Momentum: Mass x Velocity
Imagine that you are at the controls of a Union Pacific locomotive. You are the engineer. As you come around a bend in the tracks, you suddenly see a car stuck on the railroad crossing ahead. The car is about a half-mile away. Can you stop in time, or are you going to crash into it?
In 2013 alone, this very situation resulted in 2087 collisions between trains and automobiles. So hopefully the importance of stopping time and stopping distance is self-evident. (And also, be careful driving your car, especially when it comes to railroad crossings!)
We need to begin this lesson with an important physics concept called momentum. Momentum is the product of an object's mass and its velocity. The SI unit for momentum is the 
. In a math equation, the formula for momentum is:
p = mv
= momentum in kilogram∙meters per second (
)*
= mass in kilograms ( 
)
= velocity in meters per second (
)
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Note: p might seem like a strange abbreviation for momentum. Perhaps it is, but m isn't available since it already stands for mass. The concept of momentum was discovered by Sir Isaac Newton (1643-1727), English Physicist and father of modern physics. But he referred to it by the awkward phrase "quantity of motion." A contemporary of Newton's, the German mathematician Gottfried Leibniz (1646-1716), coined the more elegant term "progress" - and there lies the origin of the abbreviation p. Today we call the concept "momentum," but the p from "progress" persists as its abbreviation.
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