Question: Hi, I have been trying to find out an answer to this question ever since I did Physics O-level in 1985! If you were an average man, say 6 feet tall and weighing about 12 stone, if you took hold of a spoiler on the back of a car, how fast would the car have to go to ensure you were actually lifted off your feet and flying behind it? I know this is very hypothetical but I want to know!!! There must be a mass/speed equation going on but I am not a scientist and my two boys want to know, too! regards, Jane Jane Brittain-Long
We posed this question to Dr Hugh Hunt from the University of Cambridge...
Hugh - Well I suppose if weíre trying to figure out how fast you need to gorace car in a car and somebody grabbed a hold at the back, and youíre try to lift them up off the ground, itís all about aerodynamic drag.
The aerodynamic drag, there's a formula for it which is the half rho V-squared multiplied by frontal area, multiplied by drag coefficient. Now rho is the density of air and that's 1.2 kg/cubic metre Ė that's easy. V is the speed of the car in meters per second. A is the frontal area of the person. Now, letís make a rough guess, itís a 10-stone person, so they're reasonably slim, a couple of meters high, say on average, 20 centimetres wide, wider in the middle, thinner at the legs. Itís as good guess as any, so that gives them a frontal area of 0.4 square meters. The drag coefficient, well, we need to make another guess here because the air behind the car is very turbulent. Itís impossible to know what the drag coefficient would be, but letís say 0.5 Ė that's a reasonable figure, I think. So if you do these sums, so the drag force is roughly equal to the weight of the person, then you end up needing to drive at about 160 miles an hour.
Well, is that reasonable? Look, itís such a complicated airflow behind the car and there are questions about when does a drag become lift? Once the personís out at an angle of 45 degrees then you might start thinking, we need to calculate the lift on the person rather than the drag on a person. We could have lots of arguments over this over a few beers if you want, but at least that's my start up calculation.