image credit: Darron Spohn, © Philip Greenspun; image source; larger image

Hyperphysics: Centripetal Force

This motorcycle's path is bent into a curve by 1) the force of friction, parallel to the road's surface, between the road and tires, and 2) the force perpendicular to the road's surface that the road exerts on the tires (assuming the track is banked--if the track is horizontal, friction provides the only inward force).  To see what happens when the track is vertical, with a discussion of the physics, see Mahanakorn University of Technology's The Wall of Death.  For a summary of the physics of circular motion, take a look at Hyperphysics: Centripetal Force.

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Model of a Carousel

You can investigate circular motion with Walter Fendt's simulation Model of a Carousel by changing the variables and observing the resulting motion.  Be sure to choose the option "Carousel with forces"--the red arrow gives the direction of the net force, which is the same as the direction of the acceleration.

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image credit: John Vickery and Jim Matthes/Adam Block/NOAO/AURA/NSF; image source; larger image

Rotation Curves

This is NGC 3198, a spiral galaxy.  Astronomers originally assumed that most of the mass in this kind of galaxy was concentrated in the bright central region and, therefore, that the stars in the spiral arms moved in circular gravitational orbits--like the planets in the solar system.  But when astronomers measured how the stars move, they found something quite different--see Rotation Curves for results for NGC 3198--and they were forced to the conclusion that much of the mass in galaxies does not emit light (thus the name "dark matter").

Dark Matter

Imagine a universe where physicists understand only a small percentage of the matter in it--that's the situation today with the Missing Mass, one of the great unsolved questions in physics.  For a discussion of the observations that led to this challenging situation, visit the Wikipedia entry Dark Matter.