hello grade 12 Learners in this playlist we'll be looking at momentum and impulse and in this video we're going to be speaking about the introduction to momentum if you're new here welcome I'm Miss Martin subscribe for more physical sciences videos now let's jump right into the video what is momentum momentum can be described as mass in motion and mass in motion tells you exactly how to calculate momentum if you think about it momentum is calculated by taking the mass of an object multipli by the velocity which that object is moving now remember velocity is a vector that makes momentum a vector as well let's take a look at the formula and also let's take a look at change in momentum why would there be such a thing as change in momentum well remember velocity can change an object can speed up an object can slow down an object can change direction and if your velocity changes your momentum will change let's look at all of those things in this video first thing that you need to know is that if an object is moving it has momentum so me at the moment I have mass but I'm standing completely still I don't have a velocity I'm not moving therefore I have no momentum calculate momentum and momentum is the product of the mass and the velocity of the object so you take the mass in kilog Mass must be in kilog multiply by the Velocity in me per second and then we get momentum and I just want to show you how the unit works it says here the unit is kilog m/s if you ever forget that in your exam it's very easy to remember it the unit for mass is kilog the unit for velocity is m/ second you see that we're multiplying mass and velocity so we multiply these two units together kilog m/s that's the unit for momentum and momentum is represented by this symbol over here if you have a smaller Mass you have a smaller momentum if you have a bigger Mass you have a bigger momentum what about this over here so we've got this top scenario we've got a 15 kg little car like a little toy car if it's standing still so look at its speed velocity or speed 0 m/s to the right no momentum no speed no momentum if it's moving at 10 m/s to the right higher velocity higher momentum you see the mass is the same the whole time now it has its maximum amount of speed 20 m/s or velocity 20 m/s to the right so it has the most momentum so as you can see we another important thing that you need to know is that momentum is a vector okay it's the product of the mass of an object and the velocity now velocity is a vector which means momentum is a vector and you should know that vectors need direction so here's a very basic example a 5 kg block is moving in a straight line at 2 m per second towards the left oh goodness not right we're going this way left determine the momentum of the block so momentum is equal to the mass time velocity the mass is 5 the velocity is 2 m/s left so the momentum is 10 kg m/s left you have to include your direction you have to absolutely have to so a silly example like this probably won't happen in your exam but it would if it did it would be three marks blank naked formula substituting onto with unit and Direction now if you think about what I was saying about how momentum is all about mass and motion and it's all about an object moving we know that objects in motion objects that move their velocity can change so you can speed up you can slow down and if your velocity changes that means that your momentum will change and why would your velocity change it will only change if there's a net force acting on you so remember Newton's first law you move at a constant velocity there's no net force acting on you right but according to Newton's Second Law if a nonzero net force acts on you okay then you accelerate your velocity changes so if the velocity of an object changes its momentum will change and just so you guys can take this note done in your book as well why would the velocity change if there's an fnet acting on an object so change in momentum I hope you guys remember that change is often represented by the triangle change in the Triangle means change in momentum and remember change is always finy minus initial so if I'm working out change in momentum it's basic basically your final momentum minus your initial momentum I hope that makes sense because change is always final minus initial how can I break that formula down further how do we calculate momentum again we say mass time velocity okay so how to calculate binal momentum we would use our binal velocity then our formula our change formula says minus how do we calculate in initial momentum well momentum in general is mass time velocity and because it's initial momentum it'll be initial velocity so as you can see velocity changes Mass stays the same so we've got this formula over here and just so you know this version of the formula is the one that appears on your formula sheet now as a matric teacher I set metric exams I Mark at a matric level the rule for matric exams or exams in general is that you have to write the formula as it is given on your formula sheet so it's given in this form on your formula sheet so you are going to have to write it down in that form first but we know that the velocity changes the mass doesn't change so we can take Mass out as a highest common factor as a common factor the I okay but just keep in mind that this version of the formula is the one that you see on your formula sheet now when it comes to working out the change of momentum your sign are so important and remember in physics we use signs to indicate Direction so which way you choose as a positive direction is completely up to you you can choose to the right as positive you can choose to the left as positive as long as you state it and as long as you stick with those choices throughout your question so let's do an example we've got ignore the fact that it says example three I just pulled this from my worksheet of examples that I made so it says a car with mass 600 kg travels in an easterly Direction along a straight horizontal Road at a constant velocity of 20 m/s the driver suddenly increases his velocity so he puts his foot on the pedal he wants to go faster and he increases his velocity to 24 m per second East continues to drive