Galileo – His Experiments Physics Objective Discover that the rate at which an object falls is independent of its mass Understand that horizontal motion does not affect the rate at which an object falls vertically Learn that the speed of a falling ball is dependent on the vertical height from which it is dropped Discover that the time a ball takes to reach the bottom of an inclined plane is dependent on the slope of the plane Go to http://www.teachersdomain.org/asset/phy03_vid_galthought/ Background The motion of objects fascinated Galileo throughout his life. When he wasn't conducting real-world experiments, he developed "thought experiments" to test his ideas about the way objects move, and why. Centuries before Galileo revolutionized this area of physics, Aristotle concluded that objects of greater mass fall faster than those with less mass. This can certainly be observed with many of the objects we see around us every day. A dropped piece of paper, for example, drifts to the ground far more slowly than a falling soccer ball. However, Galileo suspected that Aristotle's universally accepted theory may not have been true. He predicted that differences in acceleration between different types of falling objects had nothing to do with either mass or Earth's gravitational force. In several experiments and "thought experiments" Galileo recorded many hypotheses and observations and found evidence to support the possibility of a universal law governing the motion of all falling objects. Galileo probably dropped and threw countless objects from high places. Certainly, he thought about doing so under a variety of conditions. Galileo also measured the time it took for pendulums of different weights to swing back and forth and the acceleration of balls rolling down inclined planes of different lengths. In the end, he concluded that the effect of gravity on earthly objects is the same, regardless of the mass of those objects. He argued that in the absence of other forces such as air resistance, all falling objects accelerate toward Earth at the same rate. In the centuries since Galileo's time, physicists have verified his conclusion and determined the rate of acceleration at Earth's surface to be 9.8 meters a second for every second the object is in free-fall. Procedure Go to http://www.pbs.org/wgbh/nova/galileo/expe_flash_1.html Analysis Why do you think Galileo's ideas and experiments seemed surprising or difficult to believe at the time? Procedure Go to http://www.teachersdomain.org/asset/phy03_vid_galileoplane/ Galileo thought a great deal about the motion of falling objects. Specifically, he was interested in the forces that acted upon objects in free-fall and whether those forces affect different types of objects differently. In Galileo's time, Aristotle's centuries-old assertion that heavier objects fall faster than lighter objects was almost universally accepted. Even today, most people would predict that a feather will drift to the ground more slowly than a hammer, but they might not be able to explain why. Galileo acknowledged this difference in falling time, but wondered if it was due to the mass of the objects and gravity's pull on that mass, or to some other force that was being overlooked. For years, he conducted real-world experiments and "thought experiments" to test his ideas. Most falling objects move quickly. Even with a modern stopwatch, it is difficult to accurately measure an object's free-fall velocity or its rate of acceleration. Having no such timepiece, Galileo might have considered a study of falling objects impossible. However, Galileo was nothing if not ingenious. He had the idea that a ball rolling down an incline would accelerate in the same way as a free-falling object, but more slowly. Using a straight, gently sloped piece of wood with a groove running down the center -- an inclined plane -- he was able to "dilute" gravity's effect. The inclined plane thus allowed Galileo to accurately measure acceleration with simple instruments and ultimately to prove that, in the absence of other forces such as air resistance, gravity causes all falling objects to accelerate toward Earth at the same rate. Analysis How did Galileo slow down motion so he could measure the motion of a falling object? Make a chart that shows the pattern Galileo discovered when he measured the units of distance a falling object covers in each unit of time. Go to http://www.teachersdomain.org/asset/phy03_vid_galmoon/ To measure the motion of an object rolling down an inclined plane, Galileo put markers along the plane to identify the points where the ball passed at equal intervals of time. Why weren't the markers placed at equal distances along the plane?
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