28 Apr, 2012
In water and air
PRESENTATION: A ping-pong ball floats attached to a string in a vessel containing water. When the vessel is accelerated the ball advances in the direction of the acceleration.
- A Simple Apparatus for Demonstrating Fluid Forces and Newton’s Third Law, Pirooz Mohazzabi and Mark C. James, Phys. Teach. 50, 537 (2012)
INTRODUCTION: The behaviour of an object immersed in a fluid depends on many factors, as for example:
- The dynamic state of the fluid.
- The relative density of the fluid and the object.
- The nature of the fluid (Newtonian or not), etc.
OBJECTIVE: To study the behaviour of an object immersed in a fluid inside an accelerated vessel.
MATERIALS: a transparent vessel, a ping-pong ball, string, adhesive tape, a Newtonian fluid denser that air (water).
SETUP: The string, whose distance must be smaller than the vessel height, is attached to the inside bottom of the vessel. The other end of the string is attached to the ball. For both experiences the vessel is displaced by a horizontal force, so it suffers certain acceleration. When the vessel contains air, the ball moves in the opposite direction than the vessel. When it contains water, the displacement is in the same direction.
EXPLANATION: The equations describing the experiences are:
[FHorizontal on the ball = – (dObject – dFluid) · aVessel · VFluid]
[FHorizontal on the ball = mObject · a Object]
In the first experience the vessel contains air and the ball oscillates freely. When the force acts on the vessel this moves with acceleration (aVessel). As the object density is larger than that of the fluid the horizontal force acting on the ball (aObject ) is contrary to that acting on the vessel, so its acceleration is in the opposite sense to that of the vessel.
In the second case, the density of the fluid (water) is higher than that of the ball, so both accelerations go in the same direction. This is verified only if the fluid used is Newtonian (as it happens with water or alcohol).
If the fluid were non-Newtonian the viscosity, would depend on the pressure, and the behaviour of the fluid would approach that of a solid body. Some examples of non-Newtonian fluids are blood or honey.
CONCEPTS: Fluid mechanics, Newtonian fluid, density ratio.
- K.U. Ingard, W.L. Kraushaar, Introducción al estudio de la mecánica, materia y ondas, Editorial Reverté, 1972.
- P.A. Tipler, Física, Editorial Reverté, 1989.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2011-2012: Roberto Agromayor, Antonio Alonso, Rosalía Alvarellos, César Álvarez
LINK pdf STUDENTS (in Spanish):