forces
rallain | May 17, 2010Galactic Binder (a Star Wars fan site) invited me to write an article for them. Obviously, I said "yes". But what to write about? I decided to write a different version of my flying R2-D2 post. What is different? In my original post, I go into a lot of details. Essentially, that post is like most of my other ones where I am thinking out loud. I used to work out the answer and then write the post, but I like it this way better.
Here is the Galactic Binder post:
Aristotle, Newton, and R2-D2
So, in this Galactic Binder post, I don't go into the details. Basically, I just use the flying…
rallain | May 16, 2010I am excited. This Wednesday, the MythBusters are doing the giant water slide jump. Maybe you are new to the internet and you haven't seen this video. Here it is:
And since it is as old as the hills, of course I have already analyzed it - actually twice. First, the video is fake - but it is an excellent fake. Here is another site with details on how this was created.
What did I look at in my previous posts? Here is a summary.
The video is difficult to analyze because of perspective changes.
Even with these problems, nothing says it has to be fake. The vertical acceleration during the…
rallain | May 13, 2010Yes, I am going to talk about the MythBusters latest duct tape episode. A couple of pre-post points:
They call it "duct" tape. I call it duck tape. It doesn't work very well with ducts. Also, it is good for water proofing stuff - you know like a duck. For the rest of this post, I am calling it duck tape. (Wikipedia agrees there is some naming problem)
You know I love the MythBusters - right? I hope you don't think I sit around waiting for them to make a mistake so I can pounce on them (I save that for ESPN Sport Science). In this case, they just made a small mistake. A mistake that…
rallain | May 6, 2010I already mentioned the MythBusters' crashing two cars episode where they correctly doubled the speed of a pendulum type object. Overall, this was a very visual (although expensive) demo. There was one part that left a sour taste in my mouth - the final explanation from the narrator. First, they showed this.
And then they had an explanation that went something very similar to to this (after restating what the sign above said)
"Although the two-car crash doubles the speed, the energy the crash is transferred to twice the mass resulting in a crash that looks like just one car hitting a wall…
rallain | April 24, 2010Check this out.
So, the guy jumps from 150 feet into some cardboard boxes. Why are the boxes important? You want something that can stop you in the largest distance to make your acceleration the smallest. Here is my Dangerous Jumping Calculator. Basically, you put in how high you will jump from and how much distance you will take to land and it tells you your acceleration.
You will probably need this G-force tolerance info from wikipedia.
One problem - this calculator doesn't really work for this case. It doesn't take into account air resistance. Does air resistance even matter in this…
rallain | April 21, 2010I want to give ESPN's Sport Science another chance. The segment on Tim Tebow didn't really have any errors. I thought, cool - what about this one on Jason Pierre-Paul? Here is the clip. Oh wait, embedding is disabled. Ok - if you want to watch go here (the image is a link)
The goal of this segment was to examine why Jason is so awesome at tackling. The answer: because he can do flips. Really, that was the answer. Let me briefly go over some of the problems with this segment.
Correlation does not mean causation
They didn't say it explicitly, but it seems the are saying the reason he…
rallain | April 19, 2010You probably already know how I feel about the "media" and their physics explanations (see attacks). Let me summarize the problem. There are a whole bunch of cool shows on tv that deal with sciency stuff - that is good. These shows then try to teach some science along with their demos and explosions and stuff. This is also good. However, they usually butcher the explanation part. Some of them (ESPN's Sport Science) must just literally make stuff up that sounds cool.
I understand that in common usage, things like "force" can be used lots of different ways. I am ok with that. Also,…
rallain | April 16, 2010If you have been on the internet, you surely have seen this website showing the sizes of different starships. Way cool. Here is a small sample - but it doesn't do the site justice. You really need to go browse around.
Of course, I can't let something like this just go. One of the things I always think is interesting is to consider objects of different size. Perhaps the general idea is that you can just scale stuff up or down as you like. But, it doesn't work this way. Let me start with my own spaceship. It is a sphere with a thruster on the back. It just holds one person.
Now, what…
rallain | April 8, 2010In the last episode of MythBusters, they tried to reproduce the following experiment. Suppose you are driving in a car at 60 mph and you shoot a ball backwards at 60 mph (with respect to the car). Will the ball just drop (with respect to the ground)? Actually, it is a cool demo - I saw some Japanese show did this a while ago.
So, what is the problem? The problem is with the MythBusters' use of their terms to explain this thing. Let me look at a couple of the things they said to explain this (surprisingly, they described it several different ways). This first one is my favorite.
Bad…
rallain | April 2, 2010In this part of the world, we have oak trees. Technically they are called live oaks - but I don't get it. Of course they are alive. I was at a soccer game and this is the tree I always look at.
