Force on a Hard Hat from a Falling Object 1

[bastolos]

We are going to do a Hard Hat safety demonstration next week and someone send us a outline on what to do, but it isn't making sense to me. The demonstration is dropping a 3.5 ounce washer 32 ft onto a balloon (filled with water) wearing a hard hat and one wearing a ball cap. The one with the ball cap should break. But they say to back this up with calculations.

They say: The force of a falling object can be calculated approx. by multiplying the weight of the object by the distance it falls. A 3.5oz washer falling 32ft will generate a force of 7ft-lbs of impact. Should this washer strike an unprotected head, the force of the blow would be equivalent to 560lbs; when a hard hat is worn, the force transmitted to the neck and spine is reduced to only 127lbs.

How to they come up with the 560lbs? I don't know if they mean 560lbs per a square unit of measure or what. There's no way dropping that washer that far will be equivalent to 560 lbs, it just doesn't make any sense. Can anyone make sense of this and lend a hand. I would appreciate it. This study came from PSU.

 

[IRstuff]

ft-lbs is a torque unit, so not clear if that makes much sense.

If you take m*g*h, which is the total potential energy, then the impact must absorb all the potential energy that you started with.

If you assume that the washer stops in 0.15 inches, then force = energy/distance, which winds up with 560 lbs-force (~2500 N). 0.15 inches is 3.8 mm, which presumably is consistent with the thickness of the skull?

Presumably, the hard-hat deforms and spreads/cushions the impact force. TTFN

 

[Metalguy]

"ft-lbs is a torque unit, so not clear if that makes much sense"

Ft-lbs is also a measure of energy (not power). Bullets are said to have so many ft. lbs. of energy (kinetic) in flight. Remember, energy increases as the square of velocity. A high-speed motorcycle can drive a slow car backwards in a head-on.

 

[IRstuff]

Sorry, you're right of course...

7 ft-lbf/0.15 in = 560 lbf TTFN

 

[Speedy]

.. also don't forget about Terminal Velocity. This will limit the kinetic energy of any object falling through a fluid. Don't know if it is relevant for this application with such a short fall though. Speedy

"Tell a man there are 300 billion stars in the universe and he'll believe you. Tell him a bench has wet paint on it and he'll have to touch to be sure."

 

[bastolos]

THANKS GUYS, THAT WAS THE INFORMATION I WAS LOOKING FOR.

BRAD

 

[Shortstub]

bastolos,

How did the HH safety demo go?

 

[bastolos]

Shortstub,
The demonstration went very well. It was kind of hard explaining the theory of this to people who have no engineering experience, but I think they got the drift of things. Thanks

 

[Shortstub]

Do you need the solution in SI units?

 

[chicopee]

The potential energy of the washer is 7 ft-lbs(=3.5ounce*32 ft./16 ounces per pounds). The striking speed of the washer neglecting air friction is 45.39 ft/sec.(=sqr(2*32.2 ft per sec^2* 32ft). The impact force of 560 lbs is an average value taking place during a very small amount of time interval between the initial velocity at contact and the final velocity when the washer is totally stopped by the hard hat. This time interval could be in the tenth, hundreth or thousanth of a second. This time element can only be measured in laboratories. To give you an idea, a rubber sponge may stop this washer in .05 second whereas the hard hat will do it in .005 second. This force is based on Newton law second law (dF=d(mv)/dt).
The third value of 127 lbs is the transmitted force to the neck muscle and spinal column however the hard hat suspension system must still absorb the remaining 433lbs of force. The suspension system with its nylon webb will stretch and absorb this remaing value.