# Homework Help: Inability to calculate kinetic energy if I use SI units

1. Jun 24, 2011

### ErikMBrewer

1. The problem statement, all variables and given/known data

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.
2. Relevant 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

3. 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.

2. Jun 24, 2011

### Staff: Mentor

If you use English units, your answer will not be in Joules. What's the English unit of energy?

3. Jun 24, 2011

### ErikMBrewer

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: Jun 24, 2011
4. Jun 24, 2011

### SteamKing

Staff Emeritus
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

5. Jun 25, 2011

### Staff: Mentor

BTU?

(which gets automatically decapitalized when posted without a comment :grumpy:)

6. Jun 25, 2011

### SteamKing

Staff Emeritus
BTUs are generally used to express the thermal equivalent or work, or energy.
1 BTU = 778 ft-lbf = 1055 joules.

7. Jun 25, 2011

### Staff: Mentor

In the OP's problem, the calculated energy using English units will be in ft-lbs, not J.