Physics 101: Blame Sir Isaac Newton for having to wear your seatbelt

When your parents nag you to fasten your seat belt, even when you’re in the back seat, don’t blame them; blame Sir Isaac Newton.

Newton, a mathematician and physicist who lived during the 1600s, developed three laws of motion, the first known as the law of inertia: An object at rest tends to stay at rest, and an object in motion tends to stay in motion, and at the same speed and in the same direction, unless the object is acted upon by an unbalanced force.

Say what?

This means that if you leave your backpack on the kitchen table overnight, it will be there when you return in the morning unless somebody takes it. If you hit a baseball, it will continue moving until it hits an outfielder’s glove or the back fence. Of course, it will eventually stop on its own even if it doesn’t get caught, because of the friction between the ball and the ground, and between the ball and the air.

Always use safety belts and the correct child restraints.

So what does Newton’s law of inertia have to do with the seat belts in your parents’ vehicle? Plenty. Have you ever been on a bicycle and stopped so suddenly that you flew over the handlebars? Hopefully not! If so, you experienced Newton's law of inertia. Your body was in motion, traveling at the same speed as the bike. But when the bike stopped, your body kept moving.

In your parents’ vehicle, seat belts prevent your body from continuing to move when the vehicle suddenly stops. They are the “unbalanced force” in Newton’s law.

Without something to hold you in place you become, in effect, a very heavy projectile. Why? Because when a vehicle stops very quickly, whether it’s because Mom or Dad slammed on the brakes or because it hit another vehicle or stationary object, like a tree, this rapid deceleration increases your “crash weight.” That’s another physics law:

force = mass x acceleration

If you weigh 60 pounds, riding in a vehicle traveling at just 30 mph, and that vehicle is involved in a sudden collision, the force of your moving body turns your crash weight into about 1,800 pounds, the weight of a small elephant. At that crash weight, you can be hurled with great force into the seats in front of you, or even through the windshield.

At 42 miles per hour, your crash weight can range from 4,500 to 9,000 pounds, the weight of an adult elephant.

Using seat belts is so easy, and they can save your life. If more people understood simple laws of physics they would see how important seat belts are, and they would wear them. Now you know. Share these safety tips with your friends and family members.

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