# Homework Help: Simple questions

1. Oct 27, 2006

### ritwik06

1. What is the effect on the density of air if humidity increases?
2. Define co-efficient of friction.

2. Oct 27, 2006

### Hootenanny

Staff Emeritus
What do you think?

3. Oct 27, 2006

### ritwik06

well, I think that:
1. with increase in humidity the density should increase.
2. co-efficient of friction is the ratio of the force of friction to normal reaction(the weight of the body in most cases)

4. Oct 28, 2006

### ritwik06

Please will anybody tell me whether I am right?

5. Oct 28, 2006

### OlderDan

Air is primarily made up of diatomic molecules of oxygen (~21%) and nitrogen (~78%), with water ranging from 0 to 7%. Now 21% + 78% is 99%, so obviously these numbers apply to dry air. When the humidity is high, water molecules are replacing the diatomic oxygen and nitrogen molecules. How does the mass of a water molecule compare to the masses of oxygen and nitrogen molecules?

#2 looks good, except I wouldn't say weight in most cases. I would perhaps say weight for horizontal surfaces.

6. Oct 29, 2006

### ritwik06

Well, of course the density water molecules is less than the density of nitrogen and oxygen molecules. But this doesnt seem to be a genuine reason. the amount of nitrogen and oxygen is fixed in air that is 99%. They arent replaced. Only the water vapour molecules take the intermolecular spaces. This should increase the density. Dont you think so?

I agree with you for the second one!! Thanx

7. Oct 29, 2006

### Hootenanny

Staff Emeritus
This is not so. Remember that a gas of a set volume always contains the same number of molecules (one mole of an ideal gas at STP occupies about 22 litres).

8. Oct 29, 2006

### ritwik06

Well what do you mean to say whether the density increases or decreases?

9. Oct 29, 2006

### Hootenanny

Staff Emeritus
Since the molecular mass of a water is less than the molecular mass of air and the number of molecules in a set volume of a gas is constant, it follows that making the air more humid (substituting water molecules for air 'molecules') would decrease that mass contained in the set volume. Hence, the density would decrease.

10. Oct 29, 2006

### ritwik06

I cant see any reason to agree to both of your posts at once. Please try to look from my point of view. You said that 1 mole of air has 99% of Nitrogen and Oxygen! 0.03% is always Carbon dioxide. Now the rest portion 0.97% consists of water vapours, dust, traces of other gases, pollen grains. etc. etc. The ratio of the last category of substances is not fixed. Now tell me if all the 0.97% of air has water vapours, ie. now the air is saturated with water. Will the density not increase???????? :grumpy:

11. Oct 29, 2006

### Hootenanny

Staff Emeritus
No, for the reasons I stated above, the density will decrease. The number of molecules in a volume of air remains constant. Therefore, if we introduce water vapor into air these molecules of water must displace air 'molecules'. Therefore, the density must decrease since the molecular mass of water is less than the molecular mass of air. I never mentioned the proportions you quote and neither me nor OlderDan stated that any of the proportions are fixed.

Last edited: Oct 29, 2006
12. Oct 29, 2006

### ritwik06

Well thanx

13. Oct 29, 2006

### Hootenanny

Staff Emeritus
Do you understand why this occurs?