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Acceleration of Water

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What kind of experiment is this?

Experimental procedure and explanation:

  • Put a styrene foam ball and a copper ball inside a plastic bottle filled with water.
  • Accelerate the bottle towards the left and observe the direction each ball moves at the instant the bottle starts to move.
  • The copper ball will be left behind while the styrene foam ball will move forward (in the direction of the acceleration).
  • This is caused by differences in density. The copper ball is heavier than water (higher density) and is harder to accelerate, so its movement is slower. When you compare water with styrene foam, the foam is lighter (lower density). When the bottle is moved, water behind the styrene ball flows towards the back of the ball, pushing it forward.
  • The force relationships here could also be explained by the pressure gradients (or pressure differences) inside the water (see reference).

[Keywords] acceleration motion of fluid, pressure gradient
[Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 90-97
Last update: 9.7.2013