Blog 9: Another xkcd-based blog

So I've been following xkcd for a while, and when we got to our section on angular momentum, I finally understood one of the comics!
 

It's got all the nerdiness of angular momentum combined with all the romance of throwing up due to nausea! Nice.

Actually, the physics here is (are?) sound enough. Angular momentum of a system (in this case, the girl and the Earth) is  constant. That means if the girl spins in the counterclockwise direction, the Earth will spin more in the clockwise direction. Time travel ahoy!

Hardly. If that worked, any time we needed more time to cram for tests, the school would be filled with kids spinning. Why doesn't it work?

Angular momentum= moment of inertia* angular velocity

Moment of inertia= mass* lever arm distance* lever arm distance. (Actually, the equation for moment of inertia varies depending on the distribution of mass, but go with me here.) The girl's mass is going to be around 60 kg, but the Earth's mass? More around 5.9 followed by twenty-three zeroes. Pretty big difference.

Who needs skin?

The Earth's huge mass means the change in angular velocity caused by the girl would be negligible. She could probably spin so quickly that her skin ripped off her body without having a discernible impact on Earth's rotation.

Clearly this girl didn't  hear, or maybe she doesn't care. Either way, she should put her arms out to increase her moment of inertia. Some bowling pins would not be amiss, either.

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 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?