that's OK though, the 10 CUBED can just be ignored and just multiply 215.9 by 1.356 to get the proper 292.72 instead of 292720.Using "engineering" units, convert mph to fps, multiplying by 22/15: 55 mph = 80 . Divide weight (in pounds) by 32.174 to get mass in slugs. Substitute these values in formula above; you should get 215.9 × 10³ (10,000???!!!) ft lb. (215900???? formula calls for 215.9 x 1 to get 292.72 result!) Finally, convert to J, multiplying this by 1.356 J/ft lb. I get 292.72 kJ.
Hey zero,
Do you ever read replies to your posts?
major
after wading through the nonsense to get to the allegedly CORRECT formula for how many kj are needed to push 2135 lb @ 55mph, the math becomes USELESS anyways because another online conversion utility says that 300kj (rounded up from 292) = just 83w!!! there's no way that can be right.
velocity (v) and mass (M) relate to energy in the form of kinetic energy (KE) or energy associated with motion, by the formula KE = ½Mv².
Watts (W) are the units for power. We all know what time is and the units are seconds (s). Any type of energy has the units of joules (j) which are defined as watt seconds (Ws). So KE = Ws.
One Watt of power is equivalent to one Joule of energy expenditure per second. It may require 300 kJ of energy to accelerate a car to 55 mph but that would generally depend on the amount of time taken to do the acceleration. Once you're moving, maintaining a given speed is dependent on your friction and drag losses, and requires a certain rate of energy expenditure - i.e. a certain amount of power.
---------------------------------1 watt-s = 1 joule. Also previously stated, a watt is instantaneous work. Add the time factor and it becomes an amount of energy.
300,000 joules = 300,000 watt-s, or 5000 watt-minutes, or 83 watt-hours.
If you were to provide 83 watts of power over the period of an hour, you'd be using the energy equivalent of 300,000 joules.
If you used a more useful period, say the acceleration time of a car going from 0-55 mph over 10 seconds, you'd have 50,000 watts over 10 seconds. That's 50,000 watts delivered for 10 seconds equaling the energy of 300,000 joules... ~65hp for 10 seconds. Reasonable for a 2000lb car.
It's all there. People haven't given you the specific answer to your numerical problem, but they've given you the tools to figure it out.
You are correct. This is incorrect. kj cannot equal w. Energy and power are different quantities. I suggest you brush up on some basic physics fundamentals and then it will become easier to correctly apply the math.another online conversion utility says that 300kj (rounded up from 292) = just 83w!!! there's no way that can be right
I mean no offense by this, but you seem to have a serious problem with units. It's either self-imposed, because you make assumptions, or you're not careful with your reading/comprehension.Using "engineering" units, convert mph to fps, multiplying by 22/15: 55 mph = 80 . Divide weight (in pounds) by 32.174 to get mass in slugs. Substitute these values in formula above; you should get 215.9 × 10³ (10,000???!!!) ft lb. (215900???? formula calls for 215.9 x 1 to get 292.72 result!) Finally, convert to J, multiplying this by 1.356 J/ft lb. I get 292.72 kJ.
trying to get the math down but that darn apples & oranges crap as well as bogus info even within a formula is holding me back. i have this INCORRECT kinetic energy formula to start with,
that's OK though, the 10 CUBED can just be ignored and just multiply 215.9 by 1.356 to get the proper 292.72 instead of 292720.Using "engineering" units, convert mph to fps, multiplying by 22/15: 55 mph = 80 . Divide weight (in pounds) by 32.174 to get mass in slugs. Substitute these values in formula above; you should get 215.9 × 10³ (10,000???!!!) ft lb. (215900???? formula calls for 215.9 x 1 to get 292.72 result!) Finally, convert to J, multiplying this by 1.356 J/ft lb. I get 292.72 kJ.
215.9 x 1.356 = 292.7604kj actually. they didn't even multiply their own numbers correctly DESPITE the erroneous cubing.
at 1:37 pm PST when at 10:16am PST I wrote,"if no one pointed it out yet, 10 cubed is 1,000 and not 10k as 10 to the first is 10."
