1029:006 — Lecture 2Mechanics (M1)Why do things move?Historical Perspective1Aristotle• 350 BC• Was the final word on any scientific question• Influenced scientific thought until the end of the 17thcentury• Believed that the natural state of an object was to be at rest—He was WRONG!2Galileo 1564-1642• To understand nature, you must first observe it• He is considered the “Father of Modern Science”• Imprisoned by Pope Urban VIII in 1633 for advocating that the earth was a planet revolving around the sun (heliocentric hypothesis)• Pope John Paul II in 1992 declared that the Church was in error regarding Galileo.3Galileo, continued• Previous thinking, accepted for 15 centuries, held that the earth was the center of the universe (geocentric hypothesis)• Invented the first useful telescope in 1609• Discovered the rings of Saturn• He performed the first experimental studies of motion4Tycho Brahe (1546-1601) and Johannes Kepler (1571-1630)Tycho Brahe compiled the first detailed observational data on planetary motion (Mars), without a telescope! No one had previously attempted to make so many planetary observations.T. BraheJ. Kepler5Johannes Kepler derived the laws of planetarymotion using the data obtained by Brahe.Isaac Newton• Born Jan 4, 1642• Published The Principia in 1687, considered the greatest scientific book ever written• Discovered the 3 laws of mechanics, known as Newton’s Laws• Based on the work of Kepler, he discovered the Law of Gravity62Newton, continued• Showed that the same laws that govern the fall of objects on earth also govern the motion of the planets.• Newton’s work followed directly from the experimental work of Galileo and Kepler’s analysis of the observations of Brahe• Scientific progress:Brahe Galileo & Kepler Newton 7Why does something move?Because nothing stops it!8Newton’s laws of motion • Newton’s 1stlaw (Galileo’s principle of inertia)“A body at rest tends to remain at rest; a body in motion tends to remain in motion.”• Newton’s 2ndlaw (law of dynamics)“The rate of change of the velocity of an object (i.e., its acceleration), is the net force exerted on it divided by its mass.”• Newton’s 3rdlaw“For every action (force) there is an equal and opposite reaction.”9Law of Inertia - examples• Pull the tablecloth out from under the dishes• Knock the card out from under the marble• Hoop and Pen• Knock the plate out under the egg • Hammer head• Shake the water off of your hands• The car on the air track keeps going• Homer not wearing his seatbelt10If you are at rest, you tend to stay at rest; if you are moving,you tend to keep moving, unless something stops you. Dogs use the principle of inertia!• When a dog is wet, he twists his body back and forth to shake off the water.• When the dog rotates his body in one direction, the water is set into motion.• When the dog twists the other way, the water drops keep moving in the original direction and fly off of him.• We do the same thing when we shake our hands after washing them.11Galileo’s principle of Inertia• A body at rest tends to remain at rest• A body in motion tends to remain in motionOr stated in another way:• You do not have to keep pushing on an object to keep it moving• If you give an object a push, and if nothing tries to stop it, (like friction) it will keep going• The “natural state” of an object is not rest123Ice Hockey 13Physics and Ice HockeyNo force is needed to keep the puck movingforward after it leaves the player’s stick.14What is inertia?• All objects have it• It is the tendency to resist changes in velocity– if an object is at rest, it stays at rest– if an object is moving, it keeps moving• Mass is a measure of the inertia of a body, in units of kilograms (kg)= 1000 grams• Mass is NOT the same as weight !15Bart is on the moving train and then jumps straight up on the moving trainwill he land:1) on the ground, or2) on the train? Bart maintains his forward motion even as he jumps up. He lands on the train.16Other examples• Having a catch on a plane, bus or train• Throwing a ball up and down while walking• Dribbling a basketball while running17Refined Law of Inertia• No force (push or pull) is needed to keep an object moving with constant velocity• Constant velocity- moving in a straight line with constant speedNo stopping or turningNote that a body at rest hasa constant velocity of zero184Concepts: speed and velocity• Speed: How fast am I going?measured in miles per hour (mph),kilometers per hour (km/h), feet per second (ft/s), meters per second (m/s), . . . distancespeed = = distance ÷timetime19Velocity includes speed and direction• Velocity includes information both about the speed (magnitude) and direction, not only how fast, but also in what direction• It is what we call a vector quantity – one having both magnitude and direction• Formula to calculate the magnitudedv = t20ExampleThe average human can walk at 5 km/hr[1 kilometer (km) = 1000 m]. If a person walks at this rate for half a day, how far would he or she travel?AnswerSince v = d / t, then d = v  t d = v  t = 5 km/hr  12 hr = 60 km21Position vs. time plots• Case A: speed is20 m/10 s = 2.0 m/s• Case B: speed is10 m/10 s = 1.0 m/s• Case C: speed is5 m/10 s = 0.5 m/sv = d / t220 1 2 3 4 5 63 m6 m0time (seconds)distance (meters)EXAMPLE• t = 0 to t = 1 s  velocity = 3 m / 1s = 3 m/s• t = 1 s to t = 3 s  velocity = 0 m/s (at rest)• t = 3 s to t = 6 s  velocity = 3 m / 3 s = 1