hello this is Mr MCM again and we're here in chapter three Newton's second law of motion all right once again we're going to go back and talk about Galileo though for a second Galileo was studying motion of objects sliding down ramps and things like that and he kind of laid the groundwork for a lot of what Newton later came in and formalized in his three laws but as he noticed things he put things on the ramp he noticed that as they went down they did not go at the same speed if you were to stop the balls at 1 second 2 seconds 3 seconds 4 seconds and 5 Seconds you would notice that they're getting farther and farther apart now this is a little difficult for him to prove because he could only take um speed have stopwatches things like that he used things like his pulse and things like that but he noticed that as they went farther down the ramp they were not moving at the same speed it wasn't constant it was increasing the speed was increasing as it went downhill and so this was acceleration and so Galileo basically came the concept that as things go downhill they would accelerate now technically you could also argue the same thing is happening in Reverse when you roll a ball uphill they would get closer and closer together as they go uphill because they're losing speed okay and they would be slowing down or decelerating all right but the uh idea of acceleration then can include speeding up or slowing down if we talk about an accelerator on your car we're usually talking about the gas pedal but technically the brake pedal is also a accelerator it would be a negative or decelerator all right but no one really calls it that so we talk about positive acceleration we're talking about speeding up which is what we normally think of when we use the word accelerate but it also can mean slowing down okay because um you're changing the velocity also there but you're slowing down and so decelerating or negative acceleration we might call it is slowing down so so uh basically the idea of acceleration based on what Galo came up with is this acceleration is a change in velocity okay if you remember from last time uh or last chapter velocity is speed and direction so if you change the speed or you change the direction then you are accelerating so that means that there are three basic ways that you can accelerate you can speed up that would be changing the speed you can slow down which would also be changing the speed but in the opposite direction and you can change direction okay and so acceleration is anytime you change velocity which you can be changing speed up or speed down and changing direction any of those things would result in a change in velocity now it's also not just the change in velocity but the change in time how quickly are we speeding up or are we slowing down talk about CS I can go from 0 to 60 that's our velocity for our speed okay how fast from 0 to 60 in how many seconds because if you can do it all cars can go from 0 to 60 my pickup can but it may take my pickup 10 seconds where it might take a race car uh 1 second or two seconds or a very small amount of time all right and so we would have different accelerations even though we're going from 0 to 60 same change in velocity uh the amount of time is going to affect which one we would say has a greater acceleration all right and so uh we can have acceleration can be speeding up positive acceleration or slowing down negative acceleration or deceleration all right or we can change directions now for example if you're a racetrack at Texas Motor Speedway going around in a circle circle or track a circular track even if they keep their speed me 150 speed are they accelerating well on the straightaways if they're going in a straight line then no they're not accelerating but every time they hit the curve even if their speed says constant say 150 mph going through the curve since they are turning they are constantly changing direction then they are constantly accelerating because that is a change in velocity because it is a change in Direction so just something keep in mind there uh you can speed change in speed or direction or you can do it to both you could slow down into the curve so you're losing speed and your changing direction you would still be accelerating just two different ways at the same time all right now um that was kind of some of the basic principles that Galileo sat down all right now Newton took this a little bit farther when he made his second law all right so if we think about an object okay it's trying to move uh let's just say we have a take some bricks here book it could be a bowling ball it could be anything you're trying to move all right um and BAS basically we know that we push on the Brick we're applying a force and that force will cause the brick to start moving change in its velocity and therefore we are accelerating the brick okay but if I push twice as hard on the brick what's going to happen to the acceleration well if I push twice as hard we would expect it to move twice as fast and therefore twice as much of a change in velocity and we would say that it accelerates um twice as much okay so it would double the acceleration right um if I put half as much force on the brick what would we expect to happen we would expect it to accelerate half as much okay so the amount of force is going to change in a direct proportional way to the amount of force we Supply so we say acceleration is little squiggle line here means approximately equal to or proportionally equal to force in other words if the force increases the acceleration will increases all right and vice versa if the force decreases or goes down then we would expect the acceleration to go down and that just makes sense okay the more Force we put on something we'd expect it to accelerate faster the more the less Force we put on it then it's going to accelerate slower assuming it's the same object all right but there's something else that affects the acceleration of an object and that is its mass okay and this is something that Newton was also trying to explain um we know that mass controls the object's inertia okay we talked about inertia in Newton's first law inertia is its resistance to change if it's moving it wants to keep moving if it's stopped it wants to say stop and we said the amount of inertia an object has depends totally upon its mass okay and so the more mass it has the more inertia now if we're trying to change the speed of an object how is that going to affect the acceleration okay now we can use the brick example here that we actually have the picture of all right so we have put the same amount of force on one brick or put the same amount of force on two bricks or put the same