Study Guide

Physics Basics - Energy and Momentum

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Energy and Momentum

Police chases make excellent reality TV—it’s all the excitement—but sooner or later things end up in a mess of bent metal and smoke. Fact: The more energy and momentum a car has, the worse the end result is likely to be.

Energy is one of those concepts that’s hard to explain, but we know it when we see it. The proper definition of energy is usually the “ability to do work.” Energy has many forms: kinetic (movement) energy, light energy, sound energy, heat energy, and potential energy. While energy is never created or destroyed, energy can change types. When a ball rolls down a hill, gravitational potential energy changes into kinetic energy, and when the car chase ends in tears, most of that kinetic energy changes into heat in the screeching breaks and smashed bodywork.

Anyone who’s seen a car rear-ended, or been tackled by a football player, knows all about momentum. Momentum is about movement and mass. The faster we move, the more momentum we’ll have, and the larger we are, the more momentum we’ll have. So make avoiding all gigantic football players’ tackles a priority.

Like energy, momentum is never created nor destroyed. That might seem like it can’t be possible: after all, what happens if we run into a brick wall (other than a broken nose and a black eye)? We stop. It’s like all our momentum vanishes. The problem is that we’re not seeing the bigger picture – like, the whole planet big. Our momentum doesn’t really vanish. Instead, the Earth itself gains that momentum.

Collisions and car crashes can be elastic or inelastic. Or put more simply, they can be bouncy or sticky. If we bounce a ball on the ground, that’s pretty elastic. But if two cars smash into each other, becoming attached at the hip as they fly into the intersection, that’s pretty inelastic. In inelastic collisions, kinetic energy is lost and gets turned into heat energy. In elastic collisions, no kinetic energy is lost.

Energy and momentum also helps us become Captain Safety. Once we know how car crashes work, we can see how safety features of cars use the laws of physics to keep us from getting hurt. Air bags, seat belts, crumple zones and collapsible steering wheels all increase the time of impact, to reduce the force on our bodies. That way, we can live long enough to learn even more physics!

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