Return to Home Page QUESTIONS AND ANSWERS Organized by Topics Select Topic Air Conditioners Airplanes Audio Amplifiers Automobiles Balloons Balls, Birdies, and Frisbees Bicycles Bouncing Balls Cameras Centrifuges and Roller Coasters Clocks Clothing and Insulation Compact Disc Players Computers Electric Motors Electric Power Distribution Electric Power Generation Electronic Air Cleaners Elevators Falling Balls Flashlights Fluorescent Lamps Incandescent Light Bulbs Knives and Steel Lasers Magnetically Levitated Trains Medical Imaging and Radiation Microwave Ovens Nuclear Reactors Nuclear Weapons Plastics Radio Ramps Rockets Seesaws Spring Scales Sunlight Tape Recorders Telescopes and Microscopes Television The Sea and Surfing Thermometers and Thermostats Vacuum Cleaners Violins and Pipe Organs Water Distribution Water Faucets Water, Steam, and Ice Wheels Windows and Glass Wood Stoves Xerographic Copiers Other Topics All Questions & Answers Ask a Question

 Question 93

 When a ball swings in a horizontal circle at the end of a string, what's the force on the ball pulling it straight? What's the force pulling it out?
Let's neglect gravity, which isn't important in this horizontal motion problem. When a ball swings in a circle at the end of a string, there is only one force on it and that force is inward (toward the center of the circle). We call such a force a centripetal force, meaning toward the center. There are many kinds of centripetal forces and the string's force is one of them. As for the ball's tendency to travel in a straight line, that's just the ball's inertia. With no forces acting on it, it will obey Newton's first law and travel in a straight line. There is no real force pulling the ball outward. But a person riding on the ball will feel pulled outward. We call this feeling a fictitious force. Fictitious forces always appear in the direction opposite an acceleration. In this case (an object traveling in a circle) the outward fictitious force is called centrifugal "force." But remember that it's not a real force; it's just the object's inertia trying to make it go in a straight line.