Lessons - Physics
Lesson 1: Modeling Free Fall
In the weeks before school was called off, we were studying Newton's 2nd law. Newton's 2nd law lets us say something about motion knowing something about net force or something about net force knowing something about motion, because a net force is what causes a mass to accelerate. For a given mass, the acceleration it experiences is directly proportional to the net force applied (and in the same direction). For a given force, the acceleration an object experiences is inversely proportional to its mass (because bigger masses have more inertia... they are "lazier" so harder to get moving). All of this is summarized with the model F = ma.
The class before school was called off was spent using our knowledge of Newton's 2nd law to improve our model of falling objects developed in "Unit 5: Acceleration due to Gravity". In Unit 5 we modeled all objects in free fall in the absence of air as experiencing a uniform acceleration of 9.81 m/s^2. While this is true, we happen to live in the presence of air, and friction, namely air resistance, has quite a significant impact on the accuracy of the predictions we might make about falling objects using the Unit 5 model. Turns out objects falling in air don't experience a uniform acceleration, but rather one that diminishes as the object falls faster and air resistance builds up, reducing the net force on the object. In fact, if an object falls long enough, it can reach a "terminal velocity" when the net force on it goes to zero (air resistance and weight balance), so it's acceleration goes to zero, it gets no faster, and spends the rest of its trip to the ground at a constant speed. We saw this clearly in the lab where dropped coffee filters.
In this lesson we will ask and answer the classic physics question about if two objects of different mass, say a feather and an elephant, are dropped in the presence of air, which will hit the ground first? What about in the absence of air? Knowing, understanding, and believing the answers to these questions is fundamental to the course.
The class before school was called off was spent using our knowledge of Newton's 2nd law to improve our model of falling objects developed in "Unit 5: Acceleration due to Gravity". In Unit 5 we modeled all objects in free fall in the absence of air as experiencing a uniform acceleration of 9.81 m/s^2. While this is true, we happen to live in the presence of air, and friction, namely air resistance, has quite a significant impact on the accuracy of the predictions we might make about falling objects using the Unit 5 model. Turns out objects falling in air don't experience a uniform acceleration, but rather one that diminishes as the object falls faster and air resistance builds up, reducing the net force on the object. In fact, if an object falls long enough, it can reach a "terminal velocity" when the net force on it goes to zero (air resistance and weight balance), so it's acceleration goes to zero, it gets no faster, and spends the rest of its trip to the ground at a constant speed. We saw this clearly in the lab where dropped coffee filters.
In this lesson we will ask and answer the classic physics question about if two objects of different mass, say a feather and an elephant, are dropped in the presence of air, which will hit the ground first? What about in the absence of air? Knowing, understanding, and believing the answers to these questions is fundamental to the course.
Video (intro)Video: Free Fall Explained 1
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Lab |
Video (debrief & deploy)Video: Free Fall Explained 2
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Practice |
Lesson 2: Modeling Inclined Planes
In this lesson we will improve our Newton's 2nd law model so that we can use it to make predictions about the forces on and the motion of objects that are on surfaces other than flat ones, what we call "inclined planes".
Video (intro)Video: Inclined Planes 1
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Lab |
Video (debrief & deploy)Video: Inclined Planes 2
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Practice |
Lesson 3: Modeling Newton's 3rd Law
Newton's 1st law describes what an object will do (or won't do) when all the forces at work on it add to zero (won't change its motion). Newton's 2nd law describes what an object will do when the forces don't add up to zero (will change its motion, aka "accelerate") and gives us a definition of what a force really is, not just a push or a pull, but that thing that accelerates a mass (F= ma). Newton's 3rd law further defines a force as not existing independently, but always as part of an interaction. In other words, forces always come in pairs. In this lesson we will model the relationship between an "action" force and a "reaction" force.
Video (intro)Video: Newton's 3rd Law & Systems 1
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Lab |
Video (debrief & deploy)Video: Newton's 3rd Law & Systems 2
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Practice |
Lesson 4: Modeling Work & Energy
Newton proposed his three laws in 1686. In the 1800's, during the industrial revolution, society began using machines to do the work of humans. Scientists became interested in determining how much work they could get a machine to do, an idea they related to "energy", and thus the "work-energy model" was born. The work-energy model can be used to answer questions about force and motion, just like Newton's, but can many times do so in a simpler fashion, especially when the path something took from point A to point B isn't known or doesn't matter. In this lesson we will define work and energy and attempt to model the relationship between them.
Video (intro)Video: Work & Energy 1
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Lab |
Video (debrief & deploy)Video: Work & Energy 2
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Practice |
Lesson 5: Modeling Energy Conservation
In the last lesson we defined quantities called "work" and "energy" and explained their relationship, namely work done on an object (or by an object) will change its energy. Then we identified some of the different forms energy comes in. In this lesson we use the idea of energy to unveil one of the most important tools physicists have to analyze their universe: Energy conservation. Conservation of energy is a rule our universe follows that has never been observed to have been broken. It states that within a system, provided there is no external work being done on or by the system, energy is neither created nor destroyed, it is conserved. Why is this useful? Well, if you have a system whose energy you know and allow it to go through some changes (like change of position, speed, condition stretched or compressed, etc....), if you know energy isn't destroyed or created in the process, you can make accurate predictions of its new state (new position, new speed, etc...).
Video (intro)Video: Conservation of. Energy 1
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Video (debrief & deploy)Video: Conservation of Energy 2
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Lesson 6: Modeling Machines
In this lesson we will use our understanding of conservation of energy to reveal the magic of simple machines, like a lever. For example, you have a nail buried deep in a piece of wood that, try as you may, you can't pull out with your bare hands. So, you grab a crowbar and soon after you have the nail popped out with ease. Crowbars don't have batteries, don't take gas... how is that just a long piece of metal can give you superhuman strength? The answer lies in an analysis of the work and energy involved in its use.
Video (intro)Video: Machines 1
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Lab |
Video (debrief & deploy)Video: Machines 2
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Practice |
Lesson 7: Modeling Energy Transfer/Transformation
Now that you know what energy is, that it comes in different forms, and it is always conserved (neither created nor destroyed) within a system, you are prepared to look at your physical universe in an entirely different, incredibly useful way. In this lesson we will model all changes that take place in our universe as a result of energy transfers and transformation. We will first learn how to make "energy chains" that model the common changes we say take place all around us. Then we will combine these chains into "energy sources" that model where all the useful energy on our planet comes from.
Video (Energy Chains)Video: Energy Chains
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Video (Energy Sources)Video: Energy Sources
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Lesson 8: Modeling Momentum Conservation
The conservation of energy model is incredibly useful for understanding our universe.
Video (intro)Video: Conservation of Momentum 1
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Video (debrief & deploy)Video: Conservation of Momentum 2
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