 In the 4 th Century B.C., the Greeks were developing the basic ideas of motion.  The foremost Greek Scientist of the time was Aristotle. 2

 Aristotle divided motion into: › Natural Motion › Violent Motion  Natural motion was limited to motion that was: › Straight up › Straight down 3

 Objects would seek their natural resting place: › A boulder would end up on the ground. › Smoke would end up in the air.  The Greeks assumed that it was natural for: › Heavy things to fall. › Light things to rise. 4

 Aristotle also extended natural motion to include circular motion in the heavens. › He thought circular motion was without beginning or end › He thought the heavens were without beginning or end. 5

 Aristotle postulated that planets and stars moved in perfect circles.  He thought since all this motion was natural, that it was not caused by forces. 6

 Violent motion was imposed motion, caused by a push or a pull. › A cart moved because of the force applied by the oxen. › A ship moved from the force of the wind › A boat move from the force applied by the oars  Violent motion was the result of some external cause. 7

 For an object to move from its natural resting place it had to be pushed or pulled.  An object moving “against its nature” was the result of an external force.  The proper state of objects was at rest.  This was believed for 2000 years, until the 16 th Century. 8

 It was believed that the Earth was in its natural resting place.  They also thought that there was no force strong enough to move the Earth.  Therefore, the Earth did not move. 9

 Copernicus was the first person to put forth the theory of the moving Earth. 10

 He thought that the easiest way to explain astronomical observations was that the Earth moved around the Sun.  He also thought the other planets moved around the Sun. 11

 This theory was very controversial.  People believed the Earth was the center of the Universe.  Copernicus had to work in secret to avoid persecution.  Only at the end of his life did he publish his ideas, the first copy reaching him the day he died. 12

13 The Lunar Crater Copernicus

 Galileo was the most important scientist in Italy in the later part of the Renaissance.  Strongly supported the ideas of Copernicus.  Was arrested as a result of these beliefs and put on trial. 14

 Galileo is given credit for getting rid of the idea that a force is necessary to keep an object moving.  Force – a push or pull  Friction › Resists motion › Acts between materials that touch and are in motion › Caused by irregularities of the surfaces of objects. 15

 Galileo said that only when friction is present do you need a force to keep an object moving.  In other words, in the absence of friction an object will keep moving forever. 16

 To prove his ideas Galileo used a ball which he rolled down an inclined ramp and then allowed the ball to continue up another inclined ramp. 17

 As the ball rolls down the ramp, it gains speed.  As the ball rolls up the second ramp, it loses speed  Without friction, it will end at the same height it started. 18

 What if there was only a ramp to roll the ball down onto a level surface.  Where will the ball stop? 19

 Where the ball stops depends on friction.  Galileo realized that the natural state was not at rest.  In the absence of friction, the ball will continue rolling forever. 20

 Galileo stated that every object resists a change in its motion.  The resistance to change in motion is called inertia 21

 Galileo addressed how things move rather than why.  He based his findings on experimentation rather than logic.  His ideas about motion and inertia discredited the work of Aristotle. 22

 Newton restated Galileo’s ideas:  Every object continues in a state of rest, or of motion in a staight line at a constant speed unless compelled to change that state by forces exerted on it. 23

 Simply stated: Objects at rest tend to stay at rest. Objects in motion tend to stay in motion. 24

 Take a table with a tablecloth and plates.  If done correctly, you can pull out the tablecloth and the plates will remain in place.  The plates are at rest, and want to stay at rest. 25

 As the tablecloth is pulled quickly away, there is some friction and the plates will move slightly.  The friction is quickly overcome and the plates remain and the tablecloth is removed. 26 Go to the Videotape

 Which slides further? › A hockey puck on a street? › A hockey puck on ice? › Why?  What would happen if the astronauts could throw an object out of the International Space Station? 27

 Objects in a force-free environment will move in a straight line, forever. 28 Go to the Videotape