## Posts filed under ‘Week 08’

### 09/11/08 [56]: first class honours

% swum: |

% days: |

totals: |
56 days, 988 lengths, 428 lengths to go, £446.74 sponsorship |

56 days, or 8 weeks, gone now – which is exactly two-thirds of the way through the allotted time for this challenge – and after 30 outdoor lengths (equaivalent to **24 lengths **of the indoor pool) before lunch today I am now 70% of the way “across”. In my line of business (Higher Education) 70% equates to the borderline between awarding a student an Upper Second or a First Class degree, so today I am going to award myself First Class Honours but remind myself that there is still a fair way to go and plenty of scope to do even better than I have already done. The timswim worm continues to creep upwards and I am particularly looking forward to crossing the 1000 length mark, sometime around 07.40 tomorrow morning…

### 08/11/08 [55]: big thank you

No swimming today or yesterday but I’d just like to put on record a big thank you to all the University of Plymouth students who contributed towards my sponsorship fund by making a donation at the end of one of my lectures yesterday. Those on the second year Coastal Oceanography module (EOE2305) donated a total of £23.33 and those on the first year Physical Oceanography modules (EOE1302, XEOE1302 and EOE1306) raised a total of £45.41 giving a grand total of **£68.74**. Many thanks to you all – my overall sponsorship total is now up to just under £445…

### 06/11/08 [53]: buoyancy + hot air

% swum: |

% days: |

totals: |
52 days, 964 lengths, 452 lengths to go, £366.00 sponsorship |

**40 lengths** this morning – another good contribution to the total which puts me over the target for the end of this week (with 3 more days to go) by 20 lengths. I thought I would write today about buoyancy, partly because it is the force that keeps me afloat and partly because I will be teaching the basic physics of buoyancy to over 150 first year degree students tomorrow morning so now seems like a good time.

When a body (for example, me) is submerged in a fluid it experience forces on its surfaces (e.g. my skin) which result from the weight of fluid directly above each point on the surface. This force *always* acts at right angles to the surfaces. We usually think of this force as a force per unit area of surface and call it the **pressure**. In a stationary fluid it is equal to the depth of the surface multiplied by the acceleration due to gravity (‘g’) multiplied by the density of the fluid (this relationship is called the **hydrostatic pressure equation**). A submerged body experiences a pressure force on all of its surfaces, but because the local pressure depends directly on the depth, the pressure forces on the lower part of the body are bigger than the pressure forces on the upper part and because the pressure always acts at right angles to the surface, the (bigger) pressure forces on the lower part of the body (e.g. my back if I am lying face-up in the water) act upwards and the (smaller) pressure forces on the upper part of the body (i.e. my front) act downards. This means that overall there is a net upward force due to the difference in pressure between my upper and lower surfaces. This upward force is called the **Upthrust** or **Buoyancy Force** and it is the force which, if you are lucky, allows you to float in a swimming pool. You can float if the magnitude of the upthrust (which acts upwards) exceeds the magnitude of your weight (which acts downwards). The basic physics of upthrust is described by **Archimedes’s Principle**, which states that “*a body which is submerged, or partially submerged, in a fluid experiences an upthrust that is equal to the weight of fluid displaced by the body*“. The first year students in my lecture tomorrow will have the pleasure of hearing me explain why this is the case…

So, let’s have a think about what this tells me. This morning I stood on our bathroom scales and they told me that I had a mass of 75kg. To find out my weight I have to multiply my mass by the acceleration due to gravity ‘g’ which, on Earth, is ~9.81 metres per second per second. To keep things simple I’ll use a value of 10. This means that I weigh 750 Newtons (the unit of force is the Newton – next time a doctor or a nurse or whoever asks you how much you weigh, multiply your mass by 10 and give them the answer in Newtons – you can sit back and enjoy watching their confusion safe in the knowledge that you are correct…). Because I can float (not easily, but I can), I know that when I float I must experience an upthrust that is at least equal to my weight, so this upthrust must be around 750 Newtons. Archimedes Principle tells me that the upthrust is equal to the weight of water that I displace, which depends on the volume of water that I displace. I estimate that when I float only about 5% of my volume is not submerged (95% is below the water surface). So if I call my volume V, then I displace a volume of water that is ~0.95V. This volume of water has a mass of 0.95V multiplied by the density of the water which is roughly 1000 kg per cubic metre, which is 950V kg and has a weight of 950V multiplied by ‘g’ or approximately 9500V Newtons. This weight of displaced fluid, which is the magnitude of the upthrust I experience, must equal my weight (750 Newtons) and so 9500V = 750 which means that my volume V = 0.08 cubic metres (or 80 litres). This also suggests that my average density (which is my mass divided by my volume) is around 75/0.08 = 940 kg per cubic metre. So, my average density is less than that of water which means that as well as being made up of water I am made up of other stuff that has a different density. Some of this other stuff will be more dense than water, but a good part of the other stuff must be less dense than water. I’d suggest that quite a lot of this must be air, and judging by how much I have written in this entry, it’s probably mostly hot air..

### 04/11/08 [51]: under five hundred

% swum: |

% days: |

totals: |
51 days, 924 lengths, 492 lengths to go, £366.00 sponsorship |

Had a bit of a bonus today, an after work family swim during which I completed **30 lengths**, considerably more than I had set out to swim. Rather satisfyingly, this session has taken my total over 900 lengths and the number of lengths remaining to under 500, an amount which almost, now that I am this far in, seems like nothing. In practice I need to do 15 or so more swims and I’ll be “en France”. One troublesome aspect at the moment is that my legs are feeling really very tired (if it is possible for a leg to feel anything…) and, in particular, my left leg, which is significantly weaker than my right after I broke my knee-cap a few years ago, is feeling very sore and uncoordinated. I suspect that now I am swimming better I am putting in more effort into my leg kick and this is working my legs much more – the first time that my left leg has been really worked hard since I injured it. Anyway, I’m sure I will survive – it’s not that bad.

### 03/11/08 [50]: faster and faster

% swum: |

% days: |

totals: |
50 days, 894 lengths, 522 lengths to go, £366.00 sponsorship |

After three non-swimming days I was a little concerned ahead of my swim this morning as to whether it might be a bit of a struggle again. Fortunately it wasn’t and despite being a little later out of the house than I should have been I put in **30 lengths **in comfortable fashion. When I started off with this challenge I was typically able to swim 32 lengths in about 45 minutes (including breaks to get my breath back). This represents an average speed of about 0.3 metres/second. Seven weeks on I find that I am completing about one length per minute throughout my swimming sessions, which gives an average speed a little over 0.4 metres/second. Overall then, my average swiming speed during a session has increased by about 40% which is a pretty amazing increase really.

Obviously I have some way to go to catch up with the fastest swimmers in the world. The men’s 50m world record for breaststroke is 58.91s (which is a speed of 0.85m/s) and for 100m it is 2:07.51 (or 0.78m/s). Both of these records are held by the Japanese swimmer Kosuke Kitajima. So, Kitajima is currently swimming about twice as fast as my average speed. But Kitajima had better watch out. if I maintain my current rate of increase between now and the 2012 Olympic Games in London I will be swimming at 3.4m/s and will absolutely smash the opposition, completing the 50m breaststroke in only 14.6 seconds. Numbers – don’t you just love abusing them?

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