## A damn car going 90km/h

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Flightless_bird
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### A damn car going 90km/h

I have no idea how to solve this. It's pretty ridiculous since it feels easy...

A car weighs 1500 kg and the friction is 8% of it's weight. The car is going 90km/h and the motors thermal efficiency is 30%. One liter of gas contains 8,7kWh. How much gas does the car need for every kilometer?

Spoiler:

EDIT: Maybe I should rephrase this. I have an idea how to solve it it's just not right. My basic idea is to just count the amount of energy the car needs and divide it by the energy that the gas gives.
Last edited by Flightless_bird on Fri Jan 15, 2010 7:51 pm UTC, edited 1 time in total.
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thoughtfully
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### Re: A damn car going 90km/h

How much work is done?

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.
-- Antoine de Saint-Exupery

GeorgeH
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### Re: A damn car going 90km/h

1) By definition, 1W = 1J/s = 1N(m/s). Use that to figure out the constant power drain (in kW) needed to move the car.

2) Now that you have the constant power drain, you can figure out the total amount of energy (or work) needed to move the car 1km from how fast your car is moving (calculate this in kWh.)

3) You know how many kWh 1L of gas gives you, and now know how many KWh you need to move the car 1km, so you can solve the problem.

As an aside, a kWh is the same type of measurement as a J (force*distance), just using different units. You could therefore have done the problem more quickly by simply converting (force of friction * 1km) to kWh, but it might not have been as illuminating.

Flightless_bird
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### Re: A damn car going 90km/h

GeorgeH wrote:1) By definition, 1W = 1J/s = 1N(m/s). Use that to figure out the constant power drain (in kW) needed to move the car.

2) Now that you have the constant power drain, you can figure out the total amount of energy (or work) needed to move the car 1km from how fast your car is moving (calculate this in kWh.)

3) You know how many kWh 1L of gas gives you, and now know how many KWh you need to move the car 1km, so you can solve the problem.

As an aside, a kWh is the same type of measurement as a J (force*distance), just using different units. You could therefore have done the problem more quickly by simply converting (force of friction * 1km) to kWh, but it might not have been as illuminating.

Trying is the first step towards failure

Zamfir
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### Re: A damn car going 90km/h

Shows us your calculations so far, that might help.

GeorgeH
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### Re: A damn car going 90km/h

1) Should get you ~30kW

2) Should get you ~0,3kWh

3) Should get you the answer

What step are you having trouble with?

Flightless_bird
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### Re: A damn car going 90km/h

Well what is the answer that you get. I mean I to calculate but I don't get that. What am I missing? What are the actual numbers that you use to get the answers at every step?
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GeorgeH
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### Re: A damn car going 90km/h

Okay, we will start with 1). What answer do you get, and how are you getting it?

Flightless_bird
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### Re: A damn car going 90km/h

1)120*9.82*1000/40=29460W=29,46kW

2)29,46*40/60*60~0,33kWh
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GeorgeH
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### Re: A damn car going 90km/h

Looks perfect so far - you are almost done. Now for 3):

Taking into account the 30% efficiency, how many kWh does 1L of gas get you? Divide that with what you got for 2), and you should have the answer. (If you are worried about units, remember that 2) was for 1km and 3) is for 1L, so you will get km/L or L/km depending on which way you divide.)

Flightless_bird
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### Re: A damn car going 90km/h

Trying is the first step towards failure

GeorgeH
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### Re: A damn car going 90km/h

You are very welcome. Just make sure you understand what you have done, if only because this sounds like a perfect test question.