00:02
All right, so you had a question about the conservation of momentum and conservation of kinetic energy and an elastic collision.
00:12
So you had two questions.
00:15
So the first one is all about the idea that kinetic energy to start with, ke sub i, must be full of the kinetic energy to finish or after the collision.
00:28
And the problem says that they're elastic.
00:34
So we can make that assumption that not only is the momentum conserved, but so is the kinetic energy.
00:43
And we know from, hopefully from previous units, that the kinetic energy is ke equals one half of b squared.
00:54
So they gave us the total mass of the system and the relative velocities of the, the two as they come closer to each other, they're decreasing their distance between each other by 0 .25.
01:10
So if the velocity was 0 .25 relative to each other to start with, then they must be equal to each other.
01:22
That same magnitude must be what we have at the end, because there's no mass being changed.
01:30
So the final velocity has to be the same just in the opposite direction.
01:35
And so we're just going to say it's negative 0 .25 because that's going to get us the same kinetic energy as before times the negative is a positive.
01:49
So we end up with the same kinetic energy.
01:50
So it has to have the same magnitude of velocity if it's a completely elastic collision.
01:57
Basically, they're bouncing off of each other and gone back away at the same speed that they came in at...