A damn car going 90km/h

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

User avatar
Flightless_bird
Posts: 150
Joined: Mon Nov 09, 2009 5:45 pm UTC
Location: Somewhere I don't want to be.

A damn car going 90km/h

Postby Flightless_bird » Fri Jan 15, 2010 6:37 pm UTC

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:
The answer is 0,13L/km (8km/L)


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.
Trying is the first step towards failure

User avatar
thoughtfully
Posts: 2253
Joined: Thu Nov 01, 2007 12:25 am UTC
Location: Minneapolis, MN
Contact:

Re: A damn car going 90km/h

Postby thoughtfully » Fri Jan 15, 2010 7:26 pm UTC

How much work is done?
Image
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
Posts: 527
Joined: Mon Aug 17, 2009 6:36 am UTC

Re: A damn car going 90km/h

Postby GeorgeH » Fri Jan 15, 2010 8:03 pm UTC

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.

User avatar
Flightless_bird
Posts: 150
Joined: Mon Nov 09, 2009 5:45 pm UTC
Location: Somewhere I don't want to be.

Re: A damn car going 90km/h

Postby Flightless_bird » Fri Jan 15, 2010 8:45 pm UTC

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.


Using your method I don't get the right answer. I don't know what I'm missing. Please help me I'm going mental here
Trying is the first step towards failure

User avatar
Zamfir
I built a novelty castle, the irony was lost on some.
Posts: 7604
Joined: Wed Aug 27, 2008 2:43 pm UTC
Location: Nederland

Re: A damn car going 90km/h

Postby Zamfir » Fri Jan 15, 2010 9:27 pm UTC

Shows us your calculations so far, that might help.

GeorgeH
Posts: 527
Joined: Mon Aug 17, 2009 6:36 am UTC

Re: A damn car going 90km/h

Postby GeorgeH » Fri Jan 15, 2010 9:42 pm UTC

1) Should get you ~30kW

2) Should get you ~0,3kWh

3) Should get you the answer

What step are you having trouble with?

User avatar
Flightless_bird
Posts: 150
Joined: Mon Nov 09, 2009 5:45 pm UTC
Location: Somewhere I don't want to be.

Re: A damn car going 90km/h

Postby Flightless_bird » Fri Jan 15, 2010 9:48 pm UTC

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?
Trying is the first step towards failure

GeorgeH
Posts: 527
Joined: Mon Aug 17, 2009 6:36 am UTC

Re: A damn car going 90km/h

Postby GeorgeH » Fri Jan 15, 2010 9:55 pm UTC

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

User avatar
Flightless_bird
Posts: 150
Joined: Mon Nov 09, 2009 5:45 pm UTC
Location: Somewhere I don't want to be.

Re: A damn car going 90km/h

Postby Flightless_bird » Fri Jan 15, 2010 10:01 pm UTC

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

2)29,46*40/60*60~0,33kWh
Trying is the first step towards failure

GeorgeH
Posts: 527
Joined: Mon Aug 17, 2009 6:36 am UTC

Re: A damn car going 90km/h

Postby GeorgeH » Fri Jan 15, 2010 10:12 pm UTC

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.)

User avatar
Flightless_bird
Posts: 150
Joined: Mon Nov 09, 2009 5:45 pm UTC
Location: Somewhere I don't want to be.

Re: A damn car going 90km/h

Postby Flightless_bird » Fri Jan 15, 2010 10:14 pm UTC

Oooooh :D Thank your so much!
Trying is the first step towards failure

GeorgeH
Posts: 527
Joined: Mon Aug 17, 2009 6:36 am UTC

Re: A damn car going 90km/h

Postby GeorgeH » Fri Jan 15, 2010 10:17 pm UTC

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


Return to “Science”

Who is online

Users browsing this forum: No registered users and 10 guests