CHAPTER ONE Unseen Forces Climb aboard a roller coaster and hang on It jerks and rattles to the top of a slope It hovers a second before plunging into a stomach thrilling drop The turns sling riders from side to side A jolting stop forces everyone backward The ride is over The thrill of a roller coaster ride comes from the different forces acting on your body as the coaster changes directions and its velocity or the speed it is moving 4
All these motions are forces Forces are actions that start stop or change the shape or motion of a body Early scientists called natural philosophers noticed patterns of motion between the Sun and Moon and the changing seasons They asked questions and tried to explain their world Galileo studied motion in the late 1500s He dropped objects of different weights from a building He discovered that objects fall at the same rate Until then people believed heavy things fell faster than light ones Galileo’s discovery energized and upset scientific and religious communities F ma F ma Force Mass x Acceleration F resultant force measured in newtons N m mass measured in kilograms kg a acceleration measured in meters per second m s2 5
Was Galileo Wrong Galileo dropped two objects off a tower Try dropping a book and a sheet of paper Do they hit the ground at the same time They don’t Was Galileo wrong Not at all A sheet of paper has more air resistance That slows its fall Heavy objects are not as affected by air resistance as light ones Do the experiment in a vacuum where there’s no air They’ll hit the ground at the same time place the paper on top of the book Let go Do they fall at the same rate The book removes the air resistance They should hit the ground at the same time Try the experiment again This time 6
Sir Isaac Newton studied nature in the late 1600s He wondered why the Moon moved and never fell because he knew that when ordinary objects fell they always hit the ground Newton believed the same force that pulled objects down might be the same force that pulled the Moon He defined this force as gravity the unseen force holding the universe together Newton’s laws of motion and gravity came from his interest in the Moon Newton said all objects have mass Mass is the amount of matter in something Mass stays the same Everything with mass exerts or applies a force on every other thing Small masses exert less force so the force isn't as noticeable Isaac Newton 1643-1727 7
