Inability to calculate kinetic energy if I use SI units

In summary, the conversation discusses the calculation of kinetic energy for two different cars with the same mass and velocity but different units. The equations used are different for English units and SI units. The English unit for energy is lb x ft or horsepower, while the SI unit is joules. The English unit of energy can also be expressed as BTUs, which is equivalent to 778 ft-lbs or 1055 joules.
  • #1
ErikMBrewer
2
0

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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  • #2
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?
 
  • #3
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:
  • #4
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
doc al said:
what's the English unit of energy?

BTU?

(which gets automatically decapitalized when posted without a comment :grumpy:)
 
  • #6
BTUs are generally used to express the thermal equivalent or work, or energy.
1 BTU = 778 ft-lbf = 1055 joules.
 
  • #7
In the OP's problem, the calculated energy using English units will be in ft-lbs, not J.
 

1. What are SI units?

SI units, also known as the International System of Units, are a standardized system of measurement used in science and engineering. They are based on seven base units: meter (length), kilogram (mass), second (time), ampere (electric current), Kelvin (temperature), mole (amount of substance), and candela (luminous intensity).

2. Why is it important to use SI units when calculating kinetic energy?

SI units are important when calculating kinetic energy because they provide a consistent and precise way to measure physical quantities. This allows for accurate calculations and comparisons between different systems and experiments.

3. What is the formula for calculating kinetic energy in SI units?

The formula for kinetic energy in SI units is KE = 1/2 * m * v^2, where KE is kinetic energy in joules (J), m is mass in kilograms (kg), and v is velocity in meters per second (m/s).

4. Can kinetic energy be calculated using other unit systems?

Yes, kinetic energy can also be calculated using other unit systems such as the Imperial system or the CGS system. However, using SI units is recommended for consistent and accurate calculations.

5. How does using incorrect units affect the calculation of kinetic energy?

Using incorrect units can lead to incorrect calculations of kinetic energy. For example, using units of feet per second instead of meters per second will result in a different value for kinetic energy. It is important to always double check and convert units to ensure accurate calculations.

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