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 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.

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?