Combat Boots

A few weeks ago I bought a pair of Magnum combat boots for various reasons (working, airsoft, hiking, etc). They are surprisingly light and comfortable. The tag on the shoe mentions that it is “slip resistant.” This means that, as a whole shoe, the boots have a very high coefficient of static friction. Because the force of static friction is directly related with normal force and the coefficient of static friction, a higher coefficient of static friction means that it takes more force to over come the force of friction and "slip". To test this, I went walking around in my bathtub with the boots on, and it was awesome. It was very hard for me to “slip” (where the force of the forward motion of my foot exceeds the force of friction provided by my boots) even in a puddle on the smooth surface of the bathtub. This made me wonder, however, why causes such high coefficient of friction. I initially thought it was the material, the rubber soles, but I also have rubber slippers, and they slip (no pun intended) very easily under the same conditions. This drove me to research a little further on the internet about friction. The answer was water dispersion. The soles of my boots are designed in such a way to disperse water over a larger area. Water acts as a lubricant, which reduces the coefficient of friction. Because my slippers are rather worn out and are almost completely smooth at the bottom, they don’t disperse water very well, and therefore get the full lubricating effect of water.

These suck at water dispersion =(

The boots, however, have many grooves in which water can go into so there is less water lubricating the parts of the sole that come in contact with the ground (or whatever surface I’m walking on).

These are awesome at water dispersion =)

Walking in a bathtub with boots on can be a fun an educational activity!

More Torques, Please

In an earlier post I wrote about forces acting on the joystick I use for flight simulation. After learning about torque, however, I noticed that there is also torque involved in manipulation of flight controls. When my hand is in the normal position I use for flying fixed winged planes, I apply a force of F, which varies depending on how much and at what rate I want to bank/yaw (as mentioned in the earlier post, the more I want to bank/yaw, the more force is required because of the spring), multiplied by the lever arm distance (about 12cm with the base acting as the fulcrum and the middle of my palm acting as point of push force), to create a torque of 0.12F Nm. When hovering in a helicopter or flying in formation, I must make very minor adjustments in my flight controls and there is no room for me to be ham-handed with my controls. I learned a while ago that it is much easier to make these fine adjustments by resting my wrist on the base of the controller (Real helicopter pilots rest their hands on their thighs, since their control stick is usually in between their legs). This not only allows me to apply much less force on the controller, but also significantly decreases the lever arm distance. Since torque is directly related with both lever arm distance and force applied, the torque I apply is also lessened, thus lessening the chance of me over controlling the aircraft.

KHAAAAAAAAAAAAAAAN!

Torque

Helicopters display alooooot of physics. I can’t actually get my hands on real helicopter, so I used the next best thing—Microsoft Flight Simulator X. Torque is a key component of flight with helicopters. The engine applies torque to the main rotor blade shaft, which in turn causes the rotor blades to rotate and generate lift. Because of Newton’s third law, however, the helicopter also experiences torque from the rotor blades which must be countered with a rear tail rotor which is controlled by anti-torque pedals The pilot of a helicopter can directly control the torque output (displayed on a gauge in the top left as a percentage of maximum torque of a specific helicopter) with a control called the collective. I decided to do a demonstration flight in Flight Simulator in an MD-500 helicopter. If you pay attention to the torque gauge in the video below, you’ll notice that there is actually a significant amount of torque even when the helicopter is resting on the ground. This is because while the helicopter is resting, the rotor blades are still rotating and fighting air resistance. Increasing the torque will cause the rotor blades to generate more lift until finally the force from lift is greater than the force from the helicopters weight (mg), thus allowing the helicopter to accelerate upwards (notice that as I increase torque during takeoff, I must apply slight pedal inputs to keep the helicopter straight because of the increased torque). Also notice that the torque does not always stay constant throughout the flight, during hovers, and especially during landings. During landings such as the one I performed, you may actually notice an increase in torque as the helicopter nears the ground. This increase in torque increases the lift force to allow the aircraft to decelerate vertically and land smoothly on the ground instead of slamming into it.

VIDEO:


NOTE:Sorry the video quality sucks, I have to find better compression software (the original file was 900+mb =/). Its also kinda hard to see the gauge. I will reshoot and upload a higher quality video when time permits.

Oops...

This weekend was a weekend full of concerts and rehearsals for me. I started with a 2 ½ hr Youth Symphony rehearsal on Friday night, a rehearsal and concert for our school’s orchestra from 3-8:30P.M/ on Saturday, and another rehearsal/concert for Youth Symphony from 11:30A.M to about 9:00P.M. (yeah, I was pretty busy). Through all this one of my most important tools is my Getzen Custom Series 3047AFR trombone. I bought it in 10th grade and its been my main concert horn ever since, travelling with me to Kaui and Japan. When I first got it, it was nice and shiny, and although it still is fairly nice and shiny, it is now showing the signs of “heavy use.” Scratches are somewhat unavoidable, but also present on my trombone are dents both big and small. The largest dent is on the top of the main tuning slide. It dropped from the top of my trombone on a really hot day once (the hotter it is, the sharper your instrument gets. For example at ideal conditions your note A might be at 440Hz, but on a really hot day, the A played in the same position might be at 444Hz). The tuning slide had a potential energy of about (0.75kg)(9.8)(1.5m)=11.025J. When it hit the ground, all the potential energy was converted to kinetic energy. The velocity at which my slide hit the ground was thus about 5.42 m/s. When the slide hit the ground, there was an impulse between my slide and the ground. Since I did measure the velocity of the tuning slide immediately after it bounced off the ground or the time it stayed in contact with the ground (No, I did not drop and dent my slide in the name of science!), I cannot calculate the impulse, but I can conclude that the impulse was strong enough to leave a sizable dent in my brass tuning slide (brass is actually quite malleable). So I guess every time my trombone gets new “battle scars,” I’ll be thinking about physics!


My trombone doesn't look like this anymore =( (image courtesy of Getzen Co.)

Toys > Cleaning

I was cleaning up around my house the other day because it was getting to the point where I had to climb over stuff to walk through the living room, and I found a toy I received as a gift a few years ago. Since cleaning isn’t exactly fun, I decided I’d play around a bit with this toy. This toy consists of a wheel with magnetic tips that rolls along two metal rods. The wheel is held to the rods because of ferromagnetism. When you place the wheel on the rods and tilt it, the wheel will begin to roll along the path of the rods (see video 1).



Tilting is required to move the wheel because the force from the weight (mass*gravity)needed to overcome the inertia, or tendency to resist changes in motion, of the wheel. Once in motion, the wheel accelerates until it hits the bottom point of the rod. Assuming I started the roll from the top of the rod (holding the rod perpendicular to the ground), the wheel will have a potential energy equal to the mass of the wheel multiplied by the height of the rod. Because of conservation of energy, the kinetic energy of the wheel while at the bottom of the rod will therefore be equal to the potential energy of the wheel while it was at the top of the rod. In an ideal situation, the wheel would then be able to rise to the same point it started after it turns around for an upward motion. My wheel does not rise to the same level again at the second try, however (see video 2). This does not disobey the conservation of energy because energy was taken away from the system by friction (there is rotational friction between the wheel’s tips and the rods).



Because mechanical energy decreases due to friction each time wheel goes back up, the wheel reaches a progressively lower height. To keep the wheel moving, you must keep tilting the rod in a way that allows gravity to provide force to the wheel to replace the energy lost from friction. The lower you tilt the rod, the greater the y-component of force from weight, and thus the greater the speed of the wheel. As you can tell from the video, I was so blown away by the sheer awesomeness of this toy that I didn't bother to finish cleaning the house. I'll finish that...soon.