physics.org | Explore | Radiation dose chart

Just came across this while looking at radiation from mobile phones on the 'physics.org' website. It shows how much radiation you can get from doing different things and loads of other random information. Thought it might be interesting - enjoy. (Click here for a bigger view.)

Elasticity - How does it physically work?

I was reading about elasticity and came across the basic principles:

• 
  
• Every material has a certain degree of elasticity
• There is a point, called the elastic limit, where the material permanently deforms and can't be returned back to it's original shape (it gains a new elastic limit as well). This is also called plastic deformation or plasticity.
• Robert Hooke described the amount stretched as directly proportional to the force it would produce and vice versa (e.g. a bigger gravitational force acting down upon the elastic material - hanging a weight off of it - would produce a bigger stretch/extension)

However, I couldn't quite understand how this actually worked on a molecular level. It took me a while to find all the information, but I eventually understood (after much searching on Google!).

So here is the explanation I collected up (from various sources):

Basically, materials can stretch due to inter-molecular forces between atoms. There are different charges in different parts of the material and these attract. If you try to squash the material like charges repel too much and so the material tends to try to stretch out again. If you stretch the material the opposite charges attract too much and the material tends to attempt to go back to its original shape. This shape is the shape in which the material is balanced: there is an equal amount of attraction and repulsion and so this is the preferred state for the material.

However, there is also the aspect of the elastic limit. This works because beyond a certain point the atoms become to far apart or close together to be pulled back into their original positions. The atoms have so much force exerted on them that they displace. However, once they have displaced they will settle in their new positions and assume a new elastic limit. This explains why elastic bands snap: the atoms move so far apart from each other that the whole material gets pulled apart!

I hope this makes things slightly clearer. If you don't understand anything just comment below!

Difference Between Centre of Mass and Centre of Gravity

Whether it makes a difference or not all depends
on size...

Now, as I mentioned in my previous post, the terms 'centre of mass' and 'centre of gravity' can be used almost interchangeably. However, this is only when we refer to relatively small objects, such as those which we find on the earth.
The difference between centre of mass and centre of gravity is fairly simple.
The centre of mass is a single point inside (or outside) any object where all of its mass effectively acts. In a regular 3D shape, such as a sphere, this would be the dead centre, however in a more complex shape it would be different.
The centre of gravity however, is where all gravity acts on an object. On earth the centre of mass is practically where all gravity acts so they can be used interchangeably. However, we know that gravitational pull depends on two things: distance between two objects and the mass of the object. The mass of the object will obviously always stay the same, however the distance from the object can change.
Consider an object as big as the moon. The distance from the earth on one side, would be much smaller than the opposite side. This means that since there is a substantial difference in pull between different parts of the 'object', the centre of gravity will be affected.
So there you go. There is always a difference between centre of mass and centre of gravity, but sometimes it is so minute that we consider it insignificant. This is due to the object being fairly small. However as the object gets larger, we begin to feel the need to distinguish between the two.
If you have any questions then, again, please feel free to leave them in the comments box below.

Unbelievable Experiment Demonstrating Centre of Gravity

What happens in the experiment:

Two forks seemingly magically balance on the rim of the cup, with the support of a mere toothpick.


Watch the video
Here is a video of an experiment I did (see it on youtube).

How it works:

This is due to the law of centre of gravity.
This law states that every objects has a point, where all gravity will effectively act. This is the midpoint of all the mass in the object. In a simple shaped structure such as a cube, this will just be in the middle, as there is an equal amount of mass in all directions of that point.
However, in the complicated, but symmetrical structure of two forks wedged together, the centre of gravity will act somewhere in the air (that is why two forks dip inwards if you hold them by the prongs)! I know this is complicated to understand, but the rest of the object does not affect the gravitational pull. If you piled up ten more forks, equally on both sides, the structure would still balance (if you had a really strong toothpick!).
Some people may use the term 'centre of mass' as well. On earth we can use these terms interchangeably, however in different gravitational fields, there is a difference.
If you are confused about the difference between centre of mass and centre of gravity, don't worry, as I will go over this in a later post, if you can't wait till then, click here.

