 So questions from the homework from last day, so number one and number two went okay Okay Anyways, if you want me to you can ask now number four a Parachute a parachutist free falls for a while I mentioned a jump out of an airplane a parachutist free falls for a while and they open up their parachute If the force of air resistance from the parachute is equal to the weight of the person There's the parachutist There's the weight of the person Apparently air resistance is exactly equal for me Yeah, and you'll notice part C is actually on the next page, and maybe that's why you couldn't the answer C is Is that what you didn't notice? Okay Yeah, it got chopped off. Oh, you know what it didn't used to do that I'm guessing I think I've told you these were typed up by a friend of my they're very good notes I bought them up a friend of mine. He typed them in word 95 for Mac. So what are we 2011? This is Office 2010 anyways some of the diagrams and some of the formatting changes periodically even when I open it So I'm gonna go. Hey, there didn't used to be a line break there. They used to be all one page number five Number five a cyclist is moving to the right steadily I would probably Brett in my mind underline the word steadily. It tells me constantly At 8.2 meters per second if the force of air drag is 130 newtons Then what's the force of propulsion from the road free body diagram? There's the biker What are the forces acting on him get the obvious ones? The really obvious ones Definitely gravity pulling him down Is the cyclist sinking into the earth? No, is he taking off into the air? Then there must be a vertical force exactly balancing out gravity same length approximately that I tried to draw it What if we call the force of a surface pushing back up counteracting gravity? Because it's normally there It's not what we call it the normal force normal actually means 90 degrees and that as well so it's 90 degrees to the surface Now what are the other forces acting on this biker tell me breath more sorry Okay, air drag and it says it's 130 newtons okay, and Then I guess propulsion from the road which is actually the force of friction from the tires, but they called it propulsion I'll just call it F What word did I underline? What does that mean my overall net acceleration is? Zero what does that mean my overall net force is? It's meant to be real obvious. You're over complicating it Force equals what times what kiddo? So if my acceleration is zero, what's my overall net force? Zero so that means everything has to be in balance My vertical forces are in balance. Yes What does propulsion have to be to be in balance here? I heard it got 130 By the way, that's what but when you're biking at a steady speed What you're really doing is you're exactly matching the force of friction between your tires Which ends up being the force of propulsion you're exactly matching that to the air resistance hitting your body That's how you travel at a steady speed But and that's also why you have to give some effort You can't just coast because there is a force slowing you down You have to continually overcome that air resistance if you're in a vacuum You'd be able to pedal just fine without air resistance slowing you down, but you'd be dead But for a split second there the biking would be really easy That okay, you see what I meant by so easy that it's tough You know what 130 if you're steady speed it's got to be 134 words to cancel 130 backwards That okay any others Hearing nothing seeing nothing Well, then Make sure your name is on it Hand it in words And let's move on to lesson two lesson two one dimensional force problems Says this important one dimensional force problems include Whoops go too far important one dimensional force problems include the elevator or apparent weight problem The air resistance problem friction problems and the many body problem First of all, let's reiterate from last year The normal force This is the contact force between the floor or a surface and the object That means if you're hanging from a rope not touching the ground. Is there ever a normal force? No, because you're not touching a surface It's the force with which the floor pushes on you and interestingly enough The normal force is also the value that a scale would read if you were standing on it I did a big speech in lesson one. We had somebody stand on a scale and do deep knee bends We said what does the scale measure doesn't measure mass Measures your normal force In fact, sometimes normal force is called the apparent weight of an object Any good amusement park ride Mitchell worth its salt any good amusement park ride messes with the normal force That's where we get our rush You want to get your heart rate rising if I change the normal force your body's not used to that you'll get your adrenaline rush That's what the elevator does on the way up you feel an increase in normal force You feel like you're being pressed down into your seat when really the seats pushing up against you harder than you're used to On the way down with the elevator you feel a decrease in normal force You feel like you're flying out of your seat actually know instead of the normal force pushing up you with what against you with One G which it does all the time they drop it to maybe point six G's So you're still accelerating down, but not as much and that's where your body guy something feels wrong cool Okay Any good amusement park ride messes with the normal force so If a person with mass M If a person with mass M and weight W is in an elevator standing on a bathroom scale Just to make it easy. Let's pretend it instead of the scale weighing in kilograms Let's pretend that it reads in Newton's so we