Dot Physics

Sport Science: Pulling and Power

I would like to continue my attack on the show Sport Science – ESPN. In this short episode, they are comparing the power of NFL player Marshawn Lynch with that of a truck. You can watch it here if you would like.

There are two things that are not quite right with this episode, first, the power thing. I will save the friction problem for another post. So, if you didn’t watch that clip, the basic idea is that Marshawn pulls some heavy tires. Sport Science then calculates the power needed to do this and then repeats a similar thing for a truck. Quick review. What is power? In short, power tells you how fast you can either do some work or change your energy.

i-58a614b07e52caf314ac3611fafa70a2-2010-02-07_la_te_xi_t_1_21.jpg

For the case of someone pulling something, I assume that the power would be based on the work this person does. Let me keep it simple. If you are pulling something in the same direction the object is moving (at a constant speed), the work done is:

i-b6882502df064d84e9c2c8e6c9b047e3-2010-02-07_la_te_xi_t_1_22.jpg

Fairly straight forward, right? Sport Science puts some motion sensors all over Marshawn’s body. They say this is so they calculate the power, but it looks like it is just to make this animated skeleton move like him.

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What actually is known about Marshwan pulling stuff?

  • He pulls some tires and a sled with a weight of 585 lbs (2600 Newtons)
  • The sled is pulled 5 yards (4.6 meters)
  • Not exactly sure about the time this takes (because they made part if it in slow motion) but I would guess it is somewhere between 5 and 11 seconds. I counted 11 in the actual clip.
  • Sport Science claims that Marshawn produces 573 Watts per kg. (this was in the online clip)

However – there is a difference in the version that was on TV. I thought I was crazy because I didn’t see this in the online version. Good thing I had taken a picture of it. Check this out.

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Yes, that says Marshawn produces 57,000 watts. They even went on and showed how many TV’s that could run. How did they get such a high number? My first thought was that they were just taking the weight of the tires times the distance for the work. This would be a pretty large error (that I will discuss in another post), but let me just assume this is what they did. How much work would it take to “lift” 585 pounds 5 yards?

i-279d6dfa6cde7d34757f61f58207f146-2010-02-07_la_te_xi_t_1_23.jpg

If I let the time be 5 seconds to pull this thing (which I pretty sure it was actually longer than that), then the power would be:

i-b1160efb8bb1ae9989a45c54bed6b8fd-2010-02-07_la_te_xi_t_1_24.jpg

Hmmmm….How can I make this work? In case you didn’t realize, 2300 watts is not quite 57,000 watts. Let me approach this from a different angle. What am I pretty sure about? I am pretty sure about the time and the distance. Let me calculate what force would be needed (what force Marshawn would have to pull with) to get that power (assuming 5 seconds, which I think is low).

i-bb1fef1b47c07a50dd7d808aa8933c5f-2010-02-07_la_te_xi_t_1_25.jpg

So, for a power of 57,000 Watts pulling something 5 yards in 5 seconds you would need a force of 62,000 Newtons (14,000 lbs). Hmmm. I am sure Marshawn is a powerful dude – but 14,000 lbs? There is something wrong.

According to Wikipedia’s page on human powered transportation, an elite sprinting cyclist can produce 2000 watts for very short times. I am sure Marshawn is elite, but not 20 times more elite in terms of power output. Something has to be wrong.

Summary

Maybe the people at ESPN were thinking: “hey attach some sensors to this guy and determine his power. Oh, just put down something huge like 57,000 watts. No one will ever check that, it will be fine.”

PS Thanks to Nick at The First Excited State for pointing out the pure awesomeness of Sport Science.

Comments

  1. #1 Coturnix
    February 7, 2010

    I think they just thought that 573 Watts per kg is the same as 573 kilowatts. Kilo-this, kilo-that, who knows the difference?

  2. #2 Jared
    February 7, 2010

    The Saints won; I don’t care what “funny math” says; it was an awesome game! Tell Greg Laden to kiss your ass ;)

  3. #3 EHR
    February 8, 2010

    Looks to me like they took the 573 W/kg (where’d the /kg come from?) from the online version and multiplied by his weight, which would be around 100kg, giving around 57000 W.

