forces
rallain | November 19, 2009Last night I saw the newest episode of MythBusters. One of the myths they revisited was the exploding water heater. Well, it turns out that I had an analysis of this first explosion, but I didn't move it over when I switched software. So, here it is.
In case you never saw the first episode of exploding water heaters, here is the important part:
If you are impatient, here are the answers (from the video analysis):
Time of flight = 11.8 seconds
Max height = 167 meters = 548 feet
Launch speed = 234 mph
Speed on impact with the ground = 76 mph
First, from re-watching the video, I can see (…
rallain | November 13, 2009Not really. Here are the details (and some data) for the Millikan Oil Drop Experiment without the oil drop that I talked about previously (originally from The Physics Teacher - lucky you, it was a featured article so it should still be available (pdf)).
The basic idea that Lowell McCann and Earl Blodgett from U of Wisconsin propose is to do an experiment similar to the oil drop experiment, but not so squinty (if you have done the oil drop experiment, you know what I mean). Instead of dropping charged oil in an electric field, they drop containers with metal nuts in water. The goal is to…
rallain | November 4, 2009I already attacked the 2008 Punkin Chunkin Show. So, now I going to give the chunkers some tips. In case you aren't familiar, the Punkin Chunkin contest has teams create devices to launch a pumpkin. They have different categories, but I am going to focus on the air-powered devices. The basic idea is to make an over sized pneumatic potato gun. Here are the things I was inspired to think about.
It seems all the canons were aimed at about the same angle. Did they guess at the angle? Or is this trial and error? What would be the best angle for a pumpkin launch?
Does the optimal angle of…
rallain | October 31, 2009Here is the deal. Why would you put a lot of effort into explaining something wrong? I just don't get it. Wouldn't it be easier and better to just not say anything? The particular show I am ranting about is the 2008 Punkin Chunkin on Discovery (I think). Fairly ok show, but they should have left the science out. The part that made me speak out was when they were talking about the different types of punkin chunkers. They have:
Compressed air
Catapult
Trebuchet
Centrifugal Force Machine
To explain these devices (the physics behind them), the show brought in the big guns. Let's bring…
rallain | October 27, 2009I know I saw this demo somewhere. Maybe it was at an AAPT conference a few years ago. I have always wanted to build this, but never got around to it. Until now. Here is the demo (it is easy, you should make one too)
So, how does this work? I think the simplest explanation is that the drinks do not spill because the string can only pull in the direction of the standing glasses. A slightly better explanation is that the string lets the tray rotate so that the sum of the acceleration and the gravitational field is in the direction of the open ending of the cup. I am still not happy with…
rallain | October 24, 2009Sometimes it is difficult to come up with new labs. Ideally, a lab should show use some of the basic physics principles as well as have something the students can measure. What to do with circular motion? I don't know how I forgot this, but here is a lab I used to do as an undergraduate student. I also like it because it doesn't really need fancy stuff like PASCO probes or anything. The basic idea is that a small mass is swung around in a circle with the tension in the string controlled by hanging a mass on the other end. Here, let me show you.
Circular Motion Lab from Rhett Allain on…
rallain | October 11, 2009The other myth the MythBusters looked at last week was the phrase "knock your socks off" (along with the dropping and shooting a bullet myth). But before that, let me complain.
Maybe it is just me, but I totally cringe when these guys use the word 'force'. Force probably isn't the best term to use to describe a collision especially when you are talking about one of the objects. "oh, we will just give this object some more force to impact with that other object". Force is not a property of an object, but rather an interaction between two objects. When two things collide, you really need…
rallain | October 9, 2009If you didn't catch the latest MythBusters (yeah! new episodes), they did something straight from the physics textbooks. Just about every text has this example of shooting a bullet horizontally and dropping a bullet from the same height. The idea is that they should hit the ground at the same time. No one but the MythBusters could actually show this demo with a real gun.
The Physics
I am going to do some calculations, but I want to first write about the physics that accompanies this idea (and you can actually do it your self without the gun). What physics principle does this demo show?…
rallain | October 6, 2009Many textbooks are pumped up about Newton's 3 laws of motion. For me, not so much. First, (as many other's point out) these are really Newton's ideas about force. Second, the first law is pretty much a special case of the second law. Here are the first two laws (in my words):
Newton's First Law:
The natural state of an object is constant motion.
Yes. I know that is not how it is normally written.
Newton's Second Law:
The rate of change of an object's speed is proportional to the amount of net force on the object and inversely proportional to the mass of the object.
This could also be…
rallain | October 6, 2009You know I like demos, right? This one is quite fun to do even in a class. The basic idea is to take a bucket of water and swing it around in a circle over your head. Simple, but if you have never done this one, it can be a little intimidating. Here is an example.
Water Demo from Rhett Allain on Vimeo.
So, how does this work? What does it show? Really, the question is: why doesn't the water fall out of the bucket?
