Homework Help: Average Speed Problem

1. Feb 13, 2005

majinknight

Hi im having problem with this question and keep getting the wrong answer.

1. A racecar driver must average 200km/h for four laps to qualify for a race. Because of engine trouble, the car averages only 170km/h over the first two laps. What average speed must be maintained for the last two laps?

Ok what i had done is this:

200=(170 +x)/2
x=230km/h
Well my teacher said this is wrong and that we need to think of this question in terms of Vavg=(d1 +d2)/(t1+t2) to figure it out but i dont understand how to do that since we dont have time or distance. Apparently we are to try and think of an equation that will have constant terms cancelling eachother out to get the right answer but i cant figure it out. If anyone could show me how to do this question i would appreciate it greatly!

2. Feb 13, 2005

dextercioby

Consider for simplicity that one lap is 200Km long...(Though it sounds unreasonably more).Then u can compute times,distances & average velocities.
Question:What is then (in this simple numerical case) the condition to qualify...??

Daniel.

3. Feb 13, 2005

Parth Dave

Remember, average speed is distance/time.

4. Feb 13, 2005

dextercioby

To be more strict,average velocity is total distance divded by total time and NOT THE AVERAGE OF VELOCITIES ON PARTS OF THE TRAJECTORY...

Do the calculations for the # that i've offered...

Daniel.

5. Feb 13, 2005

dextercioby

The answer should be roughly 243Km/h...

Daniel.

6. Feb 13, 2005

majinknight

Ok ya i figured out what u meant and i got an answer of 242.8658km/h so ya thanks, i understand completly now.

7. Feb 13, 2005

vsage

Careful with sigfigs though.

8. Feb 13, 2005

dextercioby

He probably rounded the result and wound up with 7 sign.figures.Seriously now,if the velocities are given with 3 sign.figures,then the result must be given with the same accuracy.Therefore 243Kmhr^{-1} is the correct one.

Daniel.

9. Feb 13, 2005

majinknight

Oh dont worry i have my significiant digits correct, like i didnt write them correct on here but on my sheet i have them correct.