Drag/friction on a cylinder rotating in water of different volumes

In summary, the energy required to maintain speed of the cylinders in the two tanks of water will depend on the size of the tanks and the gap between the cylinders and the tank. If the gap is small, there may be a noticeable difference in energy required. However, if the gap is larger, there will not be a significant difference in energy required between the two tanks.
  • #1
kitkatkipling
1
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This is not a homework question, but a tipsy bar question.The question being:

There are two tanks of water, one large and one small, full of water of the same density. You are using engines to rotate one cylinder in each tank, the cylinders being of equal size. Will the cylinder in the larger tank of water take more energy to maintain speed than the cylinder in the smaller tank will?
 
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  • #2
It will depend on how large your tanks are compared to the cylinders. The only thing opposing the constant angular velocity rotation of the cylinders is shear force and shear stress is proportional to the velocity gradient normal to the surface. If the gap between the cylinder is small (a few millimeters maybe) and there is a difference in the gap size between the two tanks there could be a noticeable difference. However is the gap between the cylinder and the tank is much larger (several meters) I don't think it will matter which tank the cylinder is in.
 

1. What is drag and how does it affect a rotating cylinder in water?

Drag is a force that opposes the motion of an object through a fluid, such as water. When a cylinder rotates in water, it experiences drag due to the resistance of the water molecules pushing against it. This drag force can slow down the rotation of the cylinder and reduce its overall speed.

2. How does the volume of water affect the drag on a rotating cylinder?

The volume of water surrounding a rotating cylinder can greatly affect the drag force. As the volume of water increases, the drag force also increases because there are more water molecules pushing against the cylinder. This means that a larger volume of water will result in a greater drag force on the cylinder.

3. What factors influence the amount of friction experienced by a rotating cylinder in water?

The amount of friction experienced by a rotating cylinder in water depends on a few factors. These include the speed of rotation, the surface area of the cylinder, and the viscosity (thickness) of the water. A faster rotation speed and larger surface area will result in more friction, while a higher viscosity of water will decrease the amount of friction.

4. Can the shape of the cylinder affect the drag and friction experienced in water?

Yes, the shape of the cylinder can greatly impact the drag and friction experienced in water. A streamlined shape, such as a cylinder with pointed ends, will experience less drag and friction compared to a cylinder with a flat or irregular shape. This is because a streamlined shape allows water molecules to flow more smoothly around the cylinder, reducing resistance.

5. How can the drag and friction on a rotating cylinder in water be reduced?

To reduce the drag and friction on a rotating cylinder in water, the speed of rotation can be decreased, the shape of the cylinder can be streamlined, and the viscosity of the water can be lowered. Additionally, adding a lubricant or coating to the surface of the cylinder can also help reduce friction and drag. Properly designing and shaping the cylinder can also help to minimize the effects of drag and friction in water.

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