Blog 7: An entry that actually corresponds with something we learned RECENTLY

Taken from street level

Holy stairs, Batman! That's a long trek just to get to the front door. Golly, it's a good thing I have my handy-dandy grappling hook with me!

Okay, kids, physics time. 

A force that forces an object to move in a circular path is called a centripetal force. 

centripetal force= mass* radius* angular velocity².

So if I swing my grappling hook of mass 1 kg on a (massless) line 1 m long at a rate of 2 revolutions/second, I can calculate the centripetal force.

First, we'll find the angular velocity (which needs to be in radians/second for this to work). 

(2 revolutions/second)(2Π radians/1 revolution)= 4Π radians/second

And then we can plug that in... 

yeah, we're not doing this step by step. you're smart people, you can figure it out yourselves

And we get centripetal force= 16Π² N.

Of course, at this point, all you guys are interested in is my homemade grappling hook. Take it from me, you do not want to try (emphasis on "try") to climb anything with a homemade grappling hook. Either buy a well-made one or use the stairs. (Or, well... Christmas is coming up! Y'all still have time to badger your parents.)

Blog 6: A short one

a photograph, because it is required

So sometimes I try to play tennis. It's quite fun, except for the part at the end, when we have to pick up all the balls.

Like people the world over, I use the "balance as many balls on the tennis racquet as I can and try not to drop them." And, as I'm sure one can imagine, there are a lot of tennis balls that need picking up. It's hard work!

...Or is it?

As far as physics is concerned, the part where I carry my full racquet back to the basket is no work at all.  This is because the force I'm exerting (keeping the racquet up) is perpendicular to my movement. Madness!

On the other hand, the part where I bend down to pick up more balls to add to the racquet does count as work. (I exert force vertically, and there's a vertical component to my movement.)

So picking up after a long good satisfying entertaining game of tennis is work-- but only for part of the process.

also when will i learn to spell like a normal human?

Blog 5: Coefficient of friction and a graph

Anyone in my lovely audience of four read xkcd? Well no worries if you don't. This was the comic for Friday, November 5:

As the CoF approaches zero, productivity goes negative as OTHER people get pulled into chair spinning contests.

Do you see what I see? Physics~

As we're all aware, the coefficient of friction is the ratio of friction to normal force.  (F/N)

Coefficient of friction comes in two flavors: coefficient of static friction and coefficient of kinetic friction. The former is for when the two surfaces are, well, static. That'd be, for example, when you're pushing your couch but it isn't moving yet. When it does start to slide, we're looking at coefficient of kinetic friction.

The smaller the CoF, the more easily the two materials slide past each other. A larger CoF means that more force is needed to get the desk chair spinning. If you have a very stiff desk chair, you'll have already noticed that there's a max amount of force the static friction can counteract. After you pass this max static friction, well, I think you can guess what happens next.

A Glorious Victory

The fruits of our labors.

I got the top off of a jar of pickled artichoke hearts, and let me assure you that it was no mean feat. 

The jar, you see, had been in the refrigerator  for some time, and the lid was on quite tightly. Being experienced with this sort of challenge, I first called my dad over to help, then I put some water on to boil. When Dad's good ol' meathooks didn't work (that would be "hands" to any plebs out there), we put the jar in the pot and waited.

Just put the jar in the increasingly hot water and let physics work its magic. With smaller jars, running the top under hot water suffices (special heat and all that; the metal lid gets hot faster than the glass and gets slightly bigger and it opens really nicely), but in this case, we made use of the gas laws. We all remember the gas laws and such, right? PV=nRT? In this case, the most important bits were P1T1=P2T2. The air in the top (there was not much, but enough) pushes on the lid and gives enough of a helping hand for Mom and me to enjoy a delicious treat.

Hey, look, a practical application of science! Who'd have expected it?

(Bonus advice: don't put the lid back on the jar and stick it in the fridge until after it's cooled off, okay? You'll thank me later.)

Blog three: Heads Up, We're Coming Down

"Talk to the hand."

This lunchbox part of a dramatic reenactment.

Pop quiz! What do you get when you combine my accident-prone friend, a metal lunch box, and a distracted high school student?

