Calculating Average Velocity: Family Trip from Hither to Yon

[1MileCrash]

Homework Statement

A family travels from hither to yon one afternoon. On this day, the driver spends half the distance traveling at 77 km/hr and the remaining half at 50 km/hr. What is the average speed during the trip?

Homework Equations

The Attempt at a Solution

I assumed that the trip was 2 hours.

If each speed is half the distance of the total trip, then 77 km/hr lasted for .82467 hours, and 50 km/hr lasted 1.27 hours.

I then found a weighted average, [77(.82467) + 50(1.27)] / 2

For 63.5 km/hr, which is incorrect.

What part of my process is wrong?

 

[SteamKing]

Assuming a certain amount of time (2 hours) is your problem. You know that half the distance is covered at one rate, and the other half at a different rate.

(Hint: Use the formula distance = rate * time)

 

[1MileCrash]

I don't understand how to apply D = rt, I only know rate.

I really have no idea what to do unless I assume some total time, and I didn't think it would matter..

 

[PeterO]

I don't understand how to apply D = rt, I only know rate.

I really have no idea what to do unless I assume some total time, and I didn't think it would matter..

Obviously you can't assume a total time - it gives you the wrong answer.

Try assuming a total something else.

EDIT: If the family trip was to visit someone, and come home again what would that tell you?

 

[1MileCrash]

Thanks guys, I think I got it.