# momentum principle

### Physics 2 refresher

So, I hear you are starting your second semester of physics. One of the cool things about physics is that the second semester still uses stuff from the first semester. Maybe you forgot some of that stuff, so here are the bare essentials you will need to get by (this is assuming you are in the algebra-based second semester of physics) Vectors Really, just about the entire semester course is about the electric and magnetic field. Both of these are easiest to represent as vectors. So, you pretty much need to know how to deal with vectors. Here are some reviews: How do you represent vectors…

### Physics Tips for Media

You 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,…

### Tablecloth and dishes trick - the BMW version

Hat tip to Frank for sending me a link to this video: If you have never done a demo like this (without the motorcycle), you should. It really isn't too difficult. Here is a video of my version: Inertial demo from Rhett Allain on Vimeo. So, the question is: is the motorcycle thing real or fake? First, let me talk about the key aspect of this demo. Why don't the glasses move? Well, they move - but just not very far. The demo is supposed to be an example of Newton's Second law, or you could say it is the momentum principle (which is what I will use). If a force is applied for a short time…

### Faster than terminal velocity

I 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…

### Stratos Space Jump

Red 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…

### Apolo Ohno Physics

It 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…

### R2 flying at a constant speed

Since my previous post on R2-D2 flying was so popular, I thought I would follow it up. I was going to add these two points in the comments, but a separate post seems to make more sense. Point 1 - R2 flies at a constant speed Well, he obviously doesn't always have to fly at a constant speed. However, in the clip I was looking at there are two important things in the analysis. What angle is R2's thrusters at? Is R2 flying at a constant speed? I found a much better quality version of the flying R2 from Billy Brook's site. That helped out a lot. So, on to Tracker Video Analysis (I always…

### Flying R2-D2, you are doing it wrong

You know I can't help but like Star Wars. Even with the new stuff, I watch it. Recently, I was watching the Clone Wars cartoon and noticed something odd about the way R2-D2 flies. I know what you are saying...."the odd thing is that he flies at all. Why didn't he fly in episodes 4-6?" Who knows. Here is the best image I could get of R2-D2 flying (from wookieepedia). What is wrong? Well, maybe you can't tell from the image I posted. Here is a diagram of flying R2-D2. If R2 (I can call him that because we are good friends) was flying like that, why would that be a problem? That would…

### MythBusters, Falling, Stopping, and Integration

In a MythBusters episode some time ago, Adam and Jamie jumped off a building. There was some cool stuff in this, but I want to focus on the acceleration data they collected. Before jumping into a pit of foam, they first wanted to test the set up by dropping a dummy into it and measuring the accelerations. Lucky for me, they showed a quick screen shot of their data. Note: I previously posted the calculations for jumping and stopping off of a building. For me, I see this and think - numerical integration. Before that, let me look at the physics. Here is a diagram of someone jumping off a…

### RP 11: When the centrifugal force is the centripetal force

It's odd that I have talked about these forces so much. First, I talked about how centrifugal forces were not real and the difference between centrifugal and centripetal forces. Then I talked about how sometimes, fake forces are good. Finally, I talked about the origin of the words centrifugal and centripetal. (note: "talked about" means wrote a blog post) In thinking about centripetal forces, I realized that I could come up with a situation in which the centrifugal force is the centripetal force. This is great. I can end all the confusion between centrifugal and centripetal by making a…

### RP 6: Throwing a fooball, Part II

In part I of this post, I talked about the basics of projectile motion with no air resistance. Also in that post, I showed that (without air resistance) the angle to throw a ball for maximum range is 45 degrees. When throwing a football, there is some air resistance this means that 45 degree is not necessarily the angle for the greatest range. Well, can't I just do the same thing as before? It turns out that it is a significantly different problem when air resistance is added. Without air resistance, the acceleration was constant. Not so now, my friend. The problem is that air…

### MythBusters: How do you characterize a collision?

Have I not made it clear how much I like the MythBusters? Also, I am totally aware that they are not (nor do they claim to be) scientists. Really, this is what makes their show appealing (maybe?). So here is the problem now. And, it is not just the MythBusters - I see other shows making the same mistake. If two things are colliding, how do you characterize the collision? No one really gets this right. For this particular episode, the MythBusters were looking at the collision between a bullet and gun. They wanted to see how hard it would be to shoot a gun out of someone's hand. The…

### Pendulum, let it go

This is a requested post. Clearly, I do requests. The idea here is that I am going to give all the details needed to determine the equation of motion (and then model it) for a basic pendulum. Warning: this post is a little more advanced than my normal posts. There are some prerequisites. You need to understand derivatives. I will assume that you do. Here is a pendulum. (and this time I will stick to my variables) Like I said before, this is a tricky problem unless I use some tricks. The problem is that the tension the string exerts on the mass changes. Here is my trick: think about a…

### Punkin Chunkin - they will never make a mile range

In my previous post on launching a pumpkin (punkin chunkin) I essentially just looked at what happens to the pumpkin after it is launched. How fast would you have to shoot it to go 1 mile? The answer seems to be around 1000 mph and they are currently shooting them around 600 mph. The question for this post, how fast can you launch a pumpkin so that it does not smash into smithereens? First, why would it smash at all? Here is a diagram of a pumpkin being launched while still in the tube. The pumpkin launcher works by releasing compressed air inside the tube. This means that the force…

### MythBusters and the physics of knock your socks off

The 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…

### More on Mythbusters shooting bullets in the air

There 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…

### Momentum and Football

I think we are entering a new era. An era where it is quite simple to find and get great videos. Oh, just saw a great tackle on the TV? In the old days, you would have to get that video off the TV yourself. Not anymore. Welcome to the interwebs. Also, the quality is awesome compared to 10 years ago. Here is the video. Yes, I know this is from several weeks ago - I am slow. Also, thanks to the person that put this on youtube - I edited your clip to remove the music and just look at the one collision. You did a good job though. In terms of video analysis, this isn't too bad of a video…

### How high does a bullet go?

You 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…

### 5th Gear Loop the Loop

Maybe 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…

### Parkour Physics: Wall Climb

Forgive 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,…