First, I like to talk about "fall" what does that mean? I guess that means that the object has a downward gravitational force, but no force upward to give it a zero…
rallain | October 4, 2009There were a couple of things that bothered me about the MythBusters' myth where they fired bullets in the air. The myth was that a bullet fired in the air could kill you. The first problem is that it is not a myth. There are several reported cases of people being killed from bullets that were fired in the air. The Mythbusters tested this by finding out how fast a bullet would be going if fired straight up. A couple of problems:
First, they measured the terminal velocity of a tumbling bullet, not a spinning one. I really don't know how long a bullet will stay spinning, but I guess this…
rallain | September 30, 2009This is really a lab that I have students do, but I am pretty sure they don't read this blog - so it is ok. If they are reading this, hi!
We have these projectile cannons that shoot small balls. In order to look at projectile motion, they need to first determine the launch speed of the ball. I have a great method for this. Basically, shoot the ball horizontally off the table and measure how far horizontally it goes. You can get the final location of the ball by having it hit a piece of carbon paper on top of normal paper. If you don't know what carbon paper is, you are young.
Anyway,…
rallain | September 23, 2009You know I like the Mythbusters, right? Well, I have been meaning to look at the shooting bullets in the air myth for quite some time. Now is that time. If you didn't catch that particular episode, the MythBusters wanted to see how dangerous it was to shoot a bullet straight up in the air.
I am not going to shoot any guns, or even drop bullets - that is for the MythBusters. What I will do instead is make a numerical calculation of the motion of a bullet shot into the air. Here is what Adam said about the bullets:
A .30-06 cartridge will go 10,000 feet high and take 58 seconds to come…
rallain | September 14, 2009I really shouldn't do this. I might be helping someone to set up something dangerous. But, I am going to anyway. Here is a question posted on some forum. (actually, it is from math help forum)
"I'm anticipating a good winter this year, one with lots of snow. My yard is sloped quite a bit and it would be the ideal place for a huge snowboard jump, only problem is I need to calculate how fast I will be traveling when I hit the jump, how high and what angle the jump should be, and the distance and angle of the landing ramp to optimize my range."
So, what am I going to do? I am going to give…
rallain | August 31, 2009Maybe this is a little old (in internet age), but it is a great example. Here is the Loop-the-loop stunt from the show Fifth Gear.
I like this. First, it is a bold stunt. But also, there is some good physics here. Though, most importantly, the Fifth Gear producers were kind enough to include a shot that was very compatible with video analysis.
I went to the official site of this stunt - http://looptheloop.dunlop.eu. From here I found some useful info:
Loop is 40 feet tall
The car is a Toyota Aygo
Some physics-y guy calculated that the car must go 36 mph to do the loop (I think that is…
rallain | August 28, 2009I found this in the most recent issue of The Physics Teacher (September 2009). Surprisingly, there were several good articles in this issue. One article discusses a doable version of the Millikan Oil drop experiment. Maybe you are not a (or were not) a physics major, so you might not be familiar with how cool, but tedious and squinty the oil drop experiment can be.
In the Millikan Oil drop experiment, small electrically charged drops of oil are placed in a constant electric field. It turns out that a small enough number of electrons are on each drop so that the quantization of charge can…
rallain | August 21, 2009Forgive me if I don't know the official parkour term for this move. This is where you have two walls that are close to each other and you vertically climb them. Here is a shot of Mark Witmer (from Ninja Warrior) doing the wall climb.
Doesn't look too hard, does it? Well, I think it depends on how far apart the two walls are. This is actually one parkour move that my kids like to do (Hey kids! Don't do that! Let me get my camera though because this will be perfect for my blog)
I am going to start with this second kind of wall climb. Simply because it is easier due to symmetry. So,…
rallain | August 12, 2009This Giant water slide video is extremely popular on the internets. Maybe you have not seen it (doubtful), then here it is:
This is such an incredible stunt that the very first question that comes up is - fake? or not fake? From my previous analysis, I can say:
Even though there is some slight perspective issues, the vertical acceleration seems to be constant and has a reasonable scale to give the acceleration of -9.8 m/s2
The horizontal motion is essentially constant (as a real jump would be)
The launch speed is about 19 m/s
The launch angle is 32.8 degrees
If the guy had no friction on…
rallain | August 6, 2009Thanks to Nick for showing me this video (Check out his blog - Fine Structure):
See more funny videos and TBT Videos at Today's Big Thing.
Wow. That was my first reaction. My second reaction was: no way. Is this real life? I just don't know. How hard would it be to find out exactly where to place that pool and where did they get the water from? Obviously, this one requires some analysis. First, on the VAS for this video: 4/8. Not too good. Oh here are the questions I would like to answer:
What is the guy's acceleration after he leaves the ramp?
What was his initial velocity…
rallain | August 5, 2009Parkour: the act of running and jumping like a crazy superhero. I can't do any of it. But I can analyze it. So, let me start with the wall-flip (or any kind of move that involves walking on vertical walls). This looks like a good example (there are a bajillion on youtube).
Pretty cool, huh? How do you run up a wall? Well, it has to do with friction. Remember, this is a fairly useful model for the magnitude of the frictional force on an object:
Where N is the force the surface exerts on the object (the normal force) and ?s is the coefficient of static friction. Note that this less…