air resistance) and the bullet mass (note that grains are not grams).

In real life the actual impact depends on both, energy and momentum. It is a trade-off based on the mass of the bullet. The biggest overall impact is when both values are reasonably high.

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