A few Physics questions...please help!?

Question

1. Explain why perfume can be smeeled a few distance from the person weargin it?

2. Explain why:

-Raindrops are almost spherical?

-A needle can float on water but sinks when detergent is added?

-Hairs on your head or a paintbrush cling when wet but not when dry?

-Small insects can stand on a pond of water without sinking?

-You can dry your hands with a towel but not with a sheet of polythene?

-Paint sticks to a wall?

3. Why does a bottle of lemonade alwyas have a space between the top of the liquid and the cap?

4. Why must you never put sealed bottles or 'aerosol' spray cans on fire?

5. How can u change a fire alarm to a frost alarm?

6. Explain the features which will make a thermometer quick-acting?

Answer 1

1)  Particles from the perfume travel in air, lodging themselves in your nasal cavity, allowing you to smell it.

2) Raindrops are spherical due to gravitational pull and wind resistance. As the gravitational pull is pulling the raindrop downward, wind resistance is acting in the opposite dirrection, evening/cancelling out the forces acting on it, leaving it to be a spherical shape.

A needle is too fine to break the water's surface, until a soapy substance is added, which lowers the water surface contact constant, resulting in floating objects sinking, due to breaking the water surface.

When wet, hairs have something to bond to other hairs with. Without the moisture, they rub together without fixure. It's like two pieces of tissue paper, wet them both and they will stick.

Small insects don't break the water surface because their weight is less than the water surface constant.

A towel absorbs moisture, where as polythene does not.

Paint will stick to anything, and wood absorbs moisture, in this case paint.

3. To allow for expandature. There is room in the top of the bottle is to allow some of the CO2 to be released into the air sitting there, if not the bottle would explode.

4. Heat (fire), will cause the areosol's molecules to move at a rapid rate, causing expandature, which will lead to the can exploding. The explosion will then be fueled by the contained aerosol, which is why they always warn against burning flammable cans.

5. By changing the mechanism within.

6. Alcohol substance contained within the thermometer. Alcohol is very volatile, and will move quickly.

Answer 2

Scent is caused by airborne molecules from the source. When the molecules come into contact with the hairs in your nose, a message is sent to your brain that that particular molecule is in the air.

Raindrops are spherical because of surface tension. The surface tension in water is due to the relatively strong hydrogen bonds between water molecules. These relatively strong bonds pull the surface of the drop into the smallest area possible for the volume of water. Mathematically, the smallest surface area for a constant volume is achieved by a sphere shape hence raindrops are sherical.

A needle floats on water because of the surface tension I mentioned above. It only floats when you gently place it on and not when you drop it because the force of the needle due to gravity (mass times acceleration) would break the surface tension. Adding detergent also breaks this surface tension because the detergent molecules get in between the hydrogen bonds and therefore the needle will sink.

Your next two questions can also be explained by surface tension.

I suspect that the reason for the question after that is that when you rub yourself with a towel it picks up electrostatic charges which attract the polarized water molecules whereas polyethene doesn't.

I assume paint sticks to the wall because it's sticky.

A bottle of lemonade has air at the top in case the lemonade becomes heated. This will cause it to expand and since liquids can't be compressed very much the top will pop off will dangerous consequences. Gases however can be compressed and so will stop the above from happening.

You can't put sealed canister on a fire for the reason above. The fire will quickly heat up what ever is inside causing it to expand and blow the bottle/can open.

Fire detectors work by sensing smoke in the air. Frost doesn't emit any airborne molecules and so it would be undetectable.

I'm not aware of anything that will make a thermomenter act quickly but why would you want/need to. I don't know about you but where I live the temperature doesn't change so fast that a thermometer isn't quick enough to keep up with it.

Answer 3

I can try answering the first few questions...

1. Explain why perfume can be smelled a few distance from the person wearing it?

The molecules making up the perfume disperse throughout the room as they leave the bottle, and these molecules are what smell

2. Explain why:

-Raindrops are almost spherical?

The spherical shape is made when air pushes up on all sides of the raindrop (more pushing on the bottom, so it goes around in a spherical shape)

-A needle can float on water but sinks when detergent is added?

The needle floats on the water because it has less density than the water. Adding detergent lowers the density of the water, which causes the needle to sink (because its heavier than the water)

-Hairs on your head or a paintbrush cling when wet but not when dry?

Not sure, but I believe its because the water molecules in the strands of hair attract each other, causing the strands to stick.

-Small insects can stand on a pond of water without sinking?

They're either less dense than water or they are using suction cups of air to float on the water

3. Why does a bottle of lemonade alwyas have a space between the top of the liquid and the cap?

The space is made when the cap is being put on. It forces some of the air on top of the lemonade into the bottle.

4. Why must you never put sealed bottles or 'aerosol' spray cans on fire?

The aerosol is highly pressurised, and by heating it on a flame, causes the molecules inside to move even faster, building up pressure. Eventually the pressure will be too much and the can will explode (not to mention that the aerosol is flammable, which would make a big boom)