# Friction and Scaling

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1. Nov 13, 2014

### Dave Stroud

I am looking for some advice. I am not a physicist but I have a problem that I am unsure of how to solve.

This is the scenario; I have an object that is ploughing through mud on the sea floor at a constant speed of 0.02 m per second. Prior to the contact with the sea mud it was freely moving at a constant speed in the water. What I would like to do is calculate this speed when it was freely moving in the water. Essentially I would like to create some sort of scaling figure that uses the friction with the mud and the speed it is travelling through it to estimate a constant speed prior to ploughing.

This just has to be a rough sort of estimate, nothing precise. The shape of the ploughing object is a parallelepiped. Please excuse my lack of mechanics knowledge but I am just looking to see if this is actually possible. Any help would be greatly appreciated.

Many thanks
Dave

2. Nov 13, 2014

### CWatters

I doubt I can help with a complete solution but...

Something must be pushing/pulling the object through the mud? Is that the same in both cases? For example if the object was being towed and the tension in the tow rope was say 1000N then the power needed to tow it through the mud is 1000 * 0.02 = 20 Watts. Presumably the towing device will find it easier to tow the object in water but how fast it goes depends on how the towing device works. eg would the towing device try to maintain the same velocity? or would it allow the object to speed up until the power used is the same? or ??

3. Nov 13, 2014

### Dave Stroud

Thanks for getting back to me. It is basically the water current that is moving the object. This object is moving with the current before some change in the sea floor means it is shallower and the object begins to then plough the mud. Current speed remains constant throughout but once it begins to plough the mud the friction slows the object down. I know the speed of the object as it moved through the mud (0.02) so I was wondering if it would be possible to use some sort of friction calculation that would take account of this with the speed and thus allow a rough estimate of the current to be calculated? I hope that makes sense?

4. Nov 13, 2014

### CWatters

Unfortunately I'm pretty sure it's not possible.

If it moves at a constant 0.02m/s in the mud it must still be influenced by the current otherwise it would slow down. However you have no way of knowing how fast the water is going. eg You can't tell the difference between...

a) The mud is causing very little drag and the water is going at say 0.04 M/S
b) The mud is causing a lot of drag and the water is going at 0.4 M/S.

5. Nov 13, 2014

### Dave Stroud

Ah that makes a lot of sense. I am assuming that the energy loss of the object is a function of the shear characteristics and most likely the shear strength of the mud. By calculating the shear strength of the mud do you think it would be possible to calculate how much energy was lost and use that to inform about the speed prior to ploughing?

6. Nov 14, 2014

### CWatters

If it's going at constant velocity (eg not accelerating) the drag force due to the mud and the force due to the flow of water must be equal magnitude but opposite directions (eg they sum to zero). So if you can somehow calculate the drag force due to the mud you can work out the equal force on the object due to the flow of water.

Then if you have info on the drag properties of the object in water you could in principle take a stab at calculating the velocity of the water flowing past the object.

I'm afraid hydrodynamics really isn't my field. I'm an electronics engineer. I don't know how (or even if it's possible) to calculate drag forces from shear forces.

7. Nov 26, 2014

### Dave Stroud

Many thanks for your advice. Some investigations into the geotechnics of the mud has shown that the friction when compared to the size of the object would be minimal.