# Average velocity

1. Aug 10, 2011

### 1MileCrash

1. The problem statement, all variables and given/known data

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?

2. Relevant equations

3. 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?

2. Aug 10, 2011

### SteamKing

Staff Emeritus
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)

3. Aug 10, 2011

### 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..

Last edited: Aug 10, 2011
4. Aug 10, 2011

### PeterO

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?

5. Aug 10, 2011

### 1MileCrash

Thanks guys, I think I got it.