Dot Physics

Elephants can’t jump. Really.

Posted by Rhett Allain on July 27, 2009
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Sciencegeekgirl is blogging from the AAPT. She talks about showing something interesting to get students thinking, and here is her example:

This reminds me of Dan Meyer’s What Can You Do With This stuff. Anyway, I can’t help it. I must analyze this video. Plus, Fran essentially threw down the gauntlet and called me out on this move. Another reason to analyze this movie is that it is obviously fake. Elephants are one of the few animals that can’t jump. Not even a little bit. They don’t like to have more than 1 foot off the ground. Ok, on to the analysis. As usual, I downloaded the video from youtube and used Tracker Video Analysis. Here is a plot of one series of jumps:

i-0e30371b2ba420af3fbd54fb24cf3878-data_tool.jpg

Here, the second jump gives an acceleration of -3.6 trampolines/s2. I didn’t have the scale of this video, so I measured everything in units of the size of the trampoline. Also, the elephant is not a point particle, but I used his or her head for the location of the elephant. This is not a constant position relative to the center of mass, but it is close enough.

Looking at that fit, something looks weird (other than the jumping elephant since elephants can’t jump). The motion at the top of the jump looks like it doesn’t fit. If I just use that top data to get an acceleration, it looks like this:

i-81a107e71494068b0ff7d30fb461749c-data_tool_1.jpg

This gives an acceleration of -1.8 trampolines/s2. I guess the creators of the movie wanted to add emphasis to the motion at the highest point by making it take longer.

So, how big is the elephant and the trampoline? That is difficult to answer. I could assume this is on Earth such that the acceleration would be -9.8 m/s2. But then I would not know which acceleration to match to the Earth’s.

How high did the elephant jump? From the data, it looks like about 1.6 trampolines. I suppose you could use the max height and the time of flight to determine the actual acceleration, but I am not going to do that. You can do that as a homework question (which I won’t grade).

Another interesting question: what is the acceleration of the elephant while in contact with the trampoline and is this acceleration too high to be dangerous? I would assume not, but I have not calculated it. Also, NASA does not publish data (that I am aware of) on g-tolerances of elephants.

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Comments

  1. #1 Fran
    July 27, 2009

    It is the top of the trajectories that look suspicious to me, while watching it. There’s one at 1:34 where I would swear, by eyeballing it, the acceleration is different on the way up than it is on the way down. I think, if people are going to go ahead and make CGI movies that are supposed to look real, they can by golly program in a realistic trajectory. It’s the easiest part of the programming! Don’t I sound curmudgeonly tonight! If only there were some darn kids on my lawn to yell at while I’m letting my inner geezer loose!

  2. #2 Rhett
    July 27, 2009

    @Fran,

    I am glad I was able to confirm your suspicions. The same thing happened in the movie up with the gps throw. How hard would it be to have a constant acceleration. I think it would be harder to NOT have constant acceleration.

  3. #3 Rob
    July 27, 2009

    In the early jumps where he (she?) isn’t flipping and twisting, her (his?) ears and trunk flail in the wind at zero g on the way down as they should, but his (her?) tail doesn’t, it hangs straight down. About 1:00 to 1:20. Mighty suspicious, I don’t think the jumping elephant is real.

  4. #4 Stephanie Chasteen
    July 28, 2009

    You are a compulsive analyzer, Rhett. Do we need to form a support group?

    It’s interesting, because when I watched the video, I did have this feeling that the elephant spent too much time at the top of the jump. It’s good to see the data support my gut feeling.

  5. #5 Etoyszone Trampolines
    August 25, 2009

    You are addicted to analysis, but as a Trampoline Manufacturer, I couldn’t help trying to calculate what strength of steel and springs we would need to actually make it work. It would also need to be much higher off the ground of avoid the trampoline bottoming out. Fun Video, but I won’t be able to sleep tonight trying to calculate it.

  6. #6 365 Trampolines
    December 10, 2009

    As a trampoline maufacturer Paul could you say what trampoline provides the best bounce all around ? Is it the square trampolines or the octagonal trampolines. I am not sure to be honest and think all have good features.

  7. #7 dawg
    April 29, 2010

    is dis real?

  8. #8 CherryBomb
    April 30, 2010

    Assuming the engineers at Etoyszone can design a scaled-up trampoline to deal with an elephant, the g-forces would be the same as those of a human jumping the same height on a normal trampoline. If they can’t (as their guy suspects), then the trampoline would have to be farther off the ground. This would give the springs more time to accelerate the elephant to launch velocity, meaning the g-forces would actually be lower than for a human. In any case, I hope they were not trying this experiment in a gym on the second story.