don't always have to convert to kilograms If the elevator starts at rest and accelerates up What will the scale read? W More than W or less than W. So you're ready? Once again, we're going to vote once again How high you hold your hand is how certain you are of the answer So you're standing on a scale in an elevator and the elevator is accelerating up Who says that the scale will remain unchanged? It will read W your weight Who says that it will read B more than W? Most of you almost all of you who says that it will read C less than W Joel That alone should give you pause the fact that Joel picked it might cause you to choose a different answer convince me How would I answer it first of all? I agree the answer is B Okay, in this unit I'll be talking a little bit more about those using principles of physics right to explain question By the way, we'll go over the test on a day once everyone's written and I have a short lesson I still got a couple of kids that haven't written the kinematics test once everyone's written We will go through the tests in detail. Just gotta wait till everyone writes How would I answer this I would say 90% of the times they if they give me a forces question I answer it using a free-body diagram Here's the person What are the forces acting on this person get the obvious ones? Absolutely, I have Mg down is this person in free fall No, then there has to be another force pointing upwards What force? Since we he's standing on a surface, we're going to call it the normal force now Is the normal force bigger or smaller? Which way is the elevator accelerating? The elevator is accelerating up so now I usually use fn for normal force But it looks like this time they want me to use the letter w wait for normal force fine I'll use their letters Connor who's winning in fact we would say this The weight minus gravity equals Your net force ma and since this question Mitchell is about weight. Let's get the weight by itself Mitchell how would I get the w by itself in this equation? The weight is going to be ma plus Mg now if a is zero if you're not accelerating What's ma? Brett zero your weight would just be mg That's what it is when you're on the ground or when you're traveling at a constant speed because you can't feel the acceleration If this was accelerating downwards like negative What would the weight be? Well, you'd have a negative plus a pause that weight would be smaller you feel lighter if you're accelerating downwards But in this case we're accelerating upwards positive. There's a number here. I would say weight equals ma plus mg Therefore w Must be greater because it's your original weight Plus Something positive number. It's got to be bigger There's how I'd answer that Matt with very little English Mostly equations and just thinking about a good free-body diagram example 2 says write a net force equation there We did Example 3 a 35 kilogram stands on a kilogram scale So this scale isn't kilograms if I know the normal force, how do I go from normal force to kilograms? How do I go from normal force to kilograms again? Take the normal force and? Divide by 9.8. Okay. Okay, so this time they want to go to kilos. I'll just divide by 9.8 very very in The accelerate the excel the elevator accelerates which way? Says find the normal force find the scale reading. You know the first thing I'm gonna do here is Yes, you do. What's the first thing I'm gonna do here? Free body diagram. There's the person What are the forces acting on this person get the obvious one is this person in free fall? No, then there must be a force pushing in the opposite direction. What? The normal force now I'll call it fn. I prefer that notation Oh, is the normal force bigger or smaller than gravity? Who's winning? How do you know Kara? Gotta be fussy because they're accelerating down if they were going down at a constant speed could be a tie So I'm gonna draw my normal force deliberately shorter And what I told you that we're gonna do all through this unit our vendor is we're gonna go Winner minus loser Equals my net force ma So my equation Jacob is gonna be this mg minus Normal force equals ma By doing this I'm deciding to let down be positive in this unit if I go winner minus loser I'll never put a negative 9.8 in for G. I'll take care of the signs ahead of time myself What's this question asking us to find by the way? What does a want us to find? Get the wreck get the normal force by itself. How can I do that one step? That's okay a normal force equals mg minus ma plus that to that side Minus that to that side is that okay Trevor? Okay, do I know M? Do I know G? Do I know M? Yeah? Do I know a? Oh Now it's plug-and-chug 35 Times 9.8 minus 35 times 2.25 What's the normal force that this student is feeling? 200 and I don't know what do you got? 200 mr. Do you I think is it in 200s? No? What do you get? That's my way of saying get your calculator out and try these Longer it takes the more homework you have Because lesson takes longer What'd you get? 264 so we'll go to three sig figs for our final answer units Newtons is it 264 265 rounded off properly? 