# Questions

## Actually there is no such thing as a bad question!Here are some readers GOOD questions which have cropped up with my answers:

Question 1. If I dropped a 10 kg piece of iron and a 10kg piece of cotton from a 20 storey building at the same time, would they hit the ground together?

Answer 1 The short answer is: the iron will hit the ground first. But that is only a very short answer which is not 100% correct! because there is not enough information in the question.can I now give you a longer answer:
Things fall to the ground because of gravity, we all know that. The force exerted by gravity on something is called the weight, and is measured in Newtons. Your 10kg pieces of cotton and iron have weights of about 100N (actually 98.1N). Now, gravity gives each of these weights an acceleration – the same acceleration. And any 2 object accelerating through the same distance with the same acceleration must take the same time, so they would hit the ground together. Galileo did a famous experiment by dropping a large and a small cannon ball off the leaning tower of Pisa and they hit the ground at the same time. One of the astronaughts who went to the Moon did the same experiment and dropped a feather and a hammer ant they landed at the same time. BUT……….and here is the big BUT…..this takes no account of what is called “air resistance” .
When an object goes through air it has a force on it OPPOSING its motion…if you have ever put your hand out of the window in a car you can feel a force on it (DON’T DO THIS or you may get your hand knocked off) or if you go very fast on a bicycle you can feel a force pushing back on you. The faster you go the more the air resistance. an object dropped is initially hardly moving, so there is hardly any air resistance…….but as it goes faster there is more air resistance……this means that the downwards force is being made a bit less by the air resistance so it accelerates less (ie it will take longer to hit the ground)
so lets apply this to your question:
a) if the whole experiment was done from a tall building on the Moon (where there is no air) or in a vacuum, then no matter what, both object would fall together,
b) in a real situation we have to take into account other variables which have not been mentioned like: what shape and size are the bundles of cotton and iron.
If they have the same surface area exposed to the air, (ie cotton a taller package but same base size)and they didn’t tumble, then they would both hit the ground at the same time.
if, however, they were both cubes, (AND THIS IS PROBABLY WHAT YOU WOULD THINK OF WHEN GIVEN YOUR QUESTION) or spheres of that material, then the iron would definitely hit the ground first because the cotton is less dense…that means that the bundle would be bigger so there would be more air resistance on the cotton than the iron.
It would be possible to get the cotton to hit the ground first by making it a tall thin bundle and making the iron into a big sheet and fashion it like a parachute!!!
sorry to get so complicated, but physics is a precise subject and you have to look for all the things which can affect the answer. Its like doing experiments, you can only change 2 things at once and must keep all the other variables the same otherwise you can not make sense of the results.

Question 2 From Phil. Have you ever tried pulling open envelope flap, which is stuck down with the self adhesive gum (not the kind you lick) ? When viewed in the dark the gum gives off a rapid bluish light, a bit like plasma ? I would very interested to know why this happens.
Answer 2you are right!!!! thought I would try it today in the darkroom and sure enough as you rip it open you get a nice little glow, though I thought it was greenish!!! well now! as you possibly know (I dont want to patronise you but have no idea of your physics level) light is only emitted from anything be it filaments of bulbs or silicon in LEDs because electrons jump from a higher to a lower level. To get them to a higher level you need to give them energy – in a light bulb the energy is supplied by electrons which are forced to move by a voltage – they bash the orbiting electrons into outer orbits, or else give energy to the atom which vibrates hard enough to make them go to an outer orbit. In a flame its the heat which makes the electrons jump out. When they jump back all of their own accord (because noting will stay in a higher energy state unless held there) they emit light. So what must be happening here is that the “sticky” is giving such an amount of friction that when it does actually pull one side against the other, the energy involved must be transferred to the electrons in the atoms so they can jump out. Ultimately of course, the energy has come from you.
BUT…..how did you find out that this happened…what kind of nut decided to wait until total darkness before opening his mail???

Question 3 from Sarah
Suppose you decide to weigh yourself inside an elevator. You stand on a bathroom scale and push the UP button.
A) Will your weight measured by the scale be more,less, or the same as your actual weight recorded under normal conditions? Explain.
B) When you get to the top floor and continue standing on the scale, what will your recorded weight be on the scale when you push the DOWN button and start going down? Why?
C) On your way down to the ground floor, the elevator slows to a stop. If you continue standing on the scale, what will your recorded weight be on the scale just before the elevator stops.? Why?
Answer 3: I have to add to your question a little bit..here goes: Its all about Newton’s Laws…..the first one says that if something is standing still or moving at a CONSTANT SPEED then all the forces on it are balanced…seems reasonable doesnt it! The second one goes on a bit further and basically says that if the forces on something are NOT balanced it will accelerate (or decellerate).
LIFT:
1.when stationary…up or down the forces are balanced….ie the spring in the weighing device has the same force as your weight. It records your weight.
2. If the lift is either going UP or DOWN at a CONSTANT SPEED everything is balanced so the weighing machine records your weight.
3. Just after you have pushed the UP button….the lift accelerates, that means that the lift gets YOU accelerating too through the weighing machine…ie the weighing machine ios pushing you harder than normal so it records GREATER than your weight.
4. Conversely at the top after the constant speed when the lift slows down…..you were already going up and the floor/weighing machine are pushing you less hard so it shows a value LESS than your weight.
5 going down (menswear, umbrellas etc)….after pushing the button the lift accelerates DOWN so it doesnt push so hard on you…it records LESS than your weight but
6 when you get to the bottom (you are already going down) the lift has to push you harder to stop you so the reading is more.

Question 4 from Reg
I have to exert some force on this hypothetical spring to hold it closed and then I wrap it up tight so that it won’t expand. If I drop it in an acid bath, and the spring is dissolved by the acid but the wrapping (being acid proof) is not dissolved, where does the energy go?

Answer 4:Hi Reg..great question (and a bit testing) but I shall do my best….not necessarily a perfect answer but its the best I can do. We are talking about the strain energy, since some of the energy which you used to compress it would have been dissipated as heat which went to the air after it was wrapped up. So still plenty of strain energy stored. That’s because some of the molecules have been forced closer to each other by YOU. since they are stuck there the energy remains stored…so…when the acid reacts with the metal (not just dissolves it) the molecules, when released from the rest of the body of the metal must move off with a little kinetic energy which represented the potential energy stored between the respective molecules..ie its just like a mini spring is released, but a molecule one. best wishes geoff…anyone with a better answer please write in

Question 5 From Peter
Since the time that atoms, protons etc have been identified by science, is it now possible to assess roughly how much space(area) would be taken up by all the atoms of say an average sized human being ?, plus what is the approx percentage of the area the atoms to the space that surrounds it ? What is the space that surrounds it ?, is it a void ?,