Category physics

Fake or Not: Physics analysis of “Extreme way to shop for groceries”

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Here is the video in question:

Looks too incredible to be real for me. That is when I start to question things. Is this fake or not? To answer this, I took a clip that showed a person launching a grocery item over the isle. This was a good shot to look at because it was *mostly* perpendicular to the camera view. I then used [Tracker video analysis (free) tool](http://www.cabrillo.edu/~dbrown/tracker/) to get x-y-time data for the flying projectile grocery. The scale was difficult, so I just guessed that the guy on the left was 5 foot 10 inches. Here is the vertical position data for two tosses.

![grocery](http://blog.dotphys.net/wp-content/uploads/2008/09/grocery.jpg)

A few things to notice:
– The plots are mostly parabolic. This is what you would expect from a real toss (air resistance would be small)
– The two tosses have essentially the same acceleration.
– The acceleration of these two tosses is in the ball park of -9.8 m/s2. Yes, they are not right on, but I totally guessed on the scale of the video.

So, from this, I think that shot is real. It is still incredible. Maybe they did this for like two hours to get it right. Maybe they just got lucky. Maybe they used the force and the force is strong in their family.

I was going to add an analysis of how off they could be on their throws and still make the “basket”, but I got lazy. Sorry. Maybe I will add that later.

Basics: Projectile Motion

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**pre-reqs:** [kinematics](http://blog.dotphys.net/2008/09/basics-kinematics/)

My previous “basics” post was on kinematics (in one dimension). But what about two dimensions? In particular, what about projectile motion. My motivation here is that I was about to talk about analysis of a video that involved projectile motion and I don’t want to go over all the stuff again and again.

Continue reading “Basics: Projectile Motion” →

The Sky is Falling (as always)

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One of my daughters was just reading Chicken Little to me. I don’t know if you are familiar with Chicken Little, but she is a chicken that runs around telling people “The sky is falling”. In my normal fashion, I started thinking about the plausibility of this. What would fall? What would you look for? Then I figured it out. The sky IS falling. It is ALWAYS falling and it has always been falling.

What is the sky? I am assuming the sky is the air. I will treat the air as a gas of single particles (which it isn’t, but that’s ok). So, why does this sky (air) do what it does? If you look at each individual sky particle, its motion is governed by two things.
1) Gravity. The gravitational force makes each particle fall. Without this, all the air would escape the planet. (this would be bad)
2) Collisions with other particles. This is what prevents the “sky” from collapsing.
I actually talked about this some before [when I talked about MythBusters and the Lead Balloon](http://www.dotphys.net/files/lead_balloon.html). The best way to see this interaction between gravity and collisions for a gas is with the excellent [PHET simulator](http://phet.colorado.edu/new/simulations/sims.php?sim=Balloons_and_Buoyancy)
![phet](http://blog.dotphys.net/wp-content/uploads/2008/09/phet.jpg)
If you adjust the “gravity” you can see that there are more particles down lower, but that a particle in between collisions looks like a projectile. Thus, I think it is ok to say that the sky IS falling.

Basics: Kinematics

rhettallain 1 Comment

**pre reqs:** *none*

Often I will do some type of analysis that I think is quite cool. But there is a problem. I keep having to make a choice. Either go into all the little details, or skip over them. My goal for this blog is to make each post such that someone could learn some physics, but I also don’t want it to go too long. So, instead of continually describing different aspects of basic physics – I will just do it once. Then, when there is a future post using those ideas, I can just refer to this post. Get it?

Fine. On with the first idea – kinematics. Kinematics typically means a description of motion (not what causes that motion). In particular, kinematics looks at position, velocity, and acceleration. In this post, I will try to stay in one dimension. This will make things look simpler without really losing too much. Later, when I talk about vectors, I will make it all better.

Continue reading “Basics: Kinematics” →

How about a massive catapult to replace the space shuttle

rhettallain 10 Comments

I recently saw a comment on a blog somewhere about putting satellites into space (I think it was from a post about a rocket that blew up). The poster suggested using a giant catapult to put things in space instead of rockets. Maybe he or she was kidding, or maybe not. But I have heard this idea before. Would it work?

Continue reading “How about a massive catapult to replace the space shuttle” →

Mythbusters Moon Stuff and Me

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Clearly the MythBusters did this before I did (it’s just air tonight though). I just wanted to say that I posted some videos of the Apollo “jump salute” video analysis and also sped it up to “Earth-like” accelerations. I then made a video of my daughter doing the same thing on Earth and slowed it down. Yes, the MythBusters did it better, but I just wanted to say “me too”. My analysis is here:
[Undoing Fake Moon Landing Videos](http://www.dotphys.net/files/moon-time.html)

P.S. I also slowed down a video of Kobe Bryant Jumping so he has an acceleration of that on the moon.

Heat. It’s a four letter word

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Heat. You have heard it before. You have used it. I have even used it. Do we need this word? No. Is this a useful word? No.

Let me start with the definition as usually stated in a physics type text: (this is from [dictionary.com](http://dictionary.reference.com/browse/heat))

*heat:* a nonmechanical energy transfer with reference to a temperature difference between a system and its surroundings or between two parts of the same system.
This definition is fine. It is not wrong, but is it needed? Not really. Couldn’t we just say energy transfer? Actually, I like to use this in the following equation:
![work energy](http://blog.dotphys.net/wp-content/uploads/2008/08/work-energy.jpg)
In this equation (the work-energy equation) W stands for the work, and Q is typically referred to as “heat”. This equation is used for systems of particles, if you just have one particle, then the fundamental equation is:
![work](http://blog.dotphys.net/wp-content/uploads/2008/08/work.jpg)
So, expanding to macroscopic systems, Q is still work. Work is the only way to change the energy of a system. I like to call Q (like some textbooks do) as micro-work. Q is the work done on the system due to particle collisions from another object.

The other non-sciencey definitions are clearly wrong. The common usage of the word heat is also clearly abused. This can be seen in phrases like “add heat” or “remove heat”. Another poor usage (which I have used) is “heat this thing up” which implies heat is a verb.

Ok, then if we do not use heat – what then? I think if you want to talk about transfer of energy, say transfer of energy. If you want to use Q, call it microwork. If you want to talk about the energy something has because it is hot, say thermal energy.

The Iron Cross – or: Why is Gymnastics so Darn Difficult?

rhettallain 2 Comments

I know the olympics are basically over. Really, I should have posted this earlier. Anyway, the gymnastics feat that always impresses me is the Iron Cross (I think that is what it is called). I know you have seen this, but here is a picture from wikipedia:

![Example 2ofironcross](http://blog.dotphys.net/wp-content/uploads/2008/08/example-2ofironcross.jpg)

(http://en.wikipedia.org/wiki/Rings_(gymnastics))

Why is this so impressive? Why is this so difficult? Let me start with something completely different that is exactly the same (in some ways).

Continue reading “The Iron Cross – or: Why is Gymnastics so Darn Difficult?” →

ARRRGGGGG RapidWeaver, why do you hate me?

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Dear RapidWeaver,

What did I do to you? I like you, I really do – but this is why we broke up. It’s not you, it’s me. No wait, it’s you. I am looking back through my old dotphys posts and most of my introductions are GONE. I was starting to think I was crazy when I wrote these. Take for instance [My analysis of Kobe Bryant’s Jump](http://dotphys.net/files/kobe.html). Isn’t it odd how this starts? That is because it is MISSING the first paragraph. The only reason I know I am not insane is because of Apple’s Time Machine. I went back and looked and the older file has an intro. RapidWeaver, see how you are!