Inability to calculate kinetic energy if I use SI units

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SUMMARY

The discussion centers on the calculation of kinetic energy using both English and SI units, highlighting discrepancies in results. A 2000 lb car traveling at 88 ft/s yields a kinetic energy of 240 kJ using English units, while a 909 kg car at 26.73 m/s calculates to 324.8 kJ in SI units. The confusion arises from the conversion between units and the appropriate application of formulas. The English unit of energy is identified as foot-pounds (ft-lbf), which differs from joules (J) used in SI units.

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ErikMBrewer
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Homework Statement



If a 2000 lb car with a velocity of 88 ft/s has by my calculations a kinetic energy of 240 kJ. But
for a 909kg car with a velocity of 26.73 m/s I calculate 324.7 kJ. Same velocity, same mass, different energy calculation.

Homework Equations



English units: Kinetic energy(Ek)=.5(m)v^2; m= w/g; 2000 lb/32.2= 62.1 slugs:
5(62.1)(88)^2=240 kJ
SI units: Ek=.5(m)v^2; kg-mass=kg; 1 kg/2.2lbs; 1 ft= .301 m: .5(909)(26.73)^2=
324.8 kJ

The Attempt at a Solution

Am I getting my metric mass wrong? I'm having a very hard time wrapping my head around this. Please help me.
 
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ErikMBrewer said:
English units: Kinetic energy(Ek)=.5(m)v^2; m= w/g; 2000 lb/32.2= 62.1 slugs:
5(62.1)(88)^2=240 kJ
If you use English units, your answer will not be in Joules. What's the English unit of energy?
 
I believe that the English units for work (force x distance) are lb x ft and horsepower so then they must also be the English units for kinetic energy. Thank you very much!
 
Last edited:
To wit:
1 joule = 0.7376 ft-lbf (foot-pound force) of work
1 joule/sec = 1 watt (power)
1 horsepower = 550 ft-lbs/sec (not work, but power) = 746 watts
 
doc al said:
what's the English unit of energy?

BTU?

(which gets automatically decapitalized when posted without a comment )
 
BTUs are generally used to express the thermal equivalent or work, or energy.
1 BTU = 778 ft-lbf = 1055 joules.
 
In the OP's problem, the calculated energy using English units will be in ft-lbs, not J.
 

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