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PPT Energy Developer Notes: Physics work Physiology ability to do work potential energy kinetic energy fat motion calories 840967898 calories diet metabolism Delivery We've seen that Ft = ∆(mv). What about Fd? We've seen it as in levers, and in torque. Activity - dynamics carts (can we use anything else?) hanging and on various slopes. Find the force and the distance and see if they're all the same. (Acceleration at the beginning is equaled by the deceleration at the end.) Fd is work. Fd = mad = work. Work is mad. Fd is measured in Nm, which is also called J (Joules), which is kgm2/s2. Appliance cart over curb or up ramp? No matter which way you got it there, at the end you still have the same force at the same height, able to do the same work. So now you have Fd. What is that? You can do things with it, like water above a waterfall or dam - water mills, saws, generators. Pile driver. Bucket of water over the doorway. (Can't tell the students about some of these.) Bike on top of a hill. Counter-weight on an elevator. Gravity-driven is the easiest to visualize. Takes work to get it up there, but now it can work for you. Fd is potential energy, in J. mad = kg(m/s2)m = kgm2/s2 So what happens when something falls from a height? When it gets to the bottom, we don't necessarily know where it came from. It has mass, it has velocity. At the moment it strikes, it doesn't really matter where it started. If it was shot at the same speed, who cares? So, how does that relate to its PE? The mathematical derivation is Fd = mad. d = at2/2. Substitute for d and it = ma2t2. a2 is (d/t2)2, and a t2 cancels, leaving m(d/t)2/2 = mv2/2. So, the PE turns into moving, or kinetic, energy. PE = KE. KE is also in J. kg(m/s)2 = kgm2/s2. It's nice to get it to m and v only, not a, because then you can relate things that are going a steady speed and see what energy they have, not just objects that are accelerating. Do some exercises with things falling at the acceleration of gravity. Activity - Projectile prediction with PE -> KE -> v, get time from the height to the floor (t = √(2d/a), and d = vt. Calories are another measure of energy, like Joules. The potential to do work. 1 of 3 Printed 5 PPT Physics power conservation of energy Energy Physiology strength nutrition energy in, energy out heat heat vs. temperature regulatory system? 840967898 Delivery So you can do some work, like dig a hole. Remember John Henry (can I get a recording? Lord forbid I sing!)? His was a contest of power. They wanted to dig a tunnel, and they were checking who was faster, a man or a machine. John Henry won, but he died doing it - the machine won in the long run. The question was what can do the same amount of work in less time. That's power. Power is work/t, J/s. Horsepower. There's more to life than calories. This is like conservation of momentum. There's only so much energy in the universe. 1st Law - conservation of energy. This is the biggy. ∆U = Q - W, The change in internal energy (U) of a closed system equals the heat (Q) added to the system minus the work (W) done BY the system. (But I don't totally understand this.) Total Energy = KE + PE + Q For mechanical systems. Exercises - ? Activities - ? Heat is a measure of the total amount of kinetic energy in something. activity - rub your hands together - they get warm. You need to apply a force because of the friction. So where does the work energy go? To heat. How about a car stopping? Where does its kinetic energy go? To heat. How about on a razor scooter? Do you ever step on the brake with bare feet? Heat is a by-product of work. activity / example Temperature is the average kinetic energy of the molecules in something. Heat and temperature are different, like mass and weight differ. Heat is the total kinetic energy, measured in Joules. Temperature is how hot something is, the average (not total) kinetic energy, measured in degrees,. activity / example - a bag of ice cubes vs. one. 2 of 3 Printed 5 PPT Physics entropy Energy Physiology death? maintenance energy 840967898 Delivery 2 Law - entropy. It ends up as heat. A difference makes a difference. Can't do anything without a difference. Difference is information. ∆S = Q / T, The change in entropy (S) of an object equals the amount of heat added divided by the absolute temperature (Kelvin). Activity - guess how I want these blocks put together. Assemble this electronic circuit so it works. Throw these dice until they all form a neat square. Throw the dice until they equal 12. Build a sand castle and see what it looks like tomorrow - better? Activity - Are there any heat related activities? Ice in a glass - it melts and it all turns to the same temperature - it's now useless to cool your drink. The difference in heat that drives a car engine, steam engine. Lecture - vandalism, destruction, 9/11, war, drop the clock on the wall, step on a piece of chalk - it's easier to break things than make them work. There's no pride in it. We're in a constant battle against entropy. It takes energy to put things together in a particular way there are many more ways that are wrong. Global energy nd 3 of 3 Printed 5
