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Concept #1: How to Identify the Type of Collision

Transcript

Hey guys so now that we've talked about the different types of collision you can have you should know that some questions won't ask you for the final velocity or the mass to calculate something but instead there are some conceptual questions that will ask you to identify what type of collision you have, this video I'm going to show you a step by step way to do that so let's get started, alright? So, we can use a series of simple checks to determine the type of collision now the way these questions usually work is they'll give you the masses and the velocities and based on the information you have to be able to figure out what type of collision you have, OK? And we can use very simple checks there's 3 checks you can use in this order, right? Based on and this checks are based on the key characteristics of each type for example you might remember that in elastic collisions elastic collisions kinetic energy is conserved so if you can see the kinetic energy was conserved then you can deduce you can conclude that the collision must have been elastic, OK? So the first thing you're going to check for is you're going to check if the total momentum of the system in the beginning equals the total momentum of the system at the end so you're basically checking does the left side of this equation equal the right side of this equation remember I told you that every collision question in every type of collision moment in this conserved so if you're given a problem where the left side of this equation does not match the right of the equation that violates conservation of momentum which is not possible in a collision therefore it means that this collision is not possible you can see some questions like that it never happened a collision like that would never happen and you don't have to know how to identify that so that's the first thing, now if the collision is possible it's going to be one of these three types so if it passes the first check then you're going to check for other things the second thing you're going to check for is the simplest one which is if the two objects have the same final velocity what that means is that after colliding they actually stuck together, right? If I move with a 4 to the right and this guys moves with a 4 to the right as well the means of moving together so in that case they stuck together which means that this is a completely inelastic equation pretty tricky numbers. Now the last check is you're going to check for elastic collision remember that when you have an elastic collision you can use this extra equation here this is the extra equation so what you're going to ask yourself is it true that there's a left side that equals the right side? If that's true then you have an elastic collision if that's not true then you have an inelastic collision, inelastic I call it the default one because you have no direct way of checking if it's inelastic the way you know it's inelastic is because it failed check 2 or 3, OK? So, the collision is possible but it was not this not this therefore by default has to be inelastic, OK? Very straight forward just check these 3 things is the collision possible and then you start checking that they stick together is it elastic and if it fails these two here that he has to be inelastic by default, OK? Alright so I'm going to do an example then I have a practice problem for you guys to try so here I have two blocks on a flat moving on a flat smooth surfaces so flat and smooth means it is like this and there is no friction, OK? So, object A is to the left of object B, object A has mass 2 kilograms, B has mass 1 kilogram for each set for initial and final velocity so I'm going to give you initial and final velocities and I want you to tell me what type of collision is this? And we're going to say that going to the right is the directional positive for both of these problems, Ok? So here the initial velocity Va initial is 3, Vb initial is 2, Va final 2.33 and Vb final is 3.33, OK? The first thing I have to check is this even a possible collision and to check that I have to see that the right side matches the left side of the conservation of momentum equation, M1V1=M2V2 I'm going to just put the masses here to 2 parentheses 1 parentheses 2 parentheses 1 parentheses if you're solving these questions sort of quickly you can just do this you don't have to write M1V1, alight so the initial velocity of the 2 is 3 all of them are only positive here, the initial velocity of the 1 is 2, the initial velocity...The final velocity the of the 2 is 2.33 and the final velocity of the 1, 1 is B is 3.33, OK? I'm going to fast forward if you combine all the stuff you get that 8=8 this is true which means this is possible, what does that mean? That means you keep going to try to figure out whether it's completely inelastic, elastic or inelastic in this sequence so it passed check number one let's do check number 2 are the final velocities the same? The final velocities are not the same therefore all I can tell by that is that it's not a completely inelastic which means you keep going, let's check for the third one do these velocities match? So, the two the velocities for one equal the two velocities for 2, the velocities for one are here and it's 3 kind of tight here, 3+2.33, OK? And they equal the velocities for the initial and final for the second object which are these here, 2+3.33. and I hope you see here right away that this is 5.33=5.33 it matched it is correct if they are correct notice how this is a not the same this is the same and this is same so if these match you have an elastic collision and then you are done so this is the elastic collision because of the third check was correct it was true, right? So same thing here just different numbers let's set up A and B, A B 2, 1. The velocities for A are 2 positive and 1 positive, 2 and 1 and for B it's 6 and 8 so these are initial and these are final so let's do the checks, first let's check if this is a possible collision by using the full equation, I'm going to put the velocities here this is initial of the 2, initial of the 1, initial of final of the 2, final of the 1 all the numbers are positive as well and I have 4+6=2+8 I get that 10=10 that is true so this means this equation this collision is possible this is check number one let's go to check number 2, check number 2 we're checking If V1 final = V2final they're not the same 6 is not 8 therefore I know that this is not completely inelastic the third check is whether these velocities it's this check right here, right? So, I can just put the initial and final of one add them up 2+1 is that the same as the initial and final for 2 6+8 and obviously these are not the same so you know this is not the elastic it failed the elastic test which means it default to being an inelastic collision, defaults to being inelastic collision, cool? That's it for this one, try the practice problem let me know if you guys have any questions.

Practice: Two blocks move on flat, smooth surfaces. Object A (3 kg) is to the left of object B (5 kg). For each set of initial and final velocities (all in m/s), determine the type of collision (take the right to be the direction of positive):