Dot Physics

More Dangerous Kids

My car had a flat tire. When you get a flat tire, you might as well make something useful of it – right? As I was jacking the car up, I had a great idea. Use this for one of my “Spoof Science” videos. The only problem is that this takes a ton of work to put together a short video. So, I am just going to talk about what I could have done.

Here is a quick clip of my 4 year old lifting the car.

So, he lifted the car – it maybe be difficult to tell, but he did. HE LIFTED THE CAR! Ok, I know, he only lifted part of the car. If I were to use this in a real Spoof Science video, I would have something to more clearly show that the car actually moved up.

I am going to estimate that he lifted 15% of the weight of the car. (I just made that up – it was the back end and just 1 tire off the ground). The mass of the Mazda 5 is about 1600 kg. This gives a total weight lifted of:

i-be725fb23e2ade43c0fac29b7a47eed2-2010-07-11_la_te_xi_t_1_9.jpg

Note: at this point in the video, I would completely ignore the fact that he only lifted the car less than a centimeter. Really, data like this just isn’t important when you are trying to show something cool. Now, I can extrapolate this incredible result. If this kid can lift over 2000 Newtons, what will it be like when he is full grown? How much of his body mass can he lift? If the kid has a weight of about 160 Newtons, then he is lifting about 14 times his own weight.

What if I could lift 14 times my own weight? I would be able to lift 9800 Newtons (or over 2000 pounds). Ok, so you see how I would do this, if I were to do it.

What about the real physics? If you think about it, it is still really cool. Even though he only lifts it a small amount, he still lifts it. How does this work? This works the same way all simple machines work – by work (the physics work). Recall that work done bya force is:

i-5fbc307d6634bfc24abc854477bb9b71-2010-07-11_la_te_xi_t_1_10.jpg

Where Δr is the amount the force moved the object and θ is the angle between the force and the displacement. The key is that the work done by the kid is the same as the change in energy of the car (well, car plus Earth system). The car increases in gravitational energy by an amount:

i-012b9b8ae3e8b21a15c754db2f73306b-2010-07-11_la_te_xi_t_1_11.jpg

I hope it is obvious the mcg is the weight of the car (or a portion of the car). Relating this to the work done by the kid:

i-788d75b72b921bcd5c44275dc41271f7-2010-07-11_la_te_xi_t_1_12.jpg

If I estimate that the car moved up 0.005 meters and the kid pushed down 0.3 meters, then he had to push with a force of 40 Newtons (or about 9 pounds). Pretty cool, if you think about it.

Comments

  1. #1 rob
    July 12, 2010

    sure, i suppose the physics you presented is accurate.

    however, there is another competing theory that i would like to address:

    what you actually did was breed a giant ant then dressed it in a costume of your child. the ant then actually *did* lift 14 times it’s own body weight. which really means it is a wimpy giant ant, cuz they r supposed to be able to lift hundreds of times their own weight.

  2. #2 Rhett Allain
    July 12, 2010

    @Rob,

    Wow. You figured it out. I was trying to be all deceptive, but you saw right through it. Kudos to you.

  3. #3 RM
    July 14, 2010

    Eeek! Giant insects! I banish thee with the Square-cube law!

    (Although that might explain why said giant-ant-in-guise-of-a-child appears to be so weak.)

  4. #4 Laura Sammel
    May 8, 2011

    Great stuff. Takes me back to when I used to manage a flower shop. The people we hired to do the arranging were very artistic, and not just anyone can create a good arrangement. I like the third one the best, with the little gold bird, and blue surrounding yellow.

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