You get an announcement in the Daily Bulletin asking students to please stop putting things on the railing of second floor I-building.

Good times, man. Good times.

Alright, it wasn't really good times.

But, back to physics. The lunchbox was balanced on the railing while the absent-minded student was diddling about in her locker. She (while wearing her gigantic school-issued backpack) then backed up and it was all over bar the blood and out-freaking.

In other words, it was launched horizonally, which incidentally makes these numbers a lot easier to calculate!

There is no way this could possibly go wrong.

As we're all aware, the horizontal and vertical components of a vector may be treated separately. I have a tape measure, so let's get started.

We already know

initial vertical speed = 0 m/s

vertical acceleration = -9.8 m/s/s

and I went and found out that vertical displacement = -3.27 m

So now we can find out time because (say it with me, everybody!)

displacement = initial velocity*time + 0.5*acceleration*time*time

So time = 0.58 s

And we (and by "we" I mean "I," obviously) can then figure out the horizontal velocity using the same equation, because my tape measure says that the lunchbox went 0.80 m in the horizontal direction.

horizontal velocity = 1.38 m/s

Pretty good booty bump, yes?

And finally, we can find out the lunchbox's final vertical velocity (how fast it was going when it creamed her).

instantaneous velocity = initial velocity + acceleration*time

instantaneous vertical velocity = -5.73 m/s

She's going to want to put some ice on that.

PS: I used three pictures Ms C. Do I get, like, bonus points? 

Blog 2

C'mon, Ms. C, that's a boring title...

Please ignore the glow-in-the-dark dinosaurs. Thank you.

As you can see, my door is a rather dull dark brown. I try to liven it up with some decorations. (If it looks pitiful, that's because I'm in the middle of changing over the posters and stuff.) I found this really spiffy water color painting a while back, and decided to put it up. That's all well and good, except that the door is solid wood, and my parents might take badly to me putting things up using thumbtacks and a really big hammer. Instead, I've been using blue poster tack.

However, it isn't as sticky as I'd like, and sometimes the corners come unstuck, as you can see. When that happens, it's only a matter of time until the painting falls off entirely.  Sod's Law (also called Murphy's Law) dictates that it will do so just as I'm attempting something that requires great delicacy and precision.

If we ignore wind resistance and assume the poster falls with the same orientation as above (instead of, say, turning over a few times), we can calculate the time it takes to hit the ground and the velocity it has when it does so.

Lesse... acceleration due to gravity= -9.8 m/s/s

initial velocity= 0m/s 

displacement= -1.168 m

and displacement=initial velocity*time=.5*acceleration*time*time

Plug in the information we have (paying special attention to our signs), solve for time, bake until golden brown and let stand for twenty minutes...

time= .488 seconds. (No wonder it always takes me by surprise.)

Next we can find velocity right before it hits the floor, because

instantaneous velocity=initial velocity+acceleration*time

la la la

Final velocity= -4.782 m/s

So now, when my poster falls just as I'm putting the final touches on my toothpick Eiffel Tower, at least I can console myself with knowing both the time it was falling and the velocity it had before impact. Then I can go find a really big hammer.

Total distance traveled and displacement are not the same thing.

Yes, it's one continuous line.

So last night I was drawing (as I am wont to do) and I ended up with, well... this.

And I thought, hey, cool. 

And then I thought, you know what this illustrates really well? The difference between distance traveled and displacement.

Total distance is exactly what it says on the tin. It's the distance that you have traveled, regardless of direction (toward or away from the origin).

Displacement does take into account direction of travel, and is the shortest distance between your starting point and end point. (I've circled both points in red on the off chance that you can't figure out where each one is.)

As long as I'm applying physics to doodles, I might as well add that my marker speed was pretty much constant, but my velocity was not. Speed is distance traveled over time; velocity measures speed and direction. I made a lot of crazy little turns there. (I mostly tried to avoid looks and crossing over previously established lines, though. It would have made it look confusing.) In fact there were velocity changes out, as I believe the scientific term is, the wazoo.

If you wanted to calculate my average velocity, you'd take the distance between the handily-circled points and divide it by the time it took me to draw this (a long time). If you wanted to find my average speed, you'd have to find my total distance and then divide it by time.