During the first 20 minutes of a 2.0 hr trip, a car has an average speed 39 km/h. What must the average speed in km/h of the car during the rest of the trip be if the car is to have an average speed of 74.0 km/h for the entire trip? Keep two decimal places.

Question

Physics

Rafael Monroe

25 September, 12:41

During the first 20 minutes of a 2.0 hr trip, a car has an average speed 39 km/h. What must the average speed in km/h of the car during the rest of the trip be if the car is to have an average speed of 74.0 km/h for the entire trip? Keep two decimal places.

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Answers (1)

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Zain Ochoa · Answer 1

The average speed required during the rest of the trip is 81 Km/h

Explanation:

It is 81 Km/h because we already know the average speed of 74 km/h during the 2 hours trip. Then the distance in total should be 2*74 km, then if we notice that it is undergone with two different speeds we can make the next calculation: (Speed1 * 1/3 + Speed2*5/3) = (74*2), here 1/3h corresponds to the 20 minutes at 39 km/h and the 5/3h to the 100 minutes left, both in hours, since the speed is given also in that unit of time, i. e., the speed is given in km/h. We already know Speed1=39 km/h, then Speed2*5/3=74*2-Speed1*1/3 = (148-13) km, then Speed2 = 135 Km / (5/3 h) = 135*3 km / 5 hs=81 km/h. So the average speed required afterwards to get a total average of 74km/h during the whole trip is 81 km / h.