Homework Help: Physic Help

Posted by Megan Lee on Monday, September 3, 2012 at 3:10pm.

In heavy rush-hour traffic in Atlanta, you drive in a straight line at 21 mph for 5 miles. Then you have to stop for 10 minutes and finally drive slowly by a car wreck that is 2 miles away at a steady 15 mph. After the wreck, the road is clear, and you can drive the last 12 miles of your commute at 70 mph. What was the average speed of your commute home in mph?

I tried taking the average of the time but it was not the right answer.
Thanks

Physic Help - Elena, Monday, September 3, 2012 at 3:35pm
t1=s1/v1 = 5/21=0.238 h
t2=10 min =0.167 h
t3=s3/v3=2/15=1.333 h
t4=s4/v4= 12/70=0.171 h
Total time t=t1+t2+t3+t4 =
= 0.238+0.167+1.333+0.171 =1.909 h.
Total distance s=s1+s3+s4=
=5+2+12=19 miles
Average speed is
v ave =total distance/time taken=
=s/t= 19/1.909=9.953 mph.

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