Saturday, January 8

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!