Fake vs. Real Forces

In this post, I am going to talk about real and not real forces as well as the fake centrifugal force (if you don’t like the word “fake” you could replace that with “fictitious”)

First, an example: suppose you are in a car at rest and press the gas pedal all the way down causing the car to accelerate. What does this feel like? If I weren’t skilled in the art of physics, I might draw a diagram something like this:

![Screenshot 20](http://blog.dotphys.net/wp-content/uploads/2008/10/screenshot-20.jpg)

Yes, maybe someone would add gravity and the chair pushing up, but this shows the important points. What is this force of acceleration? What causes this? This is EXACTLY the same thing as centrifugal force. If you think centrifugal force is real, this also should be real. I think this is enough discussion to show that this force (and centrifugal) is not real, but I will continue. There is another mystery: why does it feel like there is a force pushing you back when you accelerate? (if you have read all my blog posts, you may have a hint to the answer).

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Amazing Blob Jump Launch Video Analysis

Can you believe it? Have you seen this video?

Are you thinking what I am thinking? WOW. How could these people not follow my rules for cool internet video. Once again, here they are:
1 Keep the camera stationary. This way I don’t have to keep moving the origin in the movie.
2 Don’t Zoom. Same reason, this video followed that rule.
3 Include a clear and obvious calibration object. A meter stick would work, or even a Kobe Bryant (I can look up his height). Maybe it could be a Ford F-150 that has a known length. Something!
4 Include the mass and height of all people involved.
5 Use high quality video.
6 Don’t talk about fight club – oh wait, wrong list.

Despite failure to follow all these rules, I have managed to analyze this video. Really when I saw it, I said “wow” – was that real? It looked real, but who would get shot up that high? (it is on break.com, so fake is a possibility).

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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.