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JBB: An Artblog!
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@letsgethonorsphysical-blog
Our first day of service saw us in Canto Grande. This poor barrio (called a ‘pueblo joven’ in Peru) is on the outskirts of Lima. It is there that the Christian Brothers run a school called ‘Fe y Alegria 26’, one of a system of schools funded by the government but run by religious communities which...
That's a noble task you guys are doing! Best of luck to the team!
The Human Walking Machine
I was astonished when I discovered that the simple levers give humans the ability to walk and run.
Just like every lever system has the four following parts so do us,
1. a rigid bar (bone)
2. a source of force (muscle contraction)
3. A fulcrum or pivot (movable joint)
4. a resistance (the weight of the body)
Physics is part of literally everything!
Efficiency
Think about it - machines aren't perfect, so you'll never get as much work out as you put in. Efficiency will always be less than 100%. The efficiency of a machine can be calculated by dividing the AMA by the IMA. A little algebra will tell you that efficiency is also equal to the work output divided by the work input. Work is found by multiplying force by the change in distance, so both are found the same way except one involves input force and distance and the other involves output force and distance. Again, this is a ratio and is dimensionless.
Ideal Mechanical Advantage
The ideal mechanical advantage (IMA) is theoretical and is based on the system's geometry. With the AMA, friction comes into play since it deals with forces, but IMA doesn't because it focuses on the ratio of the distance in (effort distance) to the distance out (resistance distance. So: IMA = (Δd in)/(Δd out) This is also dimensionless since it's meter per meter which cancels out.
Actual Mechanical Advantage
The actual mechanical advantage, or AMA, is a ratio of the magnitude of the output force, or resistance force, to the magnitude of the input force, or effort force. So: AMA = (F out)/(F in) Since it's force divided by force, it has no units since Newtons cancel out, and is therefore dimensionless.
Totally forgot to do these posts from the last week of school before break. My bad. Anyway, we started learning about machines, which are devices for increasing a force or changing the direction of a force. The lever, the pulley, and the incline are the tree most common or basic machines. Ideally, when there is no friction, the work input equals the work output, so the input force times the change in distance because of that force equals the output force times the change in distance because of the output force.
I forgot to post this as part of my conservation of energy post, but this is some really quality physics right here. I think it's really cool how everything just comes together. Like, you're given enough variables to figure out a few unknowns, and there are so many equations you can use to find those unknowns. Yeah, it's cool, I said it.
Even though we’ve been learning about conservation of energy, in most real world scenarios friction comes into play. It is the resistance caused by the contact between two objects, at least on has to be moving. It converts some of the kinetic energy to heat causing the object to slow down.
Yeah that’s pretty true pal. Wouldn’t it be great if we could live in perfect, frictionless world? I wouldn’t constantly be blowing money on gas.
But if we lived in a perfectly frictionless world, wouldn't we never stop moving?
Mechanical Energy
Mechanical energy is the sum of the potential energy and kinetic energy of a system. The sum remains constant for all positions of an object in a system. As kinetic increases, potential decreases, and vice versa. Mechanical energy can be written as: E = KE + PE and based on the conversation of energy law: KE (before) + PE (before) = KE (after) + PE (after)
Conservation of Energy
When energy changes form there is no net loss or gain (in a closed system). The law of CoE says that energy cannot be created or destroyed; it can be transformed from one form to another, but the total amount of energy never change. This must be in a closed system though, because if forces not in the original system come into the system, there will be a different amount of energy. Yet still, that energy was always present because it is neither created nor destroyed, ever.
Potential Energy
Potential energy is the energy of position. It refers to the relative position of two objects. It is stored energy; it has the potential to do work. We mostly looked at gravitational potential energy, which is the work required to elevate an object against gravity. PEg only depends on the weight of an object and the height of its elevated position; the path taken to get there does not matter. In a formula, it can be written as: Gravitational potential energy = Work = Force • distance = mass • gravity • height Or PEg = W = F•d = m•g•h
I'm gonna go backwards now because that last post should have been after this one but I just got so excited I had to make that one first. But okay so on Friday we learned about kinetic energy, which is the energy of motion. An object with kinetic energy is moving and therefore is capable of doing work. It is measured in Joules, which is a Newton•meter. A Newton is actually a kg•m/s^2 so a N•m is kg•m^2/s^2. Kinetic energy is directly proportional to the square of velocity (based on the equation KE=(1/2)m•v^2, so 2v --> 4KE.
All this talk of work reminds me of something my elementary school textbook once said. The introduction to the went something like this: You might say “Reading a complicated book is hard work!” However, a scientist might respond “Well, that depends.” You would reply “On what?” He would say “On how many times you had to turn the pages.”
I don’t know why, but I never forgot that fictional discussion from my sixth grade textbook. It always bothered me that the scientist had to twist such an inane comment to into an annoying conversation starter. Anyway, it does provide a clear differentiation between the colloquial and scientific definitions of the word “work.” In everyday speech, we define work as anything difficult or tedious whether it be physical or mental. In science, work is the product of the force exerted on an object and the distance travelled.
I think that's a very good point by Tom. We always talk about "doing work" as basically anything that involves lifting a finger to do something, but in actuality, "work" refers to the amount of force applied multiplied by the distance that force is being exerted.
I'm gonna be honest, I get excited like Mr. Love does with how all those equations come together, so I'm gonna do another one. Friday we learned the Work-Energy theorem, which states: W=F•d W=ΔE F•d=(1/2)m•v^2 ΔE=KE=(1/2)m•v^2 AND it's not even done yet! We can move around and substitute variables in the formula: F•d=(1/2)m•v^2 F=m•a so m•a•d=(1/2)m•v^2 Then both m's cancel and then times both sides by 2 and split the velocity into final minus initial and we are left with an equation we learned in the beginning of the year: vf^2=vi^2+2a•d BOOM Derivations are awesome.
Hooray for derivations!
Today in physics we learned about work and power. Work (W) is equal to the force (F) times the distance traveled (d). Power (P) is equal to the work divided by time in seconds. Derived, power equals force times distance divided by time. Distance divided by time is velocity (v), so power also equals force times the average velocity.
I thought that this video was a great example of an inelastic collision. The energy of the ping pong ball was absorbed by the water balloon.
Thats crazy dude i’m glad my organs don’t do that when I am hit by a ping pong ball!
Look at it jiggle!
Wing Suit
When the juniors went on their trip to West Point, Mr. Love showed us a video of a man cliff diving with a wing suit and a parachute. The wing suit allowed the man to control his speed and direction through the movement of his arms and legs. The reason he was able to do this was due to air resistance. The air resistance pushing on certain parts of the suit that he was manipulating allowed him to control aspects of his flight. Though there was not enough surface area of the suit to slow him down so a parachute is needed to increase the surface area for the air resistance and decrease his speed for landing.
That sounds like a pretty cool video. How come Mr. Love didn't show us this video on Friday when the juniors weren't there?
