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 8: a frisbee ring and a tree

Jump and grab the other end. Yeah, that'll work.

So for the last day of the year 2010, my friends and I decided to go to the park and have a picnic! Indeed, there was feasting and fun for all. In accordance with tradition, one of our number chucked my frisbee ring (like a frisbee, but with a big hole in it) and it ended up in a tree. (This, by the way, was the same tree it ended up in last time we were in the park.)

After the traditional cuss words, hasty excuses to leave without helping get the blasted thing down, and general abuse, my remaining friend and I set about retrieving the ring. I tied a monkey's fist in our 20 foot rope, and we set about swinging the rope around and around. Yes, I'm sure you can see it, too: centripetal force! But I just did a post about centripetal force. Hm...

But wait, there's more! We tried to throw the rope over the branch the frisbee was caught on so we could shake it down. Using out unerring instinct for being kind of foolish, my friend and I first threw the rope over the branch near the trunk of the tree. This, of course was a bad idea, because we wanted a lot of torque acting on the branch for not too much work-- I mean, without having to apply too much force. (We're teenagers, of course.)

torque= force* distance from axis to force

We're applying the force perpendicular to the line from the axis of rotation to where the force is applied, which is convenient. Treat the base of the branch as the axis. Say the force my friend and I will be applying is going to be the same no matter where we are. See? See that? That's why we want the rope farther away from the tree trunk. Because we can have a bigger torque without having to increase our force! Hooray!

When we finally wised up, we went ahead and threw the rope over the branch farther away from the trunk. We were able to retrieve the frisbee after only one hour! (That's a vast improvement on the five hours it took last time.) Thanks, physics!

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.