Chapter 3. How Things Move. Sections. 3.1- Aristotelian Physics: A commonsense View 3.2- How Do We Know? Difficulties with Aristotelian Physics 3.3- The Law of Inertia: The Foundation of Newtonian Physics 3.4- Measuring Motion: Speed and Velocity 3.5- Measuring Motion: Acceleration
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Chapter 3 How Things Move
Sections • 3.1- Aristotelian Physics: A commonsense View • 3.2- How Do We Know? Difficulties with Aristotelian Physics • 3.3- The Law of Inertia: The Foundation of Newtonian Physics • 3.4- Measuring Motion: Speed and Velocity • 3.5- Measuring Motion: Acceleration • 3.6- Falling
3.1- Aristotelian Physics: A Commonsense View • - Aristotle’s physics- theorized that there were three kinds of motion: natural, violent and celestial motion. • - Natural motion- Unassisted motion like falling is one form of natural motion • - Violent motion- horizontal motion • - Celestial motion- Aristotle believed that the moon, sun, planets, and stars were made of a substance called ether. Ether moved in perfect circles around Earth. • -Ether- in theory ether had no weight and was incorruptible.
3.2- How Do We kow? Difficulties with Aristotelian Physics • Aristotelian physics has certain weaknesses. For Example: If a crumpled piece of paper about the same size as a baseball are dropped they fall the same speed, except when dropped for a long distance there is a small difference do to air resistance. This outcome could not be explained my Aristotle’s theory. • Air resistance- The resistance to the motion object through the air due to the object’s collisions with numerous air molecules. • When air is not present during to ball and paper experiment, the objects fall at exactly the same speed.
3.2 • Galileo’s Law of Falling- 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. • Problem with violent motion: When arrow is shot from a bow, it can travel great distances without constant external assistance. So what keeps it moving? Aristotle had great difficulty explaining this question with his theory. • Friction- The force that one surface exerts on another due to the roughness of the surfaces. • Galileo theorized that in absence of friction, a ball that once started rolling on a horizontal surface would roll forever. This statement was completely different from those made by Aristotle.
Galileo’s Methods • Experiments- Designed to test specific hypotheses. • Idealizations- Of real-world conditions, to eliminate any side effects that might obscure the main effects. • Limiting the scope of the inquiry- by considering only one question at a time. For example, Galileo separated horizontal from vertical motion, studying only one of them at a time. • Quantitative Methods- Galileo went to great lengths to measure the motion of bodies. He understood that a theory capable of making quantitative predictions was more powerful than one that could make only descriptive predictions, because quantitative predictions were more specific and could be experimentally tested in greater detail.
3.3- The Law of Inertia: The Foundation of Newtonian Physics • Rene’ Descartes a French philosopher, was the first to imagine the absence of gravity. He asked the question, “What would happen if gravity was turned off?” • His theory was that if gravity absent, an object released in mid-air would not fall to the ground. Therefore if you were to throw the object if would travel at the same speed in a straight line forever. • The Law of Inertia- A body that is subject to no external influences 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. All objects have inertia.
Space, The Final Frontier • Outer Space- refers to those regions of the universe outside Earth and outside other astronomical objects. • In outer space there is zero gravity. A good example of this is when astronauts travel to the moon, they fire their rocket boosters to achieve the desired speed, and then coast all the way to the moon. • Earth coasts in outer space. This explains what Copernicus could not, when asked how earth was to continue moving.
3.4- Measuring Motion: Speed and Velocity • In order to obtain quantitative predictions about our physical existence; further more to discover more accurate predictions, numerical methods must be used. • Average speed- An object’s average speed is its distance traveled during a time interval divided by the duration of that time interval. Measured in meters per second. • Instantaneous Speed (speed)- The average speed during a time interval that is so short that the speed hardly changes. Speedometers measure this. Measured in meters per second. • Velocity- refers to speed and direction. Speed and velocity are not interchangeable.
3.5 Measuring Motion: Acceleration • Concise Law of Inertia- A body that is subject to no external forces must maintain an unchanging velocity. • Acceleration- An accelerated object is one whose velocity is changing. Quntitatively, the acceleration is the change in velocity during a time interval divided by the duration of that time interval. It can be measured in (km/hr)/s, or in (m/s)/s=m/s^2.
3.6- Falling • Free Fall- An object like this whose falling is influenced by gravity alone. • Acceleration due to gravity- The acceleration of any freely falling object. On Earth this is about 10 m/s^2 or, more precisely, 9.8 m/s^2. • Proportional- One quantity is proportional to a second quantity if, whenever the second is multiplied by some number, the first is multiplied by the same number. One quantity is proportional to the square of a second quantity if, whenever the second is multiplied by some number, the first is multiplied by the same number squared. (HAHAHA!)