You drive your car in a straight line for 15 m/s for 10 minutes then at 25 m/s for another 10 minutes. is your average speed for the entire trip 20 m/s?

You cannot average the two speeds in conditions of equal distance. You need equal time..  

Look at it this way: 15 m/s for 10 km would take 666.67 seconds.  

25 m/s for another 10 km would take 400 seconds. Total time: 1066.67 seconds. Total distance 20 km. This average speed 20000 m/1066.67 seconds = 18.75 m/s.  

If you had gone 15 m/s for the same *time* as going at 25 m/s, then you could have averaged the speeds directly.  

To get a real feel for this, imagine going at 1 m/s for 10 km, and then going at the speed of flight for the other 10 km. The first part would take 10000 seconds (nearly 3 hours) and the second almost zero time. Would your average speed be half the speed of light?

in simple terms, the principle states that the buoyancy force on an object is equal to the weight of the fluid displaced by the object, or the density of the fluid multiplied by the submerged volume times the gravitational acceleration, g. this is also known as upthrust.

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