I have $2$ designs both moving an object through water. The standard design (design $1$) has the object with a density greater than water and so will sit on the floor of the chamber the object is contained in. In the other design (design $2$) the same shape object uses a material with a density equal to water, it is also lighter by $3$ grams ($17\,\text{g}$ as opposed to $20\,\text{g}$).

The object in design $2$ starts to move at much lower flows than design $1$.

Question - is this because it doesn't have to overcome friction as it's not 'pulled' to the floor by gravity or is it because it has less mass so easier to accelerate? or maybe both? Both objects are constrained so have minimal $50$ micron vertical clearance above and below.

  • $\begingroup$ I would assume this is to do with the mass. You have the same surface, and it should be same area to generate water-friction at. Imagine it was wind. The more mass, the harder it is to stop it. Since the object is at the same density as water, it would generate some sort of an equilibrium. if gravity pulls with the same force as the water generates lift - the vertical forces are equal to 0 and the elements pushing horisontally would much easier affect the movement. The other object would have a component of both the gravity and the other forces to move around. Perhaps floor friction too? $\endgroup$
    – denNorske
    Apr 27, 2018 at 10:52
  • $\begingroup$ Have you drawn a free body diagram showing the forces acting, and written down a horizontal force balance equation on the object? $\endgroup$ Apr 27, 2018 at 11:29
  • $\begingroup$ So if object 2 to was the same mass as object 1 but still a density greater than water so that it sank and sat on the floor it would be more difficult to move as it would have a vertical force pulling it down greater than the water generating lift. Also it would have to overcome friction as it would be in contact with the floor? $\endgroup$
    – Adrian
    May 1, 2018 at 11:56


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