00:01
So this is the first question we encounter that is about section 724 and section 7 .5.
00:09
So this is about the elastic collision of two objects.
00:12
And for these two sections, 7 .4 and 7 .5, the two equations we need to know is for any elastic collision.
00:20
Without external force, there is always conservation of momentum.
00:24
So p initial equals p final.
00:30
Or if there are two objects, a and b, that means m .a v .a plus mbvb equals m .a v .a prime plus mbvvb.
00:42
So this applies for any collision.
00:45
So this is the first equation.
00:47
And the second equation we need to know is equations that's specific to elastic collision.
00:53
So the conservation momentum is always true whether or not the collision is elastic.
00:57
But the second equation is relative velocity.
01:01
Equation or this is equation 77 and this is that the relative speed of two objects will be equal and opposite before it after the collision so with these two equations we should be able to solve any elastic collision questions okay so just a quick review now let's look at this question we have a bore of mass um a now of mass 0 .44 kilogram it's moving east um with a speed of 3 .8 meters per second, and it has a catac collision with another object that's initially at rest, that's 0 .22 kilogram.
01:44
And we want to know what is the speed in the direction of each four after the collision.
01:48
So, what we know is we know m -a, we know m -b, we know v -a, and we know v -b, and then we want to...
01:58
These are what we already know, and we want to know what is v -a -p -b -p.
02:03
So let's first look at...
02:06
So first equation, we have mava plus mbvv and block b was initially at rest, so we can ignore this time, equals ma va prime plus mbvb prime.
02:16
So the unknown, we have two unknowns here, and we have two equations, so that should be enough for we to regard the two unknowns.
02:22
So what we usually do is we use one equation to represent one unknown in terms of the other, and then we plug it into the other equation.
02:32
So i think since this equation 7 .7, since what we have here is this looks like a simpler form, we will use this to do the substitution.
02:45
So here again, vb, we can ignore vb.
02:49
From this equation, what we have is vb prime equals va plus va prime.
02:58
So i'll go it three.
03:00
I said i plug this into one.
03:03
Substitute 3 into 1...