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

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

**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)

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.