(Question answered, Mods please close/delete thread)

[Kallisti]

This is extremely embarassing, since normally my physics class is so easy it's mind-numbing. But at the moment, I'm so tired and burned-out that my mind is numb, and I just can't seem to make this one problem work.

How many kilometers per liter will a car go if its engine has 25% efficiency and it encounters an average retarding force of 1000 N at highway speed? Assume the energy content of gasoline is 40 MJ per liter.

And I plug in the efficiency and the retarding force, and get an answer in MJ/l. And I want km/l. I don't know how I'm supposed to get rid of the Newtons and get from MJ to km. I should know, but I don't, because I'm so tired I can barely read.

Please help me, smart people of the Playground. This is due tomorrow and I really want to understand it by then.

[Pyrian]

Haven't done physics in well over a decade, but it looks to me like you need the work=force*distance equality.

[Kallisti]

But how does that help me? I get down to an answer in MJ. How much of that answer is in the force and how much is in the distance? I've got nothing left to divide it by, and there are multiple possible factor pairs. So I don't know how much of w is f and how much is d, do I?

I'm sorry, but I'm so sleep-deprived I've got a functional IQ of about 77, so you'll need to walk me through the answer at least a little.

[Pyrian]

You shouldn't be getting down to an answer in MJ in the first place. I don't know how you got there. Where'd the N go?

Work is in MJ, being an energy term.

Force is in N.

Distance is in km.

Efficiency shouldn't change the units.

w=fd gets you straight to the right units. (Or, rather, d=w/f, which means X kilometers = Y MJ / Z N.) You'll have to look up the conversion factor, though.

[RS14]

But how does that help me? I get down to an answer in MJ. How much of that answer is in the force and how much is in the distance? I've got nothing left to divide it by, and there are multiple possible factor pairs. So I don't know how much of w is f and how much is d, do I?

I'm sorry, but I'm so sleep-deprived I've got a functional IQ of about 77, so you'll need to walk me through the answer at least a little.

F=1000N, by the original problem statement. So divide your MJ/L answer by this to get km/L

Although, actually, on looking back on your statement, "And I plug in the efficiency and the retarding force, and get an answer in MJ/l" doesn't seem right. That shouldn't depend on the force--You're getting 25% of 40MJ/L, so 10MJ/L actually goes into moving the car. Now divide that by the average force.

[Kallisti]

You shouldn't be getting down to an answer in MJ in the first place. I don't know how you got there. Where'd the N go?

Work is in MJ, being an energy term.

Force is in N.

Distance is in km.

Efficiency shouldn't change the units.

w=fd gets you straight to the right units. (Or, rather, d=w/f, which means X kilometers = Y MJ / Z N.) You'll have to look up the conversion factor, though.

...wow. I was being stupid because I was so tired.

You're right, I totally had the retarding force in the wrong place.

Thanks for your help. I just wish I was gettting more than three hours of sleep a night so I didn't need it:smallsigh:.

[ozyran]

http://www.cramster.com

Great resource, has helped me tons in physics this semester.

[Don Julio Anejo]

Well, Work is also W=FD.

Hence, work in MJ/L = work in N*m. Just do the unit conversions correctly.

[Kallisti]

My problem was that I stupidly tried to subtract the retarding force from the work instead of dividing by it, since it's an opposing force and I was tired enough to make that mistake. What I forgot was that since the car is miving at constant velocity the force it needs to exert will be equal and opposite to the retarding force. So I put it in the wrong place. And then I had no Newtons left to divide by.