PhySP18SS5.4.1 Time Stamp: Walkout Wednesday. Hope it doesn’t mess up the Test 5B too much.
PhySP18SS5.4.2 Time Stamp. Got to watch Big John in a lead role in the excellent Les Mis at the Sooner Theater.
PhySP18SS5.4.3 Typical day in my advisory. Cooper leads the boys in Folk Songs. They are getting really good with their harmonies.
PhySP18SS5.4.4 There is a whole big explanation of this in the Facebook Group. Search “Inertial” and “Gravitational” probably a good idea to print out that bit from the group and put it in your notebook. Might come in handy in college or the final.
PhySP18SS5.4.5 . . .
PhySP18SS5.4.6 . . .
PhySP18SS5.4.7 Some call this sort of thing a crutch. I call it just another tool for working through a fairly simple physics problem. Sometimes its the simple ones that get you.
PhySP18SS5.4.8 5.10.10a. You see that (as Arnold said), the rocket thrust has to overcome the the force caused by gravitational mass of the rocket (which we call its weight) and the inertial mass (resistance to velocity change) of the rocket represented by the “ma” part of F=ma.
PhySP18SS5.4.9 another slightly different version of 5.10.10a
PhySP18SS5.4.10 This problem is on the THT5B
PhySP18SS5.4.11 yes it IS legal for the d/dt to switch partners. That lil d is a sneaky little stinker.
PhySP18SS5.4.12 Simplified version of the Thrust Equation.
PhySP18SS5.4.13 kg/sec is a weird unit to me, but I guess it makes sense. It is how much mass is being spit out the back of a rocket per second. Usually we keep these units in kg/sec, but it could be any mass over any time.
PhySP18SS5.4.14 messing with the units,
PhySP18SS5.4.15 an example to show you the importance of realizing that the escape velocity (ve) is RELATIVE to the rocket, NOT relaive to the woman watch this whole thing from a reference point.
PhySP18SS5.4.16 All of these are example of Newton’s 2nd Law.
PhySP18SS5.4.17 Mr. Babb’s take on the development of the Thrust Equation from a rocket’s point of view. part 1
PhySP18SS5.4.18 Part 2: A little bit of mass spits out the back of the rocket (dm)
PhySP18SS5.4.19 Part 3: Here he uses Conservation of linear momentum to derive the Thrust Equation. BRILLIANT! Now I wish I had done it this way. So much cooler. I only wish he had replaced the ∆ in the the fourth line on with “lil d” to make all this instantaneous.
PhySP18SS5.4.20 Part 4: So we get the Thrust equation. I wrote about this in the Physics Facebook Group. Notice that there is a second part to the thrust equation. You will need to know this part when you work for Elon Musk at SpaceX. I envy you. He wouldn’t take me. I’m too old and I’m damaged goods. ; )
PhySP18SS5.4.21 All those mean the same thing
PhySP18SS5.4.22 How to attack 5.10.12
PhySP18SS5.4.23 Pretty straight forward 5.9.2
PhySP18SS5.4.24 How to work THT5.13
PhySP18SS5.4.25 Notice all the @s. most of those are on the equation sheets now. This is not true for every car of course. We just have to reach an agreement in the 807.
PhySP18SS5.4.26 Air drag.
PhySP18SS5.4.27 Mean old Robert Brown. Biologist whose mistake changed the world in 1820. He looked through a microscope and thought that the pollen grains were alive because they were randomly moving around. Turns out they figured out later that the pollen grains were not alive (no vis viva). They were simply being bounced around by the water molecules that surrounded them. Turns out the water molecules are ALWAYS moving and bouncing into each other in this chaotic manner. We call it, you guessed it . . . Brownian Motion.
PhySP18SS5.4.29 An illustration of Brownian Motion. High Entropy. meaning that I can not predict where the molecules will be a few seconds after this picture is taken. There is equal chance that they could be anywhere in a three dimensional sphere. This is, by definition, maximum entropy and therefore , the textbook version of Brownian Motion.
PhySP18SS5.4.28 So what does this have to do with the force of lift on a fast moving car? Okay, I’ll give you the simplified non mathematical version of this. This really requires a lot of statistics and Calculus. So I will give you the Reader’s Digest version: All this brownian motion of air molecules (O2 and N2 molecules) cause a standard air pressure (Force per unit area) of 14.7 psi (lbs per square in). This air pressure is at sea level on earth. It tells you that the air molecules have reached maximum entropy and they are bouncing all over the place in all different directions causing little tiny force vectors in all directions.
PhySP18SS5.4.30 Okay . . . so now when there is a breeze (either caused by wind blowing past an object or the object moving through “calm” air, more of the air molecules are flowing in one direction (this is called LAMINAR FLOW). When you are in a laminar flow situation, more air molecules (you can also, of course, do this with water molecules) are flowing in the same direction and the amount of Brownian Motion is reduced. There are less air molecules (or water molecules) to bounce off the sides of a tube they are in so the amount of molecules hitting the side are reduced and therefore the pressure (F/A) is reduced.
PhySP18SS5.4.31 . . .more visuals . .
PhySP18SS5.4.32 . . . more visual info about Laminar flow vs. Brownian motion.
PhySP18SS5.4.33 . . . so what does this have to do with the force of lift on a fast moving car? the faster the laminar flow there is over the top of car (becasue the air molecules have further to go) the less Brownian Motion there is on the top of the car and therefore, the less pressure. basically how an airplane wing works which is great for the airplane, but not good for a car.
PhySP18SS5.4.34 This can be demonstrated with a Venturi Tube.
PhySP18SS5.4.35 it is called the venture effect.
PhySP18SS5.4.36 an example of a venturi tube. Wish I still had mine.
PhySP18SS5.4.37 the same concept is used in perfume bottles.
PhySP18SS5.4.38 The venturi effect is used to increase the speed of escape gases in an F-15 or whatever jet this is.
PhySP18SS5.4.39 We demonstrated it with the old paper hanging from the bottom lip trick.
PhySP18SS5.4.40 Spoilers and Air Dams are how cars deal with lift.
PhySP18SS5.4.41 Cigarette Boats have a real problem with lift.
PhySP18SS5.4.42 So do dragsters
PhySP18SS5.4.43 Modifying the FBD for a car accelerating up to 120mph with the snapshot taken at 80mph
PhySP18SS5.4.44 the same car in neutral slowing down with the snap shot taken at 80mph.
PhySP18SS5.4.45 now slamming on the brakes skidding to a stop with the snapshot taken at 20mph.
PhySP18SS5.4.46 A rocket in the earths lower atmosphere traveling at 75° ALH. Notice there are three forces and a ∑F (because the rocket is Thrusting and accelerating in the DOM.
PhySP18SS5.4.47 An example of the Thrust force being a greater angle than the DOM. It HAS to be because it has to compensate for the weight of the rocket (the downward mg vector)
PhySP18SS5.4.48 another example
PhySP18SS5.4.49 From the Thursday night talk about Neutron stars being used to help us detect gravity waves.
PhySP18SS5.4.50 Most of the crew that went to the talk.