Dot Physics

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).

**Is it real?**
So, the first thing to do is take out my favorite physics tool – [Tracker Video Analysis](http://www.cabrillo.edu/~dbrown/tracker/) (free and awesome). From this I can get position-time data for the “jumper” although I wouldn’t really call it a jump. Plotting the vertical position as a function of time, I get (I use Plot – the free plotting software, it’s also pretty good).
Here is what I get:

![Screenshot 01](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-01.jpg)

First, notice the units of the position are (units). Thanks to the videographers, I don’t know the distance scale of the video. But, right away there is good evidence the video is real. When things are thrown in the air at relatively low speeds, they have a constant vertical acceleration. This would produce a parabola when plotting position vs. time. Above data looks like a parabola. I think fake jumps look much different (unless I am a complete noob when it comes to fake jumps – which I am).

Also, from this video, if I assume the jump is on Earth (a logical assumption) then I know what that acceleration is -9.8 m/s2 I can compare this to the acceleration here in units of (units/s2). The above curve fit can be compared to the following kinematic equation:

![Kinematic.jpg](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/kinematicjpg.jpg)

So in this case, the A coefficient would be equal to (1/2) times the acceleration. For the above motion, the acceleration would be:

![Screenshot 03](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-03.jpg)

This means that 1 unit would be 2.189 meters.

**How could that girl get shot up that high?**
In the video, the girl is on giant inflatable cushion. One or two people jump down on the other end and that increase the air pressure which in turn shoots her up. In the video it is difficult to tell who is jumping, from how high and how many people (note to videographer for next time). Nonetheless, the first thing I think about in this case is energy. If I take the initial jumpers (call them jumper), the earth and the launched girl (I will refer to as girl) plus the air bag as the system, then there is no work done the system so:

![Screenshot 04](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-04.jpg)

I will look at the initial position where the girl is down and the jumper(s) is up.

![Screenshot 05](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-05.jpg)

So, the energy equation for this would be:

![Screenshot 06](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-06.jpg)

Here you see that for both position 1 and 2, there is no kinetic energy. The jumper starts at rest and at the highest point, the girl is at rest. From this, I can determine the ratio of the mass of the jumper and the girl (assuming I can measure y1 and y2.

![Screenshot 07](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-07.jpg)

From the video, I have a value for y1, it is 1 unit which converts to 2.189 meters. I can get a value of the maximum height of the girl from position graph. This gives y2 = 4.15 units = 9.08 meters (or 28.9 feet).

Now for the girl – I measured the length of the torso + legs as 0.602 units or 1.32 meters (4.3 feet). I could look up some average weight-height chart, but I am going to totally ballpark her weight at 90 lbs (40.8 kg). This means that the jumper has a mass of

![Screenshot 08](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-08.jpg)

**note** I am using pounds and kg both as a unit of mass – which it isn’t (I know).
This seems doable. Either one huge dude or two big dudes. Most likely it was two dudes around 170 pounds. Actually, the mass would have to be a little larger because there would sure to be some energy loss such that E1 would be a little greater than E2. Also, clearly some estimates were made – but this seems reasonable.

So, the next time you do something crazy like this and you video tape it, please remember the video analysis rules I posted.

**UPDATE:**
I still think it is two guys – look at this:

![Screenshot 12](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/09/screenshot-12.jpg)

Comments

  1. #1 crazytrain
    April 4, 2011

    I think there is one thing thats banging my head. Your initial assumption that there is no work done by the blob, earth and jumpers system so you can equal the energies may be wrong. The blob will continue to depress into the water which means water will be displaced and that will be work done (some amount of energy).
    You may have to take the jumper’s initial speed when contacting the blob, and the final speed when the blob is maximally depressed. Then you can get the jumper’s deceleration over the depressing distance. From that you can get the force applied to them. There should be equal and opposite force applied to the girl over the same depressing distance d. So you can find her acceleration and final speed. What do you think?

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