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Measuring the Speed of a Bullet with a Ballistic Pendulum

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Our experiment today is to measure the speed of a bullet from a Remington .222 rifle. We are doing this in the woods of North Carolina for safety purposes. This is obviously something you would not do at home. In order to measure the speed of the bullet, we will be using a device called a ballistic pendulum. This is composed of a large mass; we will be using this log right here. The mass will be suspended by ropes in a pendulum arrangement, and the gun will be fired right into the end of the log. When the bullet hits and embeds, the log will swing a little bit, and we will measure that distance it swings back and use that to calculate the velocity of the bullet as it struck.

Now, we will need several measurements in order to carry this out, and one of them is the mass of the log itself. For that, we will put it on a bathroom scale, and get the weight in pounds, later you will be able to convert that to kilograms.

Another measurement we will need is something we can use as a scale factor, because we are going to use a video tape to determine the horizontal distance the log swings back. That is going to require a scale factor, so for the scale factor we will use the length of the log itself. This measurement is in inches, so later you will need to convert that to meters.

We will also need to know the mass of the cartridge itself. These .222 cartridges, when the gun is fired, this jacket will separate from the end of the cartridge and the only part that will enter is the tip. So, what we need to know is the mass of just the part that goes into the end of the log. To get that, we will use the manufactured specifications. Those are given on the end of the box here. And if you look down there, you will see about 55 gr, that stands for grain. And later you will need to get a conversion factor for gr to kg, so that your calculations will be in international units.

We have suspended the log in this particular arrangement for stability. When the bullet hits the log, we want it to swing straight back, we don't want it to swing like this. So with the ropes going off at angles like this, we should be able to achieve that. We are going to take a couple measurements of distances, which you may need in your calculations later. One measurement is the distance between the two ropes at the top, which is 72 inches, and the other is the vertical measurement down to the rope goes through the hook. In order to measurement the horizontal deflection of the log, without having to get close to it, we are going to use a video camera. The video camera is off to the side; we are using the telephoto setting in order to get close up views.

I am going to try to keep the gun level while firing. But first a little safety protection.

(The gun is fired several times.)

 



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