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:
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:
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 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:
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.