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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