Natural Questions. About 2000 years ago Greek scientists were confused about motion. and developed a theory of motion

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1 Natural Questions First natural question: Next question: What these things made of? Why and how things move? About 2000 years ago Greek scientists were confused about motion. Aristotle --- First to study motion seriously and developed a theory of motion In 1600 to1700 Galileo and Newton Aristotle s theory is inadequate. A new powerful theory was proposed by Newton and others. In 1900 (beginning), relativity and Quantum theories altered the Newtonian physics. But Newton theory is still useful.

2 Aristotle s theory Agrees with most people s common sense. Very ignorant. Motion Natural Violent Falling Flowing Rising Leaping Natural motion occurs b/c every thing on the earth is made of FOUR different elements. Every element has its natural rest place and strives towards that. Earth Water Air Fire Chinese have 5th element Wood

3 All natural motions are vertical Horizontal motion Violent Motion on earth is either natural or violent Motion in the heaven is entirely different Moon, stars, sun, etc are made of fifth element Ether Ether No weight Eternal (incorruptible) Its natural place is heaven It naturally moves in perfect circles around the earth (Celestial motion) All motions are due to nature of the moving objects or due to push or pull. If the object is at its proper place, it will not move unless subjected to force. The normal state is rest. But celestial object are exception Beyond question for 2000 years

4 Difficulties with Aristotle s theory Does crumbled paper fall faster than flat sheet? How can it be explained using Aristotle s theory? How will you get around this argument? Objects of same shapes but different weights. Aristotle s Prediction? Today s theory prediction: Galileo Galilei( ) --- dropping objects of various weights from Tower of Pisa Air resistance! Which object will fall faster? A stone twice as heavy as another Theory of moving earth. Nicolaus Copernicus( ) A Polish May 24, 1543 Muslims contributions (600 to 1500) especially in astronomy and optics Galileo was first to provide conclusive evidences against Aristotle s theory

5 Summary Aristotle theory of Motion Motion on earth is either natural or violent. Motion in the heaven is entirely different. Natural motion occurs b/c every thing on the earth is made of FOUR different elements. Every element has its natural rest place and strives towards that. Earth does not move rather, all celestial objects move around the earth. Difficulties Heavier objects do not take less fall time. Earth does move around the sun.

6 Aristotle was brilliant observer Vacuum was impossible Basic to Aristotle an object requires a push or pull to keep it moving A resistive medium is always involved in motion Galileo If there is no interference, the moving object will keep on moving in a straight line for ever; no push or pull in required. Ball roll down an incline plane What happens on horizontal plane? Galileo roughness Friction

7 On frictionless surface: Ball would speed up on any downward incline Ball would slow down on any upward incline On a perfect horizontal plane... Frictionless surface --- no friction, no air resistance, no gravity Rene Descartes What if we turn off the gravity? Stone will hang in mid-air. If we flick a stone with our fingure? An object at rest will be there unless someone moves it, And a moving object tends to move with same speed in same direction Tendency to maintain its state of motion is INERTIA Law of Inertia In the absence of any external force, an object at rest will stay at rest, and a moving object will keep on moving with constant (unchanged) speed in a straight line.

8 Restating Law of Inertia In the absence of any external force, an object at rest will stay at rest, and a moving object will keep on moving with constant (unchanged) speed in a straight line. Constant speed in a straight line constant velocity Remaining at rest constant velocity (zero) In the absence of external forces an object must maintain a constant (unchanged) velocity. How can an external force effect an object s motion? Law of Inertia External force must change the velocity Whenever there is change in velocity there must be external force/s The heart of Newtonian Physics

9 Aristotle's view: Brief Comparison Aristotle Galileo Newton A force had to act for an object to move and that the velocity of an object is proportional to the strength of the force. A heavier object falls more quickly than lighter. He was also aware of resistance. He never distinguished acceleration from velocity. Galileo s view: Falling objects. He argued that the natural tendency of a moving object is to continue moving. He developed the mathematical description of motion Newton s Contributions: Newton s Thee laws of motion Law of universal gravitation

10 Laws: Newtonian Physics Galileo s Law of Falling All the objects regardless of their shapes or mass, when dropped on the surface of earth fall with same constant acceleration Three Newton s Laws of Motion First Law or Law of Inertia In absence of any external force, an object will not be accelerated 2 nd Law acceleration = Force / mass Law of Force Pairs or 3 rd Law Newton s Law of Universal gravitation Concepts: Action and reaction are always equal but opposite in direction Centripetal Acceleration F = G Centripetal force m 1 m 2 r 2 Inertia Speed Velocity Acceleration Mass Force weight

11 Idea of Gravity Newton saw an apple falling from tree while moon stayed in the sky Both moon and apple were more or less round Apple is on earth and moon is in the sky Apple falls to the ground while moon stays aloft But Newton saw the similarity Although velocities of apple and moon are different, forces on two may be similar. V, a, F All towards the center of earth We are interested in Force Aristotle No force is required, How do the forces compare? b/c it is moon s nature Law of Inertia Force is required in order for moon to deviate from straight line motion Moon would What is the direction of force? If there were no force move straight Moon does two things: Its inertia keeps it moving forward in straight line Simultaneously pulled inward: It also falls Force is required, which must be directed towards earth just like the force on apple. Both forces must have same source --- Gravitational attraction

12 Although the velocity of a falling apple and the moon are quite different, the acceleration of both are directed towards the center. True for all objects moving in circle Then why moon or other objects stay up? Newton s Law of Universal Gravitation If you throw a ball horizontally, what will be its path straight curved Larger the horizontal velocity, farther from the base, the ball will land At very large velocities the curvature of earth becomes significant. If the launch velocity is large enough, the projectile would never reach the surface of the earth. It keeps falling, but earth s curvature falls away too. You need a speed of 8 km/s (29000 MPH). Newton s view: Moon is actually falling, just as a projectile does. The same force that accounts for the acceleration of object near the surface of earth, explains the orbit of the moon.

13 Moon is planet of earth and Earth is planet of sun Planets keep moving forward because of inertia. The sun s gravitational pull bends their orbit into ellipses Law of force pairs every satellite must also pull back. That is gravitational pulls on both bodies All the satellites must also be pulling each other (Earth and Mars) Why the gravity be restricted to astronomical bodies. It should also be between smaller bodies? Gravitational force is universal

14 Limitations of Newtonian Physics 18 th and 19 th Centuries Newtonian Physics was accepted as an ultimate truth Science is never absolute Hundreds of experiments can t prove my theory right but only one experiment can prove it wrong. Einstein Last two decades of 19 th century Some experimental results appear those were impossible to reconcile with Newtonian Physics. In the beginning of 20 th century, three new theories were developed: Special theory of relativity General theory of relativity Quantum theory Newtonian Physics give incorrect predictions in four types of situation When speeds are very high Special theory of relativity deals with this When gravitational forces are very strong When distances are extremely small Quantum theory deals with this When distances are very large At speed of 30, 000 km/s Roughly around the world in 1 second), 1 % off. At speed of 290, 000 km/s (very close to speed of light), 1500 % off General theory of relativity deals with this. Comparable to atomic size An object can not be both small and slow moving General theory of relativity deals with this. Newtonian physics is good approximation to reality