1/22/14 Lecture PowerPoint Chapter 3 Chapter 3 How Things Move: Galileo Asks the Right Questions Physics: Concepts & Connections 4th Edition Art Hobson © 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Units of Chapter 3 Aristotelian Physics: A Commonsense View How Do We Know? Difficulties with Aristotelian Physics The Law of Inertia: The Foundation of Newtonian Physics Measuring Motion: Speed and Velocity Measuring Motion: Acceleration Falling 3.1 Aristotelian Physics: A Commonsense View Aristotle distinguished three different kinds of motion. Natural motion: falling objects and liquids, rising air and flames Violent motion: needing a constant push or pull to continue Celestial motion: motion of the moon, planets, sun, and stars 1 1/22/14 3.2 How Do We Know? Difficulties with Aristotelian Physics 3.2 How Do We Know? Difficulties with Aristotelian Physics Take a flat piece of paper and drop it. Then wad the paper up and drop it again. Aristotle predicts both fall at the same speed. Galileo did detailed studies of falling and rolling objects, and formulated his Law of Falling: Aristotle also predicts that heavier objects will fall faster than lighter ones. Is this true? 3.2 How Do We Know? Difficulties with Aristotelian Physics Now, throw a ball across the room. Once it leaves your hand, what keeps it moving? Aristotle says there must be a constant force to keep it in motion. Galileo’s thought experiment: Let a ball roll down an incline; it will speed up. Let it roll up the incline; it will slow down. In between, on a perfectly flat surface with no friction, the ball will keep rolling at a constant speed forever. If air resistance is negligible, then any two objects that are dropped together will fall together, regardless of their weights and their shapes, and regardless of the substances of which they are made. 3.2 How Do We Know? Difficulties with Aristotelian Physics Galileo’s method: Experimentation, to test a specific hypothesis Idealization, to eliminate side effects Consider only one question at a time Quantitative methods: precise measurement 2 1/22/14 3.3 The Law of Inertia: The Foundation of Newtonian Physics Imagine we could turn off air resistance, friction, and gravity. How would things move? Descartes had the answer (which Newton took as his first principle of motion): The Law of Inertia: A body that is subject to no external influences (also called external forces) will stay at rest if it was at rest to begin with and will keep moving if it was moving to begin with; in the latter case, its motion will be in a straight line at an unchanging speed. In other words, all bodies have inertia. 3.3 The Law of Inertia: The Foundation of Newtonian Physics 3.3 The Law of Inertia: The Foundation of Newtonian Physics For example, if you are driving a car on an icy road – a VERY icy road – and try to stop or turn, you will find that your car continues to go in a straight line, as there is little friction. You can also watch videos of astronauts – things will float around until somebody grabs them, or they run into the wall. 3.3 The Law of Inertia: The Foundation of Newtonian Physics Where does outer space begin? Astronauts in “outer space” still move under the influence of gravity; they are basically falling all the time, and thus experience apparent weightlessness. 3 1/22/14 3.4 Measuring Motion: Speed and Velocity Speed is the distance an object moves divided by the time it takes to move. This coaster is moving at 35.2 cm/s. 3.4 Measuring Motion: Speed and Velocity Suppose you drive 100 km in one hour. Your speed would be 100 km/h. But this is your average speed over the whole trip – how likely is it that you were traveling at exactly 100 km/h the whole time? Your instantaneous speed is your speed at one particular moment; this is what your speedometer measures. 3.4 Measuring Motion: Speed and Velocity So, are speed and velocity the same thing? Not in physics! Speed is what you would read on a speedometer; velocity is the combination of speed and direction. So if I tell you I am driving 100 km/h, that is my speed; if I tell you I am driving 100 km/h north, that is my velocity. 3.5 Measuring Motion: Acceleration The law of inertia tells us that an undisturbed object will keep moving with a constant velocity. If an object’s velocity is changing, it is accelerating. Acceleration is the rate of change of velocity: acceleration = (change in velocity)/time Acceleration is measured in (m/s)/s, or m/s2. 4 1/22/14 3.6 Falling Falling objects speed up as they fall. How do we know this? Hold your book above the floor and let it drop. What is its speed right when you let it go? (Don’t throw it!) What is its speed when it hits the floor? 3.6 Falling We see that the speed is proportional to the time – 10 m/s at 1 s, 20 m/s at 2 s, and so forth. What about the distance? Clearly it is not proportional to the time. However, if we look at the pattern of how the distance changes from second to second, we can see that the distance is proportional to the square of the time. 3.6 Falling This diagram shows an object falling. Note that both its distance and its velocity are increasing. From one second to the next, the distance traveled increases, but the change in velocity is the same. 3.6 Falling The object in the diagram is changing its speed at a rate of 10 m/s per second. Therefore, its acceleration is 10 m/s2. This is (approximately) the acceleration due to gravity anywhere on the Earth. 5