264 okay, we still have that number on our calculator B. How can I find what the scale will read in kilograms if I know the normal force? How do I change that to kilograms divide by? So since you have that number on your calculator divide by 9.8 What's this person's scale reading right now? 27 even 26 point. Oh So it is 27 Okay 27.0 kilograms Care is that okay? Care is that okay? Kayla, sorry my bad care is over there Kayla. Is that okay good interest? Thank you. Are you talking physics? Yes, okay, but still just so there's the famous elevator question. I like that question. I like that question Air resistance Air resistance is the force of air molecules Bumping into an object that's moving through the air the force of air resistance has as its direction Opposite to the direction of motion Example four says draw force diagrams for each of the following a ball being thrown and then once it leaves your hand a Ball going up and a ball going down. We're talking about this three sets While I'm throwing it When it leaves my hand on the way up and then on the way down is part C So while it's in my hand while I'm throwing it part a what are the forces acting on it get the obvious ones? Absolutely What else? You know what we call it normal force. I'm actually because it's an outside person Applying a force. I'm gonna call it F applied the applied force and if you said normal force, I'd take that but I don't consider myself a surface I consider myself an outside being By the way, why don't I draw the arrow so much longer? What you gotta be waiting, right? Now you could argue when it's in my hand as I'm throwing it There is probably a small amount of air resistance as well And it would be in the opposite direction of motion if I'm throwing it up while it's in my hand the air is pressing against it Now on the way up What are the forces acting on it get the obvious ones? gravity What else air resistance which way up or down? Which way is the ball moving up? Air resistance acts in the direction opposite to motion. There would be also a Small f air air resistance. Why small? This is pretty aerodynamic. How can I make air resistance larger? Yeah, increase the surface area. It's a parachute does right piece of paper That's why piece of paper doesn't drop very well. It tends to float big surface area small man's on the way down What are the forces acting on this ball get the obvious one? gravity What else air resistance which way this time? Yeah Having said that Most of the time we're going to choose to ignore air resistance because we're going to assume that our projectiles are pretty Aerodynamic just like we did last unit example 4b. It says this In example 4b, and I'll turn back to it one second while you guys stay on this page How can the ball be rising? Yeah, you up on the page. There we go Now you're gonna yawn. It was that much work to turn the page really An example 4b How can the ball be rising if all of the forces acting on it are downwards? Explained so you guys stay there 4b Both forces are downwards. How the heck could it be going up because it is how the heck can it be going up? Or maybe it's not going up. Maybe we're just maybe our free-body diagram is wrong Was that a hand up the cold look like a hand up to me? I don't know I saw this out of the corner of my eye pretty sure My way of saying can that maybe be put away later or something. Thank you Kayla. I've got your name right now Matt okay force tells us well force equals what times what what does the a stand for? Yeah force involves acceleration not velocity and Velocity and acceleration are direction are independent of each other now Eventually if you accelerate for a long time your velocity will be in the same direction of your as your acceleration eventually, but Here's all I would write v and And a Are independent of each other You can accelerate backwards and be moving forwards. No problem That you do that every time when you come to a stop at a red light you're traveling forward, but you're decelerating you're accelerating backwards But we call it slowing down Same thing is happening, but vertically it's slowing down Example 6 says if the falling ball in example 4c is speeding up If the falling ball in example 4c is speeding up write a net force equation you guys stay here here was our diagram All right, we want to write a net force equation for this who's winning who's winning gravity Who's losing by the way in this equate if I asked you to do this equation Who's winning who's winning? Gravity and Air you have winner plus winner no loser Because they're both in the same direction Clearly gravity is bigger Joel, but it's winner plus winner Anyways our equation would be this winner minus loser equals ma my net force Did they say an example 6 that they wanted me to solve this? No, they just asked for the equation good example 7 a 90 kilogram parachutist eventually that jumped out of an airplane a 90 kilogram parachutist free falls for a time and then opens a parachute if The parachutist slows down at a rate of 1.5 meters per second squared a Compare the forces B write a force equation and see find the force of air resistance on the parachute system Okay You know what the first thing I'm gonna do here is What do you think say I? Think I'm gonna do a free body diagram There's me did I mention that I jumped out of her phone What are the forces on me get the obvious ones by the way those you didn't have me last year the reason I always say what are the ones get the obvious ones that way you've got something on your diagram You don't feel so stressed. You're more relaxed. So I always When Brett earlier started giving me the tougher ones. I'll never do it They'll always get the obvious ones done out of the way also that way I don't forget the obvious ones. So what are the forces acting on this parachutist get the obvious ones Absolutely gravity What else I heard it Kara I Think you said air resistance, but it's hard for me to say because you said it the same volume twice in a row Yes, did you say air resistance now? Here's the real question Bigger or smaller well first of all can they be Mitchell? Thank you. Can they be the same size? Why do I know they can't be the same size I Have an unbalanced acceleration just like your biker question earlier, but here I know it's not balanced