View us on YouTube!

I have made a YouTube profile on which I will be posting a variety of different videos and experiments relating to this blog. Feel free to check it out now at http://www.youtube.com/user/Rockhoppable?feature=watch. I'll post new videos on the blog as well so you don't miss out!

In my next post, I'll tell you about center of mass, and how to do an amazing experiment using this simple law!

How to improve your life with a 'P'...

I've thought up a little system composed of ten words. These words describe and include ten things that apply to everyone in the world, and will make your life better if you improve on them. I know it is quite random, but oh well.

1) Politeness - be nice to other people!
2) Punctuality - be on time, don't be late and get yourself organised!
3) Procrastination - stop putting off tasks you should be doing. It will make life much easier.
4) Personal Hygiene - this is obviously essential...
5) Prayer - may only apply to those who are religious and actually pray, but communication with a divine being is proven to make you feel a lot more satisfied!
6) Priority - if you get these straight, then workloads and commitments seem to sort themselves out...
7) People skills - your social life is obviously important, and being able to communicate with others effectively is a must.
8) Perseverance - when you get things done there is that great sense of accomplishment.
9) Perspiration - this can refer to both exercise (keep a healthy lifestyle), and working hard in whatever you do, to do it the best you can.
10) Personality - probably the most important! Be yourself and be a good person!

Improve on these ten 'P's and I can guarantee everything will seem a lot better!

Where does the energy come from in our food?

This may look fairly complicated, but actually its simply just the composition of the elements. Here it is in word equation form.

Although this may be slightly biology related it is also chemistry related AND physics related. So we know that when we eat food, to get energy we require the substance glucose (which is a molecule in the food we eat) to be burned in oxygen to produce carbon dioxide and water as by-products. However, most importantly this reaction produces energy, namely in the form of heat. Firstly here is the chemical equation:

C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) ΔHc -2880 kJ



Glucose + Oxygen → Carbon Dioxide + Water (+ENERGY:- 2880 kJ to be precise)

kJ or kilojoules are basically a unit to measure energy in (1000 joules, as the prefix "kilo" implies, as in kilometres or kilograms). You may have also heard of calories being used, especially in terms of food. They can be directly compared.

4.1868 kilojoules = 1 Calorie


One calorie is the energy needed to raise the temperature of exactly one gram of water by one degree Celsius. So, one kilojoule is just 1/4 of that amount of energy approximately.

So, anyway, we know that when these materials are transported to the cells, this chemical reaction occurs, producing heat energy. The equations are balanced. The question is, where did the heat energy come from?

We know that it is theoretically possible to convert matter into energy (as described by Einstein's equation of E=MC^2), but obviously no matter has been converted into energy for three reasons:-

• The equations are balanced
• You'd need immense heats for this to happen which we can't even produce on earth. Imagine this type of heat in your body!
• If we actually could produce anything close to these types of heat in our bodies, it wouldn't exactly be gaining energy would it?

So just what happens to release this energy?

This question can be answered simply if you know about atoms. Atoms always try to gain the lowest energy levels possible, as energy is stored in their chemical bonds, to keep them joined together in compounds. Glucose is fairly happy as glucose and oxygen is quite content with itself too. Unfortunately for them, when they come together, they realise that there is a another state they would prefer to be in, namely in the rearranged forms of water and carbon dioxide. You can of course probably guess why...

They need less energy!

Another law states that energy cannot be destroyed (although it can be theoretically created from matter). So where has all the excess energy gone that is not needed any more?

It's released as heat energy in your body!

So that answers the question of where the energy came from, but also describes sort of what chemical energy is, and how that becomes heat energy. If you have any questions or corrections, just leave them in the comments.