Practical Coilgun Design 

How to Measure Speed

Pendulum PhysicsThis page is reproduced from "Backyard Ballistics" by permission of William Gurstelle. Pendulum PhysicsEarlier in this book, Sir Isaac Newton's three laws of motion were discussed. Let's apply those laws of motion to determine the velocity of a bullet. Physicists would say that the collision between the spud and the pendulum is "perfectly inelastic" because the waddedup newspapers allow for no bounce or rebounding whatsoever when the potato hits the pendulum. In this type of collision, physicists would say that "momentum is conserve." However, in this type of collision, where there is no bounce or elasticity between the colliding objects, those same scientists would note that the kinetic energy is not conserved. Energy and mass conservation laws are the basis of anlysis that scientists use to relate mass, velocity, distance and time. Through careful choice of the variables involved, and mathematical manipulation of the physics equations that describe the process, an equation can be written to let us determine the speed of the projectile from the easily measured variables discussed earler. V potato = (1 + ^{M}/_{m}) sqrt(2gh) It is beyond the scope of this book to derive the equations for muzzle velocity. Most beginning physics textbooks discuss the physics of the ballistic pendulum when onedimensional particle kinetics and energy and momentum conservation laws are introduced. For now, take it on faith that the muzzle velocity of the potato is given by this equation.
If we insert the constants, we can make a fairly easytouse equation to determine the speed of any projectile shot into our pendulum: V potato = (1 + ^{M}/_{m}) sqrt(43.9 x h)
To find the muzzle velocity of any projectile, substitute your figures in the equation given. Tips and Troubleshooting




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