Units conversion for impact test - ft.lb to Joules

[gareth71]

I've got a US spec which asks for a 100ft.lb impact test done by a drop tube. I almost can't bare to ask this, but what is the height and weight for that test?

If it's really a 1 lb weight from 100 feet, or 100lb from 1 foot, that's (please check!) a 5kg weight from 2.8m or 137 Joules.

The reason I ask is a European spec asks for 20J impact which I'm confident we can meet, so I'm really hoping I've miscalculated this 100ft.lb

 

[robyengIT]

1J=1N * 1m so 100 ft*lb = 13.825 kg*m = 13.825*9.81 N*m = 135.6 N*m = 135.6 J

 

[gareth71]

Yes, that's the same as I got with a couple of my rounding errors.

I was really hoping to be wrong by a factor of 10, something to do with lb weight and lb force....

 

[rb1957]

ther are a bizillion unit convertors online.

another day in paradise, or is paradise one day closer ?

 

[zdas04]

Unit conversion doesn't seem to be the problem here. The problem seems to be order of magnitude. Why is the U.S. spec 135 J and the ISO spec 20 J? Is one of them a typo or is something really different?

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[gareth71]

I know there are a bizillion converters online, but I don't necessarily trust them.

zdas, I've got two different specs for a similar product, one says 100ft.lb and the other says 20J. By my (and RobyengIT's) calculations they are very, very different. I was looking for someone to say I had made a mistake and that 100ft.lb was around 20 Joules

 

[drawoh]

gareth71,

Your phrasing makes me nervous. You can generate 100ft.lb energy by lifting a 100lb weight 1ft, or by lifting a 1lb weight 100ft. When the weight arrives back at the ground, the 100ft.lb will be in the form of kinetic energy, less air resistance.

100ft.lb[×]4.45N/lb[×].3048m/ft=136N.m, as noted above.

--
JHG

 

[gareth71]

ok, thanks everyone. It looks like my answer was correct and we'll be non-compliant on that part of the spec. 136 Joules is an enormous impact energy to survive, especially as the other requirement is only 20J.

But thank you for your help!