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# Tutor profile: Katie H.

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Katie H.
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## Questions

### Subject:Physics (Fluid Mechanics)

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Question:

What does the Reynolds number represent?

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Katie H.

This number is defined as the ratio between the intertial and viscous forces in a fluid. The formla is Reynold number, Re = (v*L)/nu where v is a typical velocity scale in the fluid, L a length scale and nu a measure of the viscosity of the fluid. A high Reynolds number means high inertial forces and so turbulent flow is likely. Conversely at low Reynolds number, viscous forces dominate and flows tend to be laminar.

### Subject:Astrophysics

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Question:

What is a black hole?

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Katie H.

A black hole is a region of space where gravity is so strong that not even the massless components of light (photons) can escape! It consists of a 'singularity' at the centre and an 'event horizon' which is defined as a sphere within which no photon is escaping.

### Subject:Physics

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Question:

Estimate how many large textbooks you could pile on a roof before the roof collapsed.

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Katie H.

There are several things you have to consider when solving this. A good place to start is to work out the force each book will exert on the floor. You might estimate that each book is around 0.5kg and you must times this by the graviational acceleration which is approximately 10ms^2 to give a force of 5N. However this force will be projected on a limitted area. So we should consider the force per unit area, or pressure, which assuming a book area of about 0.5m^2 gives a pressure of [5*(no.books)]/0.5=10*(no.books)Nm^-2. If you look up the typical design load of a roof you will find it is around 200Nm^-2. This means you could build your pile 20 books high!

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