Journey Into Space: Gravity, Orbits, and Collisions
Space Gravity
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Thrills and Spills
Kim Y Masibay 

Are stomach-flipping roller-coaster loops your idea of fun? Then here's a question you probably don't want to ask yourself mid-loop: If gravity causes objects to fall toward Earth, why doesn't your coaster train plunge to the ground? The answer begins with Newton's first law of motion, or the law of inertia, which states that moving objects keep moving in a straight line unless acted on by an outside force. The outside force in this case? To find out, take a closer look at the physics of looping.

“Vertical looping is all about inertia and acceleration,” explains Jeff Pike, a design engineer at Great Coasters International in Sunbury, Penn. Inertia is the tendency of a body-a roller-coaster train, for example-to resist a change in motion. “Once the train enters the vertical loop, the train wants to keep going straight,” says Pike. “But the track gets in the way.” Its upward curve interferes with the train's inertia. So, according to Newton's first law, the track is the outside force that makes the train change direction and speed-a movement called acceleration. Add a curved path and you get centripetal acceleration: As the train speeds into the loop, it accelerates because the track forces it upward. (As the train climbs higher it also loses speed, but regains speed when it plunges down the loop's other side.) Since the train wants to keep going straight-not up-you're pushed into the track.

When you're at the top of the loop, gravity wants to yank you to the ground, but centripetal acceleration makes the train hug the track and inertia pushes you into your seat-even when you're upside down! The force your body feels is measured in g's: one g equals the force of gravity. (Standing on the ground, you feel the force of one g.) At the top of a vertical loop, the centripetal force that glues you to your seat must be equal to or greater than one g to counterbalance gravity's downward tug. “As long as the centripetal acceleration is equal to the force of gravity, you can flip head over heels without crashing,” Pike says. So you can save the “spills” for lunch!
   
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