amount of force on three bricks well The more mass we have and we put the same amount of force on it is it going to accelerate faster or is it going to accelerate slower and the fact is that if we put the same amount of force then The more mass we have or the more bricks we go to two to three it's going to accelerate less okay we put the same force on one brick it's going to accelerate a certain amount but we do have the same amount of force on two bricks which is twice as much mass then it's going to Accel cerate half as much okay if we do it with three bricks has three times as much mass it's going to accelerate only 13 as much mass all right and this had Newton came up with the relationship between acceleration and mass he says acceleration is inversely proportional one over the mass in other words if this number gets bigger the mass gets bigger then the acceleration is going to be equal to less okay just like 100th is less than 1/10th the the larger the number on the bottom of the fraction the smaller it's worth okay one piece out of more pieces is going to be worth a lot less so basically as the mass number gets bigger the acceleration is going to go slower assuming you apply the same amount of force all right so we have basically these uh two laws and Newton took these two together and put them together into this formula which says acceleration is equal to net force divided by mass all right and basically this is just a combination of these two proportions in other words if this number gets bigger the amount of force gets bigger and the mass stays the same as this number gets bigger the top number of the fraction we would expect the acceleration to be bigger also and it'll accelerate faster if the force stays the same and we increase the mass then it's going to be inversely the bigger the object with the same amount of force it's going to uh accelerate much slower and so that's basically what Newton's law is saying is that as you increase the force keeping the mass constant the acceleration will increase proportionally however if you keep the force the same and increase the mass then it will decelerate or only accelerate at a slower rate or inversely so basically say acceleration has a positive correlation with net force if it increases and so will acceleration and it has an inverse relationship to the mass of the object which means the bigger the mass the less acceleration or the smaller the object the less Mass the greater the acceleration it will do the opposite all right and that's basically what Newton's first law established now before we go I like talk a little bit more about Mass just to make sure we are clear on mass and what mass is mass is the amount of matter or atoms that make up an object how much matter or stuff makes up the object okay uh it is not volume does not have to do with how big it is okay uh for example we could have an iron anvil which is uh not real big and it could weigh as much or more than a whole person could depending on how how big the Anvil is but this takes up a lot less space than the person yet they could have exactly the same mass all right that has to do with density okay this Anvil has a lot of mass and a small amount of space so we say it is very dense the person is less dense because uh it takes a lot more volume of their body to equal the same amount of matter or mass as the um Anvil so when we talk about Mass we're just talking about how much matter there is not how much space it takes up that's when we're getting into density and we're not really talking about that all right and so um we are also not talking about weight weight and mass are similar okay but they're not the same mass is how much matter there is all right now if something has a lot of matter is it going to weigh more good question it all depends on gravity okay weight is totally dependent on gravity if we have this Anvil and if we have gravity say it was on Earth instead of out in outer space here if if it was on Earth the gravity would be pulling it down toward the Earth and it would have a certain weight okay that's why we measure um weight with scales or excuse me or scale where you step on it and then gravity pulls you down or if you're in the supermarket you put your fruit on the little tray and gravity pulls it down and the scale measures how much does it get pulled pull down by gravity and that's basically your weight now if an object has more mass is gravity going to pull it down harder yes there's more mass for it to act on and so the weight will also be good so they're directly proportional if you increase the mass you also increase the weight assuming that you're on Earth where gravity stays the same now if we were to go to say we were to fly up to the Moon if we got out on top of the moon is there any less Mass to our body than there was on Earth Earth no we still have the same amount of mass so our mass does not change but if we're on the Moon the Moon is a lot smaller than the earth and it would pull down or have less gravity pulling on us so we would way less and that's why when you see the astronauts kind of bouncing around up there because there's not as much force of gravity pulling down but their mass is still the same there's still as much matter making up their body as there ever was okay so if we took things up to the Earth Earth say we had two objects that had the same mass okay if we took them up to the Moon they would way less but if you put them on scales okay balancing a scale with a little thing here in the middle they would still balance because they had the same amount of mass and so there's less gravity pushing down on this Mass so it weighs less but there's also less gravity on the moon pushing down on this side so it also weighs less and they will always balance and that's why Mass is measured in balances because mass is always the same and no matter where the gravity is it'll change on both sides equally weigh they have weigh less or if you went to Jupiter they would weigh more and they would still balance no matter where you're at and so we measure mass on the balance weight we measure on scales weight depends totally on gravity okay and mass does not mass is the same no matter what so instead of saying oh I need to watch my weight maybe we need to just go someplace that has less gravity and then we wouldn't have to worry about that now the last thing is we talk about forces and mass and and weight um we need to know the equivalence of it one kilogram we say you have one kilogram that is a mass certain amount of mass uh looks like we'll have to finish this up on the next one all right so we'll stop right there and we'll see you in the next part of our lecture shortly