Posted by **Mandy** on Tuesday, January 19, 2010 at 6:58pm.

A car is driven from point A to B at a constant speed of 25.0 m/s along a straight line, and then back from point B to A at a constant speed of 20.0 m/s. The values of the average speed over the entire trip is?

- Physics -
**drwls**, Tuesday, January 19, 2010 at 8:47pm
Let the distance from A to B (in meters) be L.

The total distance travelled is then 2L

The times spend going back and forth are L/20 and L/25 s.

Divide 2L by [L/20 + L/25] for the average speed. Note that the L cancels out.

Vav = 2/[1/20 + 1/25]

= 2/(9/100) = ___ m/s

Note that the average speed Vav is NOT the average of the two speeds.

Instead, 1/Vav = (1/2)[1/V1 + 1/V2]

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