Solve Average Speed Problem: How to Qualify for a Race with Engine Trouble?

AI Thread Summary
A racecar driver needs to average 200 km/h for four laps but only averages 170 km/h for the first two laps due to engine trouble. To qualify, the driver must achieve an average speed of approximately 243 km/h for the last two laps. The correct approach involves using the formula for average speed, which is total distance divided by total time, rather than averaging the speeds of individual segments. After recalculating, the driver confirmed the necessary speed to qualify and acknowledged the importance of significant figures in reporting the final answer. Understanding the correct method clarified the problem for the driver.
majinknight
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Hi I am 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 don't understand how to do that since we don't have time or distance. Apparently we are to try and think of an equation that will have constant terms cancelling each other out to get the right answer but i can't figure it out. If anyone could show me how to do this question i would appreciate it greatly!
 
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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.
 
Remember, average speed is distance/time.
 
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.
 
The answer should be roughly 243Km/h...


Daniel.
 
Ok you i figured out what u meant and i got an answer of 242.8658km/h so you thanks, i understand completely now.
 
majinknight said:
Ok you i figured out what u meant and i got an answer of 242.8658km/h so you thanks, i understand completely now.

Careful with sigfigs though.
 
vsage said:
Careful with sigfigs though.

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.
 
Oh don't worry i have my significiant digits correct, like i didnt write them correct on here but on my sheet i have them correct.
 
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