Inability to calculate kinetic energy if I use SI units

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Homework Help Overview

The discussion revolves around the calculation of kinetic energy using both English and SI units. The original poster presents a scenario involving a car's mass and velocity, leading to differing kinetic energy results based on the unit system used.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the conversion of mass and velocity between unit systems, questioning the correctness of the original poster's calculations. There is discussion about the appropriate units for energy in both English and SI systems, with some participants suggesting that the English unit of energy may not be Joules.

Discussion Status

Participants are actively engaging in clarifying the differences between English and SI units, particularly regarding energy calculations. Some guidance has been offered regarding the conversion factors and the implications of using different units, but no consensus has been reached on the original poster's calculations.

Contextual Notes

There is an ongoing examination of unit conversions and the definitions of energy in different systems, with some participants noting the potential confusion arising from using English units for kinetic energy.

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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