# momentum

### What if everyone jumped?

Might as well jump. Jump. Go ahead, jump. - Van Halen Suppose everyone in the world got together and jumped. Would the Earth move? Yes. Would it be noticeable? Time for a calculation. Note: I am almost certain that I have done this before, but I can't find where. Starting assumptions. 7 billion people. Average weight: 50 kg (you know, kids and stuff) Average vertical jump (center of mass): 0.3 meters - and I think that is generous. Mass of the Earth: 6 x 1024 kg Gravitational field near the surface of the Earth is constant with a magnitude of 9.8 N/kg Ignore the interaction with the…

### Collisions: Kinetic energy or momentum?

In the last episode of MythBusters, they wanted to see if a tornado could make some glass cut off a person's head. The first attempt was just to take some glass and through it at a simulated human neck. Clearly, this wasn't quite the same as a tornado. So, here was their plan. If they want to simulate glass moving at 300 mph, they could get a bigger piece of glass and put it on a truck moving at 80 mph. The result would give a piece of glass with the same kinetic energy as a smaller piece moving at 300 mph. Their calculations look to be correct. However, the question is: would this make…

### MythBusters' energy explanation

I 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…

### 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,…

### MythBusters and physics terms

In 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…

### An example of two collisions

Suppose you want to move an empty paper clip box by shooting it with a toy dart gun. Why would you want to do this? Don't worry about that - this is my example and I am sticking with it. Should you shoot a dart that sticks to the box or should you shoot one that bounces off? I made a video of this exact situation. Note: you could obviously come up with other objects to do this with, but I always like to use more normal stuff. In case it wasn't clear, the first dart bounced back and made the box go much faster (and farther) than the dart that stuck (inside) the box. The usual question is…

### 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…

### Fight Science = Bad Science

Fight Science is an entertaining show. Great graphics. The basic idea is to look at the science in different fighting styles. They had a clip-style commercial on it during a MythBusters episode I was watching. And from that, I can say that the kicking looked cool, but the science needs some work. The Setup The basic idea is that they wanted to compare kicks from different fighting styles. From what I can gather, they collected data by having some dudes kick this "kicking bag". During the kick, they measured the force exerted on the bag and they had a sensor on the kicker's leg - I…

### 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…

### 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…

### MythBusters: Crashing two moving cars, or one

I am so pumped up that MythBusters is back on. Not only do I like the show, but it offers so many blogging opportunities. Their latest show featured car crashing myths. One of the myths from the episode was a redo of the myth where two trucks crash head on simultaneously crushing a smaller car in the middle. The first test was very similar to the previous time they tested this, but faster. They towed two 18 wheelers to crash together around 50 mph and smash a stationary car. The results were impressive. However, they did not have the result of the car completely contained in the…

### LA LA LA LA LA - NY Times and the physics of football

People say I am picky. Ok, sometimes I am. But somebody has to stand up for what is right and just. Maybe I am that person. Please stop using the word force if you don't know what it is. There. I said it. You can attack me now. It wasn't just one thing that got me fired up. It was two things. First, I read this article on physics and football (Physics of 'The Hit' from the NY Times). If it was just this article, I would have let it go and moved on. But no. One of my kids just happened to be watching MythBusters (We all love MythBusters) and there was a discussion that used the term…

### Physics of the Water Rocket

I said I would come back to this, and I am. I am a man of my word. Yesterday, I posted a link to a video of this really cool water rocket thingy. How does this work? What is the physics going on here? I think this can be best explained with the momentum principle. Let me start by pretending like I have some object that shoots out a piece of water (or really it could be anything). Also, let me pretend like this is in space ore something where there are no external forces. Above is a before and after picture. Initially, there is something (the box) with water inside. Through some process…

### Real Water Rocket Guy - Analysis to come

I saw this on Hack a Day, but I am sure it has been posted in other places also (seems like something that would be on Swans on Tea). Check it out. Amazing New Water-Powered Jet Pack - Watch more free videos Some pretty cool stuff. I am pondering the questions to be answered right now.

First, don't play with fire. Fire is dangerous. You can get hurt. Here is a video of two guys trying to (presumably) make the Miami Heat flaming basketball. Note: the above is just a picture link to the actual movie. Embedding the movie was doing some weird stuff. In the movie, two guys pour gasoline on a basketball and then light it on fire. One of the guys then gives the ball a good kick. The result is that some of the flaming gas stays in place and some goes with the ball (and some gets on the guy's foot). This could be a good example of the momentum principle (also know as Newton…

### Time Warp Samurai guy - waves of kinetic energy?

I am sure I have mentioned the Discovery show Time Warp before. The basic idea of the show is to find how many cool things they can look at with an ultra slow motion video camera (btw - this is on my christmas list). So, I have seen this Samurai guy on the show twice. Yes, he has a big sword. Yes, he could utterly destroy me if he wished. However, I can not handle him talking about physics. Mr. Samurai guy, please stop using the words "kinetic energy" until you know what they mean (I said please). Here is my first problem. Samurai guy is showing how to chop people down. He plans to…

### Basics: Collisions (Interactions between two objects)

Pre Reqs: momentum principle, forces,energy, vectors Really, there is not much new here. This is an introduction to objects that interact. To describe this, I will need to pull several different ideas together (that you have probably already looked at). Let me start with a simple case. Suppose I have two objects, maybe they are two asteroids in space. I will call them asteroid A and B: In this situation, the two objects have different momentums but one interaction between them. Notice that the gravitational force on asteroid A is the same magnitude but opposite direction as the force…

### Physics and directly downwind faster than the wind (DWFTTW) vehicles

A commenter (Joe) suggested I look at vehicles powered by the wind that can move faster than the wind. This also came up on Swans on Tea. One of Joe's points was that I should do this BEFORE MythBuster's gets to it. And so I shall. I know this is one of those topics that can never really be won - like "the moon landings were fake" or "you can run your car on water". I will try anyway. If you are not sure what this "faster than wind" thing is all about, check the Swans on Tea link or this boing boing link. There has been a ton of stuff posted about DWFTTW vehicles. Literally a ton. If…

### MythBusters - Testing Bullet Proof and Bullet Speeds

On MythBusters this week, Adam and Jamie tested the bullet-proofness of various objects. The one that sticks in my mind is the ipod. The said there was a report of a solider being shot by an AK-47, but he was saved because the bullet hit his ipod. To test this, Adam shot an AK-47 at an ipod and it went through. Their conclusion was that he was also wearing body armor. I am not sure I like that conclusion. Why would someone report that the ipod saved him if he was also wearing body armor? Maybe they would, but not sure. I was thinking, maybe the bullet went through the ipod because they…

### Spring Motion and Numerical Calculations

Maybe you know I like numerical calculations, well I do. I think they are swell. [VPython](http://vpython.org) is my tool of choice. In the post [Basics: Numerical Calculations](http://scienceblogs.com/dotphysics/2008/10/basics-numerical-calculation…) I used vpython and excel to do something simple. I will do that again today (in that this problem could also be solved analytically). However, there is one big difference. This problem has a non-constant forces. Suppose I have a mass that is connected by a spring to a wall. This mass-spring is sitting on a table with no friction. ![…