Okay Right when I pulled my parachute eventually I jumped out of airplane right when I pulled my parachute Did I slow down or speed up? Slow down are you saying I was decelerating? That must have meant that for a split second the force in that direction must have been bigger when I pulled my shoot Now a parachute is actually a very very complicated. Do I talk about it later? Oh? I do okay air resistance is really complicated because it's not constant air resistance Changes depending on how fast you're moving What's the air resistance of this piece of paper right now? Zero You know why? Now moving if I went really fast That's a fairly big air resistance in fact it bent the paper I could feel it in my hand if I move the paper fairly slowly I can't even feel it there is an air resistance although. I'm sure there is so air resistance rolls and very very complicated force You really need calculus to analyze it properly. Do we have calculus at our fingertips here? No, so that's why we're looking at this very specific situation That's why I had to tell you I there is a net acceleration and it's upwards. You know how I know it's upwards Slowing down so a said compare the forces really that's the physics speak for do a free-body diagram B. They want the equation Andrew. Who's winning? So the equation is going to be winner minus who's losing? loser the force not you equals ma B said write a force equation. Have I written a force equation good? What does he want me to do? Okay, get the air resistance by itself. How I think air resistance is going to be ma plus mg Let's double check Connor did they give me the mass? Check do I know a Check do I know and oh you already said that do I know G? Oh? Now it's plug-in Chuck Times 1.5 Plus 90 Times 9.8 Mitch. What do you get for the air resistance? It's my way of saying pick up your calculator and start doing some typing 1017 anybody else so I'm gonna write 1017 except Mitch. How many sig figs is that? Don't like that 1.02 1,020 is what we're around 1.02 times 10 to the what Mitchell units Units don't even have to think about it. It's Newton's force. Okay now as Soon as I look up as soon as I slow down This number will get smaller Because air resistance will get smaller in fact as I slow down more and more and more I'll reach the magic number where my acceleration is zero where air resistance exactly balances out gravity And that's how hard how fast I'm gonna hit the ground that But with a parachute, it's a survivable velocity And in fact in a parachute In a parachute what they do at the very very end is they change your angle of descent They fill the shoot at the last split second and increase the air resistance for a split second and they time it Just right you'll be almost at a stop and then just drop to the ground barely on the next page It says the force of air resistance actually not a constant value the force gets larger as speed increases and in fact A falling object will eventually reach the point where the force of it says force if this should be the force of Right here. This should be the force of Air resistance equals the weight of the object and the fancy word for that You may have heard this term before we call that terminal velocity. You're not gonna speed up any faster So when I jumped out of my airplane and I was in free fall I'm actually I don't have an airplane. I I hit terminal velocity Terminal velocity. I think is around 230 meters per second example eight a Parachute is jumps out of an airplane and free falls They reach terminal speed and they maintain that speed for a while Then they open their parachute. They quickly reach a different terminal speed and descend to the ground sketch of velocity time graph, okay? Do it Yes Shouldn't this technically all be negative because you're going down it should you know what I'm gonna graph a speed versus time graph Or I'm just gonna let down be positive Okay, and we're only talking about vertical speed not horizontal speed because when you're in the airplane before you jump You are traveling horizontally, but here's what I'm gonna say At the beginning you're in the airplane. What's your vertical speed? So for a while you're traveling at zero and then you jump What happens to your speed? hmm When you jump what happens to your speed it would increase and in fact it's gonna do this It's gonna increase, but it's slowly gonna level off as you fall faster and faster You hit terminal velocity. There's terminal velocity for a little bit Then you pull your shoot what happens to your speed and you hit a new terminal velocity Lower survivable. Hopefully you keep this up, and then you hit the ground when you hit the ground you come to a stop I think it would look something like that jump Freefall While you're in freefall you hit terminal velocity for a few seconds Pull your parachute slow down slow down hit a new terminal velocity level off for a while The times are all skewed because you're only in freefall for about 30 seconds when it takes you about four minutes to actually get down Once the parachute is open whatever I'm not that interested in the scale. I'm more interested in the heights speaking of freefall. Mr. Duke has a video Normally here. I show my jumping out of an airplane video. Have I mentioned that I jumped out of an airplane? I haven't well. I did have I showed you the video already. I got a couple of more to show you but first of all homework number one Number two number three Number four number five Six is good Seven is good. So so far. I've assigned everything Nine is good. Oh heck eight. You say sure why not and 10 skip 11 12 They go pretty quick 13 is a review of projectiles. I'm gonna pass on that 14 is I'm gonna pass on that