Professor L. Franklin Modern Physics

Transcription

1 SCIT 1418 Professor L. Franklin Modern Physics Textbook : Conceptual Physics Twelfth Edition Author = Paul G. Hewitt SCIT Applied Physics I Houston Community College Fall 2017 Lecture/Laboratory Science and Technology NE, Room number 106 leon.franklin@hccs.edu HCC/NE Month Day Assignment: Chapter(s), Tests, Labs, Final, etc. September 1 INTRO September 8 Chapter 2, 3 September 15 Chapter 4, 5 September 22 Test #1 (Chapter 2, 3, 4, 5) September 29 Lab #1 October 6 Chapter 6, 7 October 13 Chapter 8,9 October 20 Test #2 (Chapter 6, 7, 8, 9) October 27 Lab #2 November 3 Chapter 10, 12 November 10 Chapter 13, 14, 15 November 17 Test #3 (Chapter 10, 12, 13, 14, 15) November 24 Lab #3 December 1 Chapter 16, 17 December 8 Chapter 18 December 15 Final

2 The following at home projects and assignments were added to compensate for the late start due to Hurricane Harvey 1. Energy Class group project Group 1 do all objects around us have energy if they are not moving? 2. Answers : Also apart from mechanical energy: yes. All kinds of energy: nuclear energy from the forces that hold the atomic nucleus together, chemical energy from the energy levels of the electrons that do or do not bind to other atoms. heat energy from the tiny motion and vibration of the atoms and so forth. What is Newton's third law? Learn about the fact that forces come in pairs. What is Newton's third law? You probably know that the Earth pulls down on you. What you might not realize is that you are also pulling up on the Earth. For example, if the Earth is pulling down on you with a gravitational force of 500 N, you are also pulling up on the Earth with a gravitational force of 500 N. This remarkable fact is a consequence of Newton's third law. Newton's third law: If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A. This law represents a certain symmetry in nature: forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as action-reaction, where the force exerted is the action and the force experienced as a consequence is the reaction.

3 We can readily see Newton s third law at work by taking a look at how people move about. Consider a swimmer pushing off from the side of a pool, as illustrated below. A swimmer pushes on the wall with her feet, which causes the wall to push back on her feet due to Newton's third law. Image credit: Adapted from Openstax College Physics The swimmer pushes against the pool wall with her feet and accelerates in the direction opposite to that of her push. The wall has exerted an equal and opposite force back on the swimmer. You might think that two equal and opposite forces would cancel, but they do not because they act on different systems. In this case, there are two systems that we could investigate: the swimmer or the wall. If we select the swimmer to be the system of interest, as in the image below, then Fwall on feetf_{\text{wall on feet}}fwall on feetf, start subscript, w, a, l, l, space, o, n, space, f, e, e, t, end subscript is an external force on this system and affects its motion. The swimmer moves in the direction of Fwall on feetf_{\text{wall on feet}}fwall on feetf, start subscript, w, a, l, l, space, o, n, space, f, e, e, t, end subscript. In contrast, the force Ffeet on wallf_{\text{feet on wall}}ffeet on wallf, start subscript, f, e, e, t, space, o, n, space, w, a, l, l, end subscript acts on the wall and not on our system of interest. Thus Ffeet on wallf_{\text{feet on wall}}ffeet on wallf, start subscript, f, e, e, t, space, o, n, space, w, a, l, l, end subscript does not directly affect the motion of the system and does not cancel Fwall on feetf_{\text{wall on feet}}fwall on feetf, start subscript, w, a, l, l, space, o, n, space, f, e, e, t, end subscript. Note that the swimmer pushes in the direction opposite to that in which she wishes to move. The reaction to her push is thus in the desired direction. What are other examples of Newton's third law?