in the original Direction determine the change in momentum of the car now notice how when I went through the question I highlighted the variables I hope at this stage that that is something that you also do so I've got my mask I've got my direction East so I'm going to take East as positive I've got my initial velocity that's VI and I've got my final velocity because they want change of momentum I need to write out the formula first so like I said your formula sheet the formula looks like this m VF minus M VI let's substitute the mass is 600 VF final velocity is 24 m/s in the orig original Direction which is east and remember we said East is positive so we're going to substitute 24 in is a positive minus 600 time initial velocity is 20 also East and we said that East was positive so we leaving it as a positive 20 and then we just work that out quickly I got 2,400 but you cannot leave your answer like that first of all what is your unit your unit is kilog me m/ second and then are we done no we're not done we need a direction East okay it's a positive answer which means our change of momentum is in the positive direction which is East easy enough let's take a look at another one a 3 Kg block so there's my mass slides along a horizontal frictionless path at a constant velocity of 1.5 m/s it passes over a small rough patch so picture this in your mind it's going over of a frictionless path and then it encounters a rough patch so what do you think is going to happen to it it's going to slow down okay so it continues along the original motion so say it was going to the right it's continuing to go to the right but now the velocity is 1 m/ second it was 1.5 now it's 1 they want the change of momentum right so Formula First always Formula First and I get it from my formula sheets you write it down exactly as is if you want to take math out in the next step you absolutely can but I'm writing it down as it is on the formula sheet first okay my Mass is 3 Kg then my final velocity this one is your final velocity is one but it's going in the in the original direction do you see in the question they don't say um to the right or east or left it's just they don't mention the direction they just say original Direction so let's say this way is the original Direction your initial velocity is going 1.5 m in that direction so it's positive your final velocity is still in the same direction so it's 1 m/ second it's still positive I'm going to put a minus over here because my formula says minus and then 1.5 I hope you can see what we're going to get as an answer now I got a negative it makes sense that I would get a negative because my final velocity was smaller than my initial veloc velocity therefore 1 - 1.5 that gave me negative right 0.5 so I would get a negative answer but in physics do we leave our answers as negative absolutely not absolutely not so if you get a negative answer what does that mean okay it means that the change in momentum is not in the original Direction so our answer must include Direction so you'll say 1.5 kg m/s in the opposite direction or in the negative direction again how do I know it's in the opposite direction or in the negative Direction because I got a negative in my answer but you can't leave your answer is negative you have to rewrite it as a positive but then your direction must just say in the opposite direction Let's Pretend in the question they told me that the original Direction was to the right then in my answer I could say to the left if they told me the original Direction was North then I I'd get a negative which means it's not North it's South so because they don't tell me right or north or whatever I'm just saying in the opposite direction okay I hope that makes sense let's do one more right in our next example we have a 100 G tennis ball already your the alarm Bells should be going off in your head 100 G our Mass must be in kilogram we'll convert it in a second though 100 G tennis ball strikes a wall at 10 m/s and bounces back in the opposite direction at 8 m/s now this is a very important example let's draw a little picture to help us understand this is my wall over here we've got 100 G tennis ball going towards the wall like that at 10 m/s then it hits the wall and it bounces back in the opposite direction at 8 m/ second can you see that this is the first example where we're changing direction that's very important and in physics we always need to choose a positive direction so let's say towards the wall is positive okay so I'm going to write here towards the wall is positive which means away from the wall would be negative so the 10 is towards the wall it'll be a positive 10 the eight is away from the wall we will substitute it in as a negative change of momentum is what they want so let's write our formula mvf minus MV I you can substitute directly into this formula but if you want to take Mass out first you may do that you don't have to okay I'll Mass 100 G you should know that you need to convert that to kilogram and it's very important to know your conversions this is why it's very important to learn it in grade 8 to9 math and keep it somewhere in your brain but if you forgot how to convert from kilogram from grams to kilogram there's 1,000 G in 1 kilogram so to convert from G to kilog we have to divide by 1,000 so it will be 0a 1 kg okay divide 100 by 1,000 our final velocity remember 10 is our initial it's initially going towards the wall so that's the I the 8 is our final velocity but remember it's going away from the wall so it has to be 8 so the negative is because of the direction then this minus over here is from the formula minus postive 10 okay I hope that makes sense and we get Negative 1.8 now remember what I told you guys about a negative okay we can't leave our answer is a negative so it's 1.8 kilog m/s now what direction it came out as a negative positive was towards the wall so negative would be away from the wall away from the wall and there we go that is how you calculate change of momentum it's very very important that you know how to calculate change of momentum because we're going to be using it in follow-up sections like impulse an Impulse momentum I'll see you guys in the next video bye everybody