It's also hard to point out the difference between a Joule and Kilojoule when the answer makes the OP feel stupid. But how much simpler can the explanation get? It was not my intention to make you feel stupid. It was my intention to point out that 1000 joules = 1kJ... and countless other relationships."Let's look at what you found... 215.9 x 10³. First off, 10³ = 1000, not 10,000."
We're not being difficult. Do you read the replies to your posts?why does EVERYONE in this forum have to be so difficult?
We have posted regarding this several times. I will put it as simple as I can. MASS times VELOCITY equals NOTHING !!!!!!!!!!! Meaning that the product of mass and velocity does not result in any useful quantity in any known system of physics or units in this universe, as far as I am aware.*why on earth should i have to study physics when all i want is a simple formula (or group thereof) that converts mass & velocity into watts? that's ALL i'm looking for eg. mass x velocity x .1234 = ergs, ergs divided by 4.321 = horsepower & horsepower x 9.876 = watts. that's all i'm trying to get to, but can't find the COMPLETE formulas. if KJ can't be converted to watts, then that's the wrong formula.
Yep... It's like watching someone try to fit a square peg in the round hole.This is by far the most bizarre (yet entertaining) thread I have EVER read on ANY forum period...
Hey Major,We're not being difficult. Do you read the replies to your posts?
We have posted regarding this several times. I will put it as simple as I can. MASS times VELOCITY equals NOTHING !!!!!!!!!!! Meaning that the product of mass and velocity does not result in any useful quantity in any known system of physics or units in this universe, as far as I am aware.
Furthermore, an erg = 100 nanojoules, which is a unit of energy. ENERGY is not POWER. So you can NOT convert units of energy to units of power.
No simple factor (number) can be used to make a KJ into a watt.
Got that?
major
Good point. I stand corrected. Momentum. Not something I use much. I double checked several places before posting that and could not see where the unit of kg*m/s was listed anywhere. Also did not see N*s. Oh wellHey Major,
I'm not trying to quibble here but Mass times velocity equals momentum.
p = mv
My God. I think you have read a replythe round peg in a square hole analogy is very appropriate for this thread.
Please read the EV Information section of this site. http://www.diyelectriccar.com/forums/showthread.php?t=669now if only i could find the formulas for moving that object at 55mph in relation to planet earth i'd use the word progress.
1. The POWER requirements of your car at a particular speed is:
Power in Watts = ((Mass in kg) (9.8m/s²) (Velocity in m/s) (Rolling Resistance)) + ((0.6465) (Coefficient of Drag) (Area in m²) (Velocity^3))
The 9.8m/s is acceleration due to gravity, the 0.6465 is 0.5 times the density of air in kg/m^3. If you enter the required numbers it will give you your power consumption in watts.
If you want to get range with your design then figure out a way to put fenders on it (open wheels are very bad for air resistance), and find some narrow, smoother tires. Big knobbies are very bad for rolling resistance.my entire purpose in asking questions is to read replies, but not when they take on a condescending tone. i just cannot get along with that personality type at all.
too bad i don't know either the Cd or rolling resistance of my design. i will take a look at it and see if it's possible to ignore those 2 varables and get results.
rough draft of my "cheap, light & efficient" design
http://i188.photobucket.com/albums/z157/zeroemission/electrobug.png
if serial hybrids didn't guzzle pretty much the same amount of gas as a standard ICE car, it would be possible to turn a design like that into an under 500 pound vehicle with a very low Cd profile.
i'm not looking for Nth degree accuracy, just a ballpark estimate to design around. the best i've been able to do is imagine a space in between motorcycle conversions and typical EV conversions, moreso motorcyles as my nylon skinned tube chassis concept is closer to them in weight and drag coefficient.
if one can get better performance than a typical EV conversion building from scratch, but for the same price or less, unless you're given a donor car, it makes more sense to build an "electric skateboard" from scratch to me. i'm not a fan of imitating the look of ICE either unless one gets EV vanity plates to advertise the fact.