4 Other examples of Newton s third law are easy to find. As a professor paces in front of a whiteboard, she exerts a force backward on the floor. The floor exerts a reaction force forward on the professor that causes her to accelerate forward. Similarly, a car accelerates because the ground pushes forward on the drive wheels in reaction to the drive wheels pushing backward on the ground. You can see evidence of the wheels pushing backward when tires spin on a gravel road and throw rocks backward. In another example, rockets move forward by expelling gas backward at high velocity. This means the rocket exerts a large backward force on the gas in the rocket combustion chamber, and the gas therefore exerts a large reaction force forward on the rocket. This reaction force is called thrust. It is a common misconception that rockets propel themselves by pushing on the ground or on the air behind them. They actually work better in a vacuum, where they can more readily expel the exhaust gases. Helicopters similarly create lift by pushing air down, thereby experiencing an upward reaction force. Birds and airplanes also fly by exerting force on air in a direction opposite to that of whatever force they need. For example, the wings of a bird force air downward and backward in order to get lift and forward motion. [Hide explanation.] This article was adapted form the following article: 1. "Newton s Third Law of Motion: Symmetry in Forces" from Openstax College Physics. Download the original article free at 6c8ed997733a@9.1:26/Newtons-Third-Law-of-Motion-Sy. What do examples involving Newton's third law look like? Example 1: Fridge push A person drives a cart, Cart 1, to the right while pushing another cart, Cart 2, that has a massive refrigerator on it. The total mass of Cart 2, cart plus fridge, is three times the total mass of Cart 1, cart plus person. If the person is driving with enough force that the two carts accelerate to the right, what can be said for sure about the magnitudes of the forces on the carts? Choose 1 answer: Choose 1 answer: A

5 The force on Cart 2 exerted by Cart 1 is larger in magnitude than the force on Cart 1 exerted by Cart 2. B The force on Cart 2 exerted by Cart 1 is smaller in magnitude than the force on Cart 1 exerted by Cart 2. C The force on Cart 2 exerted by Cart 1 is equal in magnitude to the force on Cart 1 exerted by Cart 2. D The force on Cart 2 exerted by Cart 1 could be larger or smaller depending on the size of the acceleration. The correct answer is that the force on Cart 2 exerted by Cart 1 is equal in magnitude to the force on Cart 1 exerted by Cart 2. It doesn't matter whether there is acceleration or whether the mass of one object is larger than another object. Force reaction pairs are always equal in magnitude. We know the forces in this question are force reaction pairs since exchanging the order of the objects in the phrase "force exerted on *Cart 2 by Cart 1* gives us "force exerted on *Cart 1 by Cart 2*. Exchanging the order of the objects involved in a force i.e., the object exerting the force and the object the force is exerted on is an easy way to identify a third law force pair. Example 2: Third-law-force pairs A box sits at rest on a table as seen in the image below. Various forces are listed in the table below the image. Drag the forces in the right column so that they're lined up with their Newton's third law partner force in the left column. First force of a third-law pair Second force of a third-law pair

6 To find the Newton's third law partner force, we can simply reverse the order of the objects in the force description. For example, the pairs listed below all form Newton's third law force pairs. The partner force for the downward force of gravity on the box exerted by Earth would be the upward force of gravity on Earth exerted by the box. The partner force for the downward force on the table exerted by the box would be the upward force on the box exerted by the table. The partner force for the downward force on the ground exerted by the table would be the upward force on the table exerted by the ground. The partner force for the downward force of gravity on the table exerted by Earth would be the upward force of gravity on Earth exerted by the table. 3. Satelite in motion - How does the craft in space get up to that speed without the jets and how does it maintain that speed? The craft uses a propulsive engine a bit like a jet to escape earth's atmosphere and get to the right height and then it lets gravity accelerate it back to earth. But it uses its engines to give it enough momentum to miss earth and so fall constantly in an orbit. Its speed is maintained purely by gravity, maybe using it's engines every so often if its speed deviates too much. 4. The Atomic Nature of Matter History of the atom Atomic Nature of Matter. All matter is made up of tiny building blocks called atoms which combine to form molecules which

7 combine to form compounds.... Atomic mass is equal to the sum of the masses of all atomic particles (protons, neutrons, electrons) found in the atoms of the same element Solids 6. Liquids