00:01
Problem 8 .43, we have a 12 gram bullet hitting a ballistic pendulum, 6 kilograms.
00:11
It's going 380 meters per second immediately before the collision.
00:17
The pendulum is suspended from a 70 centimeter cord.
00:28
And the first thing we want to find is through what height does the pendulum move after the bullet hits? it swings backward.
00:46
The second thing we want to find is the initial kinetic energy of the bullet before the impact.
01:04
And the third thing is what is the total kinetic energy right before or right after the impact, but before the pendulum has started swinging and turning some of some of that kinetic energy into potential energy and eventually it gets to the top of its swing and it.
01:20
It all becomes potential energy.
01:27
So, like i just said, if we want to use the conservation of energy to figure out how high the pendulum goes, and we need to know what its kinetic energy is, and in order to do that, we needed speed, for which we can use the conservation of momentum.
01:51
So the initial momentum just consists of the bullet moving, and afterwards the bullet lodges into the pendulum, and they both start moving together.
02:15
So we have the combined mass, pendulum, and bullet.
02:26
And for the sake of brevity, we'll call the speed here the speed of the pendulum, even though the bullet is also lodged in it.
02:38
So the speed of the pendulum is going to be equal to this ratio of the masses of the things that are moving before and after the collision.
02:58
Times the initial speed of the bullet.
03:05
And now this works out to be 0 .758 meters per second...