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Mr. Incredible Stops Crooks' Car With a Tree, Crooks Keep Going

The car stops, the bad guys keep flying forward.

Screen capture by HowStuffWorks staff

To foil a group of crooks, Mr. Incredible drops a tree trunk in front of their speeding car. The car comes to a metal-crunching halt, but the bad guys continue moving forward until they smack into the dashboard and windshield, incapacitated. This is Newton's first law at work. It says that an object will keep doing whatever it's already doing until some force makes it do something else. You may have heard it phrased, "An object at rest tends to stay at rest; an object in motion tends to stay in motion," or just the law of inertia.

This law can seem counterintuitive at first, because here on Earth there are a bunch of invisible forces acting on objects at all times that cause them to seemingly violate Newton's first law. If you throw a ball, shouldn't it keep going forever? It would in space, but on Earth the ball is slowed by friction from passing through air, and eventually gravity makes it fall to the ground (where even more friction makes it come to a stop).

In the case of the guys in the car, they're moving forward, pushed by the normal force of their seatbacks. When Mr. Incredible brings the car to a sudden stop, the crooks keep moving forward, in accordance with Newton's first law. However, it isn't air resistance or gravity that slows them down, it's the solid objects in front of them -- the dashboard and windshield. Those objects impose a force on the baddies, causing them to stop moving (and, as is the case with the car, the sudden deceleration causes some physical damage).

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