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# How fast do you need to go

last edited by 2 years, 6 months ago

3.2.1.P8

A. I need to drive from A to C via B at total distance of ΔL. The distance from A to B is ΔL1, from B to C is ΔL2. I have time Δt to get from A to C, so I figure I can make it if I average <v> = ΔL/Δt for the entire trip. Going from A to B, the traffic is heavy and I can only average a speed v1. How fast do I have to go from B to C in order to average <v> for the whole trip? Write an equation expressing v2 (the speed I need to average going from B to C) in terms of the other symbols given: v1, ΔL1L2, and Δt.

B. I need to drive from A to C via B. The distance from A to B is 25 miles, from B to C is 75 miles. I have 2 hours to get from A to C, so I figure I can make it if I average 50 mi/h for the entire trip. Going from A to B the traffic is heavy and I can only average 25 mi/h. How fast do I have to go from B to C in order to average 50 mi/h for the whole trip?

1. 58 mi/h
2. 60 mi/h
3. 75 mi/h
4. 100 mi/h
5. Something else
6. You can’t get it from the information given.

Joe Redish 9/11/16