# Physics speed mph question

1. Dec 12, 2004

I thought the answer was 90mph, but my friend says it's wrong. I just can't figure out any other answer.

----
You drive to the store which is a 2 mile trip. In the first mile you average 30 mph, how fast must you drive the second mile in order to average 60mph for the entire 2 mile trip?
----

thnx
Jennifer

2. Dec 12, 2004

### Zurtex

I would have thought it would have been an average 90mph in the 2nd mile as well

3. Dec 12, 2004

Nobody?

4. Dec 12, 2004

### master_coda

If your average speed for the trip were 60 mph, it would take you 2 minutes to complete the entire trip. Thus you have to drive fast enough on the second leg to complete the trip in 2 minutes.

However if you only drive at 30 mph for the first mile then it you will already have taken 2 minutes; no matter how fast you drive for the second mile, you'll take longer than 2 minutes so you can't actually average 60 mph for the whole trip.

Note that this is because "average speed = distance travelled / travel time" and not the average of all the speeds over each segment.

5. Dec 12, 2004