A rotating stone flys off tangentially when we cut the string and not radially. So how does centrifugal force "throw" the fluid outward (i.e radially) in case of centrifugal pumps? Also please explain the reason responsible for outward motion of fluid particle in case of centrifugal pumps.

  • $\begingroup$ Who says the fluid flow leaving a centrifugal pump is radially outward? $\endgroup$ Jun 11, 2017 at 16:28
  • $\begingroup$ So what are you saying? The fluid flow coming out of a centrifugal pump is not tangential? $\endgroup$ Jun 12, 2017 at 2:22
  • $\begingroup$ Its not completely tangential neither radial...I am saying that the centrifugal force pushes it radially upward(so it flows along the curved blades) and the rotational flow gives it a tangential direction...you can check the velocity triangle and see the direction of absolute velocity. $\endgroup$
    – user158324
    Jun 12, 2017 at 3:04
  • $\begingroup$ @chester maybe what you're referring to is the relative component of velocity and I am to the absolute... But what I am trying to understand is what is the reason for this relative component of velocity or what is the reason why the fluid moves in the upward direction(from eye of the impeller to exiting the impeller into the casing)... And I conclude that centrifugal force is responsible for it...Please correct me if I am wrong. $\endgroup$
    – user158324
    Jun 12, 2017 at 3:16

1 Answer 1


In circular motion, the reason the particle moves in a circle is because there is a net force acting inwards towards the centre of the circle. The object is accelerating (although its speed remains constant) as its direction of motion is changing. In the case of the string being cut, it travels in a straight line as the resultant force acting on the stone (i.e. the tension in the string) is removed, hence it follows its original direction of motion(i.e. tangential to the circle). In the case of the centrifuge, the reaction force of the walls of the container is what's responsible for the circular motion of the particle, hence the fluid is pressed against the walls of the container.

  • $\begingroup$ So in case of a centrifuge there is no force to counter the centrifugal force that's why the particles move towards the wall...because in case of circular motion of stone there is centripetal force acting on the string,so that stops the stone from going radially outward (during the circular motion and not when the string has been cut because then there would be no circular motion only inertia ) ? $\endgroup$
    – user158324
    Jun 11, 2017 at 17:06
  • $\begingroup$ The idea of "centrifugal force" is actually a misconception, as there is actually no "force" pulling the body away from the circle. The inertia of the body causes it to attempt to move in a straight line, but due to the change in direction of motion of the body it instead gets "pressed" against the container walls. Similarly, the centripetal force is actually not some "special force" that pulls the body in towards the centre of the circular motion, but is instead the net force acting on the body. $\endgroup$ Jun 11, 2017 at 17:19
  • $\begingroup$ So in the case of the stone, when the tension in the string (the "centripetal force") is removed, there is no net force acting on the body and hence it moves in a straight line. $\endgroup$ Jun 11, 2017 at 17:20

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