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How does the placement of a car's center of gravity affect it? - SM, York Beach, Maine

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Pop Quiz: If both of these vehicles were taking this turn at a high speed, which one would be more likely tip over?

The higher a car's center of gravity, the more likely it is to tip over during a high-speed turn. That's because the horizontal frictional forces that change a car's path as you turn are all exerted on the bottoms of the tires. The farther those horizontal forces are from the car's center of gravity, the more effective they are at flipping the car over.

A stationary car is naturally stable. Tipping it causes its center of gravity to rise, a process that requires energy. If you tip a car slightly and then let go, it doesn't flip over. Instead, the car returns to its ordinary upright orientation and releases this extra energy. It rocks back and forth briefly as it turns its extra energy into thermal energy and then comes to rest. You'd have to tip the car quite far before it would tip over when released.

But during a high-speed turn, the ground pushes the tires sideways so hard that the car's natural stability may not prevent it from flipping. Friction from the roadway can actually push the tires out from underneath the car's center of gravity so that the car tips over. You can see this same behavior by pushing a block of wood sideway. If you push the bottom of the block sideways gently, the block will move sideways without flipping. But if you push hard, the block's bottom will move out from under its center of gravity and the block will tip over. The taller the block, the easier it is to push the bottom out from under the center of gravity.

Keeping a car's center of gravity low is important to avoiding disaster during turns. Cars that have high centers of gravity and narrow wheelbases are susceptible to flipping and several production cars have been found unsafe in this respect. Cars and trucks that have been raised up for aesthetic reasons are similarly unsafe and even some fully loaded passenger vans become relatively unstable during turns because the occupants themselves raise the van's center of gravity.

Answered by Louis A. Bloomfield of the University of Virginia