Average velocity from multiple displacement/velocities

1. Nov 27, 2009

BogMonkey

Heres the problem:
"A car travels at a constant speed of 60kph for 30km, 40kph for another 30km and 50kph for the final 30km. What is the average speed of the car."

I know that in this case I can just add the velocities and divide by 3 since the displacements are equal and get 50kph as the average speed but when I tried to do it the long way I didn't get the same answer. First I calculated the total time elapsed like this d1/v1 + d2/v2 + d3/v3 and got t = 1.85h. Then to get the average speed I divided the total displacement by time (90/1.85) and got 48.65kph.

Theres a pattern there I've tried this with various problems and I always a little less than the right answer. Whats going on here?

2. Nov 27, 2009

Staff: Mentor

Although it might seem right, it's not. You really need to find total displacement over time.

Now if the time intervals were equal, then it would work.