# The Naked Scientists Forum

### Author Topic: QotW - 11.02.20 - How fast do I have to go to be lifted off the ground?  (Read 1517 times)

#### thedoc

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##### QotW - 11.02.20 - How fast do I have to go to be lifted off the ground?
« on: 13/02/2011 19:47:42 »
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

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« Last Edit: 13/02/2011 19:47:42 by _system »

#### Soul Surfer

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##### How fast do I have to go to be lifted off the ground?
« Reply #1 on: 06/12/2010 23:54:53 »
The airflow around the back of a car is complicated and I am not sure it would work like that  you can test this by tying a ribbon to a spoiler and driving along to see the way the ribbon flies.

Maybe a simpler way of expressing what you are asking is if you were to hang on to a flag pole with both hands (and maybe a safety harness to keep the health and safety people happy).  How strong would the wind have to be before you flew like a flag?

The detailed calculations are probably very difficult but there may be a way of approaching this via some known facts.

Like a windsock the angle of the dangle is dependant on the velocity of the wind so to fly out straigt the wind would have to be very strong.

I have read that the terminal velocity of a skydiver is around 120 MPH and that at that speed he is going forward about as fast as he is going along and skydivers look to be angled at around 30 degrees to the horizontal so if the wind was about 120MPH you would probably fly at around 30 degrees.

Taking another look at the problem if you are trying to walk into a strong wind 30MPH is quite difficult and you really have to lean into a 60mph wind far further than you would be stable if the wind stopped.

#### spinfun

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##### How fast do I have to go to be lifted off the ground?
« Reply #2 on: 09/12/2010 08:48:54 »
Funny you should ask this - I've been doing some wind-tunnel tests the last few days to work out a very similar question for drag on the string of a very high balloon. It's not straightforward.

A rough order-of-magnitude answer would be that the aero drag will be similar in magnitude to the weight.  Aero drag is roughly .5 * rho * v^2 * A * Cd where rho = 1.2 kg/m3 for air, A = frontal area = 0.4 m2 say for an adult, Cd is the drag coefficient = 0.5, say for a typical bluff body.  The man weighs 76kg == 760N so speed v = sqrt(760/(.5 * 1.2 * .4 * .5))  = 80 m/s == 150mph - not far from Soul Surfer's answer based on skydivers, because after all this is how you calculate terminal velocity.

But as Soul Surfer says, it's tricky to estimate what will really happen in the turbulent wake behind a car.  Some may suggest that the lift coefficient should be used, but at what angle does drag become lift?  And what is the lift coefficient for a human-body-shaped object?

Any use?

Hugh

#### chris

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##### How fast do I have to go to be lifted off the ground?
« Reply #3 on: 09/12/2010 08:56:15 »
Thanks, both of you, for these thorough answers.

Chris

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##### How fast do I have to go to be lifted off the ground?
« Reply #3 on: 09/12/2010 08:56:15 »