At the carnival, a swing ride lifts people high in the air. As it spins, the riders fly out to the sides. They whirl above the crowds, a laughing blur. Now, imagine if the tower suddenly disappeared. What would happen to the carnival-goers? Would they fall straight to the ground? Would they keep flying in a circle? Would they soar off in random directions?
Going Off on Tangents
When an object moves in a circle, it experiences centrifugal force, centripetal force, and angular momentum. Centrifugal force flings the object outward. Centripetal force makes it want to move toward the center. Since these two forces are equal, the object moves in a circle. Angular momentum makes it want to move in a straight line. If the other two forces disappeared, this momentum would make the object travel at a tangent to the circle.
So, what would happen to the riders if the swing ride disappears? Each would fly off along a tangent to the circle. Each person would have his or her own tangent depending on his or her seat location, so they'd all soar in different directions before gravity brought them back down to Earth.
NASA uses tangents to figure out how to launch space probes. A probe must travel fast enough to escape the pull of Earth's gravity. NASA needs to know where and when to launch the probe so that it travels in the correct direction once it leaves orbit.
See for yourself: http://www.youtube.com/watch?v=lTEK7HhFLMw
NASA scientists are so good at calculating tangents that they can fire a probe from Earth and know where it will land on Mars seven months later. Watch the videos below to follow the adventures of two robotic rovers sent to explore the Red Planet.