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Mental Health Report
This week you can't even call it a week. Im so glad to be at the end of the course. Not because I didn't enjoy it, but because my summer will finally start and my husband won't be so lonely. Im really glad that we aren't trying to fit new material into these last few days because I have no idea how I would ever get it done. As it stands now I feel like I will be pulling at least one all nighter to get it all in. Im concerned about the video because while Im sure my laptop has video editing softwear, I have never once opened it.
Our P3 glog and work is coming along nicely. I feel like I might have taken on a bit more than I should have this time by agreeing to write all the physics bits. There seems to be alot more than last time and more of it is new. Good job Sarah, way to drown yourself just before you get to the finish line!
Neat Video 8
Ever wished you could make your own ballistics gel at home? I have, so I went searching and this was the simplest recipie I found. Something about melting and rehardening the gelatin that makes the structure stronger the second time around. Still, I wouldn't recommend trying it until the weather cools a bit.
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Since this week I'm creating a tribute to my dogs, I thought it might be fun to add a video of some dogs. These dogs obviously aren't familiar with the laws of physics. For example, when running on ice or trampolines, friction is often negligible...
While sitting in the chamber of the gun the bullet has no initial velocity and a constant downward acceleration due to gravity of -9.8 m/s/s (y axis). However, this acceleration has a negligible effect on the first stage of the bullets flight since the gun itself resists this force on the bullet. Potential energy is stored in the gunpowder inside the bullet casing. This state remains until the trigger is pulled and the igniting of the gunpowder causes gases within the casing to expand. This sudden conversion of potential energy into kinetic energy in the form of heat increases the pressure and forces the bullet out of the barrel. This energy can be calculated since it is equal to the bullet’s mass times the force of the gases times time length of time the force is applied. During this phase the gunpowder’s energy is transferred to the bullet through heat and dramatic pressure increases, giving it a horizontal velocity and acceleration (x axis). In our tests the bullet had an average horizontal velocity of ~7637ft/s. As the bullet leaves the barrel gravity’s force begins to affect its flight. This now gives the bullet a vertical velocity and an acceleration that continues to be -9.8 m/s/s. In our tests the vertical velocity was an average of ~893 ft/s. At a glance, it would seem that the bullet travels in a straight line after leaving the barrel but this is untrue. While the bullet does have a much higher velocity along the x axis the y axis still affects its flight and the bullet immediately succumbs to the negative acceleration in the y axis. This creates a flight path that follows a sloped downward curve. As the kinetic energy driving the bullet along the x axis dissipates, the negative acceleration from the y axis increases the velocity in that direction.
In our examples however, this isn’t readily apparent since a soda can often interrupts the bullets path. The moment the bullet makes contact with the soda can its kinetic energy is transferred from the bullet itself to the can. This energy is evident in the hole split in the can’s side. This occurs because of the high velocity of the bullet and its small surface area. The energy is dissipated very quickly into a small point.
Newton’s laws and bullet phases...
When looking at the physics of firearms, Newton’s 3rd law becomes very important to remember. Often in movies we see two cowboys (of equal mass) standing ten paces apart in a shoot out. The town folk are scared as they watch from the window of the general store and a mother hides her baby’s eyes as a breeze rolls a tumble weed across the dirt road. The two cowboys turn and shoot. One of them is thrown back off his feet because the power of his opponent’s colt .45 is too much as the bullet hits him in the shoulder! Oh the drama of it all!
In reality this would never happen because even though the bullet transfers all of its kinetic energy into the cowboy as it hits him and penetrates his shoulder, it is still no more force than it exerted on the first cowboy as it left the barrel of the gun. Newton’s 3rd law states this very plainly. For every action there is an opposite and equal reaction. When leaving the gun barrel, the cowboy and his gun pushed on the bullet with the same amount of force as the bullet pushed back with. By extension (and Newton’s 1st law), without any outside force adding to the bullet’s velocity during its flight, it will not increase in energy and the bullet will only exert as much force on the second cowboy as the first cowboy exerted on it. Since the first cowboy wasn’t thrown back just by shooting the gun, and no force acted on the bullet during its flight, there is no way the bullet could throw the second cowboy off his feet.
In the case of our soda cans, this is also true. The can pushes back with the same amount of force that the bullet pushes on the can with. The difference is that the can has considerably less mass and thus the energy transfer looks more dramatic when the bullet strikes it. Even so, the same amount of force is exerted whether the bullet strikes a 12 ounce soda can or a 200 pound cowboy.