Essay on Drag on Sphere

1780 Words May 12th, 2016 8 Pages
Objectives
This lab helps students to understand the drag on objects in a fluid with the same Reynolds number. We select spheres of different masses and diameters to get a wide range for Reynolds number. We also compare the values found in literature to the values calculated through experiment to determine experimental uncertainty. Keeping all in mind our main objective is to design an experiment to reduce uncertainty.

Background and Theory
When a dense particle falls through a fluid of lesser density, the gravitational force F_g causing the particle to fall is opposed by a buoyancy force F_b and a drag force F_d as shown in figure 1. If the particle falls at a constant velocity the forces can be expressed as-
F_g= F_d+F_b………(1)
We
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Then we determine the viscosity of each liquid. For this first we obtain room temperature from the thermometer in the room. Then we determine the viscosity of each liquid by interpolation from the table below

Table 1

The measurements First we selected sphere of three different materials and different diameters. We don’t want to select spheres of small diameters because it can lead to small drop times, which can increase the uncertainty. Then we measure the diameters of these spheres. Using the digital mass balance we measure the mass of these spheres. Now we make the marks on the test column at a specific distance between these columns we take measurements. For water test column we make three marks and we measure time between these three marks because the viscosity of water is very less which leads tolow coefficient of drag. Now we start the most important part of experiments that is taking the reading. First we select one liquid and drop these spheres with the help of tweezers. Using the stopwatch we measure the time a sphere takes to pass through both these marks. To recover the spheres there are two valves at the bottom of the drop tubes, which allows the sphere to be recovered. They must never be opened at the same time. Close the bottom valve and open the top valve to allow sphere to drop in the space between the valves. Then close the top valve and open the bottom valve to recover the sphere. Repeat this procedure for each

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