Look how far those limbs extend horizontally. That branch is about 12 meters long. Why is this amazing? Have you ever tried to hold an 8 foot 2 x 4 board horizontally by holding one end? Pretty tough. How about I calculate the forces needed to hold that branch in place? I will do a simple model and then maybe later I can make it more complicated. Suppose I replace that limb with one straight…
rallain | March 24, 2010How does a suction cup work? It is all about the atmosphere. Here is a demo. Take some type of "suction cup" device. In this case, I used a toy dart. Stick it to something smooth and lift it up. Like this:
What lifts up the metal block? The atmosphere. Diagram time:
But this isn't a very realistic diagram. Actually, the suction cup would be pushing down on the block because the force from the atmosphere would be too large to balance with the weight. Let me put some numbers in here. Suppose this is an aluminum block - I just going to pretend it is 4cm on a side (and a cube). In…
rallain | March 23, 2010I happened to catch two parts of two different episodes of Meteorite Men - a show about two guys that look for meteorites. In both of the snippets I saw, they were talking about a debris field for a meteor that breaks up. In these fields, the larger chunks of the meteorite are further down in the field. Why is this?
Let me approach this first from a terminal velocity view. This requires a model for air resistance. I will use the following:
Where:
rho is the density of air
A is the cross sectional area of the object
C is a drag coefficient that depends on the shape of the object
v is…
rallain | March 22, 2010This is a topic that I am going to talk about in my physical science class. Might as well make a post about it, right?
Here is the deal. You are in a pool. You drop a quarter in the deep end and swim down to get it. I know the first thing you are going to ask: Why do I have a quarter in the pool? Does it matter. What matters is that your ears are killing you. Boy, that hurts. Why do your ears hurt and what can you do about it?
Pressure and depth
When you put a fluid in a gravitational field (like on Earth), the pressure in that fluid (or gas) increases as you go down. Why? There are…
rallain | February 28, 2010I ride my bike and mostly the wind makes me unhappy. On a very few days the wind is with me on the way to work and then changes so that it is with me again. But most days the wind is fairly constant. So, if the wind is constant then shouldn't everything even out? (Even Stephen).
Assumptions:
Let me start with the assumption that I (a mere mortal) can output at a constant power (but not 57,000 Watts like some people). I will also assume an air resistance force that is proportional to the square of the relative air speed. Here is a diagram.
A couple of quick things to point out. First,…
rallain | February 23, 2010I had so much fun creating graphs for the Red Bull Stratos Space Jump calculation, that I figured I should make some more.
Can you fall faster than terminal velocity? That is the question.
Air Resistance
Air resistance is a force exerted on an object as it moves through some stuff - air in this case. The magnitude is usually modeled as:
Rho is the density of the stuff the object is moving through
A is the cross sectional area of the object
C is the drag coefficient of the object - this depends on the shape (a cone would be different than a flat disk)
v is the magnitude of the velocity of…
rallain | February 19, 2010Red Bull is sponsoring this sky dive from really really really high up - Stratos: Mission to the Edge of Space. Seems dangerous. The basic idea is that Felix Baumgartner will take a balloon ride up to 120,000 feet and jump out. Here are some questions:
Will he reach supersonic speeds?
The Red Bull site says: "can Felix react to a 35 second acceleration to mach 1?"
How about the claim that he will free fall for 5 minutes and 35 seconds? That seems pretty short.
In 1960, Joe Kittinger jumped from 102,800 feet. Will 20,000 feet make a large difference?
Assumptions
Clearly, this can be a…
rallain | February 17, 2010It is winter Olympics time and time for physics. The event that I always gets me thinking about physics is short track speed skating. It is quite interesting to see these skaters turn and lean at such high angles. All it needs is a little sprinkling of physics for flavor.
Check out this image of Apolo (apparently, it is not Apollo).
How about I start with a force diagram?
I know what you are thinking...Fcent....what force is that? Yes, I am going to use the centrifugal force in this case - but remember that sometimes fake forces are awesome. In short, if I want to pretend like Apolo…
rallain | February 16, 2010Do you need some physics to go with your beads? Well, here it is. Actually, I will let you do the physics (for now - meaning if you don't analyze these, I will).
Here are some pictures of a broken strand of beads. I hung them on my fence - you know, for display. Click the image to get a larger analyzable version. Oh - I just happened to include a meter stick in the image. Also, just for show I included a level (that is level).
Now, I have two videos. In both of these videos, I again included a meter stick. You know...because I like meter sticks. Also, I am posting these videos on…
rallain | February 16, 2010Now I get to do something with that force scale I built.
I had a request some time ago to talk about friction. Friction is surprisingly complicated. When two surfaces rub against each other, why is there a friction force? The basic answer is that the stuff the two surfaces are made of (atoms) are interacting with each other. If you like, you could think of the bumps on one surface hitting the bumps on the other surface. I know I said it was complicated and that doesn't seem to complicated - does it? The complication comes when you try to model this interaction by looking at either all…
rallain | February 14, 2010I was thinking about some experiments that deal with friction and I wanted to show something with a force probe. The problem is that most people don't have one of these. So, I decided to try and make one out of simple things. In this case, I am using some straws, a rubber band and some paper clips. Let me draw a little sketch of how this thing works.
The basic idea is to use the rubber band to measure the force (by measuring the amount the rubber band stretches). The two paper clips do two things. First, it allows you to hook up the device to something (like hanging some Lego bricks on…