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 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.)