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Tractor Talk Discussion Forum

True or False.... Ft/Lbs.

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Just Curious

12-03-2003 14:03:04




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If you drop a 100 lb. weight 2 feet will it generate a 200 lb. force? If you drop it 3 feet will it land with a 300 lb. force?




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buickanddeere

12-06-2003 06:11:46




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 Re: True or False.... Ft/Lbs. in reply to Just Curious, 12-03-2003 14:03:04  
You are thinking linear. Gravity accelerates mass at 32 ft per second squared. Double the speed and four times the energy. Triple the speed and nine times the energy.



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Bill 52 8n

12-03-2003 21:36:20




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 Re: True or False.... Ft/Lbs. in reply to Just Curious, 12-03-2003 14:03:04  
It's potential energy would be equal to mass x gravity x height. 100lb=45.359Kg, 2ft=.6096m
45.359 x 9.81 x .609 = 271.256N or 60.98 lb*ft So i guess it would be about 122lbs of force when it hits the ground from 2ft. Seem about right? sorry for the SI conversion, but it's all they teach us in physics.

Bill



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Rod (NH)

12-03-2003 16:18:51




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 Re: True or False.... Ft/Lbs. in reply to Just Curious, 12-03-2003 14:03:04  
Hi Just Curious,

Odd name you got there :o). Here's another opinion. There is no valid term as ft/lb. What I think you mean is ft-lb which is an energy term. It represents a force acting THROUGH a distance on an object. Despite common usage, ft-lb is NOT a term that properly designates torque. Torque is a force acting AT a distance on an object not THROUGH a distance. Big difference. To acknowledge that difference, torque is properly termed as lb-ft and not ft-lb.

The samples you provide develop an energy (kinetic energy) of 200 ft-lb and 300 ft-lb respectively. Weight times height. This is not a force. The actual force that is developed when the weight lands depends entirely on the relative stiffness of the weight and whatever the weight lands on. That amount of energy has to be dissipated (usually by deformation of one or both objects) upon landing in order to bring the weight to rest. Extremely high forces can be developed when hard weights land on hard surfaces. A steel cannon ball on a concrete surface for instance. In comparison, the force developed by that same cannon ball when landing on a pile of modelling clay would be much less due to the relative ease of deformation of the clay.

The above is true if one neglects the effects of any change of gravity with height and the resistance of the air. Both are negligeable in the context of your question, I believe.

third party image Rod

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HTR

12-03-2003 17:38:58




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 Re: Re: True or False.... Ft/Lbs. in reply to Rod (NH), 12-03-2003 16:18:51  
Yea, Yea, I know. I have manuals that go back to the 20s. They all use foot pounds of torque, if you have a torque wrench that is a few years old, it says foot pounds on it. A few years ago the powers that be decided that you can't have foot pounds of torque, some genius might get it confused with a foot pound of energy. I don't think any mechanic ever got the two confused. I can see it now, it says fifty foot pounds, get the crane boys we have to raise her fifty feet to get it tight, ok hook the chain up she goes fifty feet. Ok let her down and hook to the next one. Hey, that one is still finger tight, I don't understand why we put fifty foot pounds of energy to it. I am not very smart but I never got the two confused. I do know to get one feet pound, you can put one pound on bar one feet from the pivot, and you have It. I just don't like to change things just because somebody smarter than I am says so. Remindes me of the old fellow in the VA hospital, over heard telling the doctor. My foots hurt, doctor says which foot hurts, patient, well both foots hurt, but my right feet hurts worse than my left feet.

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thurlow

12-03-2003 16:43:01




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 Re: Re: True or False.... Ft/Lbs. in reply to Rod (NH), 12-03-2003 16:18:51  
Always wondered why the guy on Motor Trend (TV Show) said lb-ft in discussing vehicles; first time I've ever seen it explained.....



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Dan in Ore

12-03-2003 15:50:41




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 Re: True or False.... Ft/Lbs. in reply to Just Curious, 12-03-2003 14:03:04  
A foot pound is the amount of energy required to raise a 1 pound weight 1 foot.

That has nothing at all to do with a falling object.

Dan



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RAB

12-03-2003 14:48:59




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 Re: True or False.... Ft/Lbs. in reply to Just Curious, 12-03-2003 14:03:04  
Forces accelerate things. Gravity (weight) is a force. This object under free-fall will gain kinetic energy as it accelerates (due to gravity), and this energy will be converted into some other form when it lands. Of course, it has no weight at all while accelerating under free-fall.

The equation for kinetic energy is: 0.5 x mass x the square of the velocity

In the S.I. system, unit of mass is the kilogram and the velocity would be measured in metres per second in the direction of motion.
Regards,RAB

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Kevin (FL)

12-03-2003 14:22:07




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 Re: True or False.... Ft/Lbs. in reply to Just Curious, 12-03-2003 14:03:04  
Answer: No. Torque is measured in foot-pounds--a constant force applied at a specific distance from rotation. A dropped item does not apply torque--only pressure equal to the item's weight divided by surface area contacted and slightly increased due to acceleration of gravity at 32 feet per second squared. Greater heights will result in higher "g-forces" which will increase the force at impact, but not in the factors of 100 pounds per foot as you mentioned.

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maybe?

12-03-2003 14:11:58




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 Re: True or False.... Ft/Lbs. in reply to Just Curious, 12-03-2003 14:03:04  
JC, Ft-lbs is a measure of Torque, not of an impact force, which depends on the terminal velocity and acceleration(usually 1 "G" or the force of gravity) of the object. Still confused? Go to www.askjeeves.com



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