    Given the calculations above, it seems more likely to me that he actually generated 573 Watts during the attempt (and therefore, took about 20 seconds, hence the editing play with slow motion and probably different camera angles to make things look more dramatic). Then they somehow justified the /kg to themselves, since 573 seems too small.

  4. #4 rob
    February 8, 2010

    you are all getting it wrong. he didn’t actually pull the tires. he was actually pushing the earth in the opposite direction. its all about the reference frame! since the earth has a mass of 6 x 10^24 kg, they really ought to have taken 573 W/kg and multiplied by the earth’s mass.

    so his power output was really 3.4 x 10^27 Watts.

    hmmm…wait…that is ten times the output of the sun.

    i must have dropped a factor of pi somewhere.

  5. #5 Rhett Allain
    February 8, 2010

    @Coturnix

    I like the theory of 573 watts with the “per kg” left off. However, in the tv-version, they said both 57000 watts AND 573 watts per kg. Also, they had a graphic where they showed all the tvs this could power.

  6. #6 Bill
    February 9, 2010

    Way to make a mistake correcting someone else.

    Work = (2600N)(5.4m) = 11,600J???
    Try putting that in a calculator, it’s 11960J.

    Which then gives 2392Watts

  7. #7 Eric
    February 9, 2010

    @Rob
    Jesus, when will YOU get it right? You have to take into account whether he was pushing the earth with or against the rotation of the earth. If he pushed the earth against the rotation then holy god you need to add a couple factors of pi.

  8. #8 bratty
    February 9, 2010

    There’s a mistake in your calculations too, IMHO… You are assuming that the guy performs work against the weight of the sled (2600 N), but it’s not true.

    He performs work against the force of friction produced by the sled. This force is related to the weight of the sled through the (unknown) coefficient of friction.

  9. #9 wisher
    February 9, 2010

    As idiotic as the episode was, I realize that it accurately reflects the gullibility of most people.

    The actual work done by the athlete (aside from carrying his own bad ass) was simply 2600N*4.6m*(coefficient of friction between tires and ground). Assuming a very generous dynamic coefficient of friction of .3, the dude did around 3.6 kJ of work in 11s, an average of around 330 watts or about .44 horsepower. Lance Armstrong can sustain 1/4 HP throughout a full day race, and can accomplish 1/2 hp hill climbs for an hour or more. While exceptional, .44 hp for 11 seconds does not rank. It simply doesn’t. His static strength might be above average, but a strong man it does not make.

    Even more embarrassing was the fact that the truck they used spun out. Assuming it weighs 6000lbs, and only 2000lbs are on the rear axle, and a dynamic coefficient of friction of perhaps .15 while the rubber was burning, and with a speedometer that indicated perhaps 40 mph during the charade, the horsepower being delivered by the truck was only around 24 kW of power. The truck’s engine was probably a 220hp at a minimum, and so was only able to deliver no more than 32/220=15% of the horsepower available. In addition, the friction coefficient of the concrete blocks may or may not have been comparable to the tires’ values, and the tow setup might have not been comparable.

    A ridiculous way to misinform. Just about par for the three sheets to the wind ESPN crowd.

  10. #10 wisher
    February 9, 2010

    Correction. This accurately reflects the gullibility of a *faculty* professor of *physics* at Southeastern Louisiana University. Seriously? OMG.

    We are screwed.

    Or at least our fellow citizens at Southeastern Louisiana University.

  11. #11 Rob Cockerham
    February 9, 2010

    It would be easier to calculate the power needed to pull those tires 10m up a ladder, but I’d be amazed if the total was above 300 watts, which, in this age of LCDs, powers a lot more tvs than it used to.

  12. #12 Excited State
    February 11, 2010

    Hi Rhett,

    Nice writeup. The episode I saw compared “the greatest Super Bowl-winning catches” by allegedly adding up all of the forces involved in each catch. Of course, they added the forces as scalars, and didn’t consider the same forces for each. It was ridiculous.

    Thanks for the hat-tip and the link, but I must point out that I’m not Nick. The First Excited State is currently a pseudonym, although I may change that if I find the time to start blogging regularly again. (Maybe you were thinking of Nick O’Neill of finestructure.com ?)

  13. #13 Rhett Allain
    February 12, 2010

    @Excited State – sorry for the error (and sorry Nick for the confusion).