Transcript Chapter 12
Chapter 12 Mechanics: Physics of Motion The Path to Modern Physics Greek Philosophy: Science began with the Greeks… However…the Greek philosophers rarely checked their ideas experimentally and so had false conclusions… The Path to Modern Physics In the 1600’s Italian astronomer, Galileo began to question the truthfulness of Aristotle’s ideas about falling objects… Galileo viewed things as special creations of God and expected the universe to be orderly…and to operate under specific laws… The Path to Modern Physics Galileo greatly influenced an English scientist named Sir Issac Newton… Also guided by a desire to show God’s order in the universe, Newton built on Galileo’s ideas… Newton discovered the laws of motion and of gravity… Modern Physics We are now in the modern age of physics.. Scientists like Robert Boyle, Daniel Bernoulli, and Albert Einstein… Mechanics: The Science of Motion • Our world is in constant motion… • Our earth moves, our moon moves… and our galaxy moves… • The same laws of physics that govern the motion of the planets and stars controls the motion of objects here on earth like the motion of cars… • Mechanics: the study of objects in motion Mechanics: The Science of Motion • All motion is relative to some point…called a reference point. • The term “position” refers to the location of the object. • If an object is found at one position and later at another, we say the object has moved, or changed position. • When the object is undergoing a continuous change in position, we say the object is in motion. Mechanics: The Science of Motion • In order to measure motion, scientists must be able to describe it. • They use three terms to describe motion: 1. speed 2. velocity 3. acceleration Speed: Rate of Motion • Speed is the distance traveled in a given time. • The equation to calculate speed is: speed = distance/time s = d/t • Usually you are calculating for average speed… Velocity: Speed and Direction • In physics, speed and velocity are NOT the same thing… • Speed refers to the distance covered by an object in a given time. • Velocity takes into account direction as well… • Velocity is a “vector” quantity…which means it includes magnitude and direction… Velocity: Speed and Direction • Physical quantities with only magnitude are called scalar quantities… • A vector quantity is signified by an arrow… Velocity: Speed and Direction While on vacation, Lisa Carr traveled a total distance of 440 miles. Her trip took 8 hours. What was her average speed? Velocity Vectors Combining Velocities: If an object is moving in more than one direction we can find a resultant vector velocity using Pythagorean theorem… a2 + b2 = c2 Velocity Vectors Velocity Vectors Forces and Motion • Objects do not move on their own… • They need a push or a pull to change their motion… • That push or pull is called a FORCE… • Without a force, their can be no change in motion… Acceleration: A Change in Motion • A change in velocity is called acceleration. • There are three ways to change acceleration: 1. speed up 2. slow down 3. change direction Acceleration: A Change in Motion • Acceleration can only occur if a force acts on it… • Without a force…there can be no acceleration!! Measuring Acceleration The unit for acceleration is m/s2 Newton’s First Law of Motion Inertia causes an object to resist change in its state of motion… Newton’s First Law An object in motion stays in motion, an object at rest stays at rest unless acted on by an outside force. Newton’s 1st Law An example of this law is the wearing of seat belts… When a car crashes, your body’s inertia carries it forward as the car stops…an outside force (a seatbelt) is the only thing that will cause a change in your bodies motion! Inertia Newton’s 2nd Law The value of an unbalanced force is equal to its mass multiplied by its acceleration. Or F = ma Force = mass x acceleration The unit of force is the Newton Newton’s 2nd Law An object accelerates in the direction of the force applied. It takes more force to get a big car rolling and likewise to stop it than a smaller car. Newton’s 3rd Law For every action there is an equal and opposite reaction. This is known as the action-reaction law. Newton’s 3rd Law Force always comes in pairs… if there is a push on an object the object will push back. Example: A rocket Newton’s 3rd Law Forces A force is a push or a pull… Forces always come in pairs… Remember…equal and opposite. Momentum Momentum (p) is defined as mass (m) times velocity (v). p = mv Momentum is the quantity of motion. The greater the mass, the greater the momentum the moving object has. Momentum The unit for momentum is kg·m/s. A small bullet can have a large momentum because of its velocity… A large truck could have a small amount of momentum if moving slowly. Conservation of Momentum Momentum is conserved. In a collision, the momentum of one object is transferred to another so that the total momentum of the two objects after a collision is the same as before. Gravity Gravity is a distance force (or as your book says “action-at-a-distance force”). No one really knows how distance forces work or why… It is still hotly debated and researched by scientists today. Universal Law of Gravity Gravitational forces are directly related to the mass of the objects. Gravity Falling bodies: All objects near the earth fall at the same rate… Acceleration due to gravity is 9.8 m/s2 Air Resistance So…Why don’t the elephant and feather fall at the same rate? Air Resistance!! Distance of a Falling Object For a falling object, the distance that it travels can be calculated with the following equation: d = ½ a·t2 distance = ½ acceleration x time2 For a falling object acceleration is gravity so it is 9.8 m/s2. Drag and Terminal Velocity When an object falls through the atmosphere, gravity is pulling it down… air is pushing up on it… eventually, these two forces equal out and the object no longer falls any faster. Drag and Terminal Velocity When this happens, it is called… Terminal Velocity The object is still falling, it just is not gaining speed. It is falling at a constant speed. Weight as a Force Weight is a measurement of the pull of gravity on a mass. W = mg Weight = mass x acceleration due to gravity W = m(9.8m/s2) Weight as a Force Remember, mass and weight are two different things!! Mass is the amount of matter an object has and weight is the pull of gravity on the mass. If you were placed on the moon, your mass would not change! But, your weight would! Forces Two basic types of forces: 1. Contact Forces Example: Friction 2. Distance Forces: Example: Gravity and Nuclear Forces General Types of Forces Gravity Friction Nuclear Air Resistance Magnetic Electrical Forces Only an unbalanced force (net force) causes a change in the motion of the object. A balanced force does not cause change.