Accrington Web - mechanical aptitude

Quote:

Originally Posted by jambutty (Post 480764)

Basically it amounts to - when you remove air or a liquid from somewhere you suck it out. When you put air or liquid into a container of some sort you pump it in.

In fact the title of the article is interesting because, as it mentions, a vacuum cleaner will not work on the moon.

Here is a quote from this teaching website Workbench: You Can’t Vacuum the Moon

Your comments about liquids are also incorrect. It is the pushing force of gasses that cause the movement of liquids in all the cases that have so far been mentioned. Unless of course you are pressurising the liquid then it too will move from an area of high pressure to one of low pressure ( a hose pipe would be a simple example)

If you don't want to believe what it says that is fine by me. I am surprised by the way you let your stubbornness make you appear ignorant to the physics in question. I must have been wrong when I assumed you to be an intelligent man.

Quote:

* Any situation in which suction seems to be apparent can now be explained properly.
* The expansion of sealed snack bags or the explosion of shampoo in suitcases of airplane passengers can be explained by the changes in air pressure

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Science Concepts

Gases exert a pressure through the collective impacts of their atoms on the surface of an object or container. Because of this, pressure is always a positive value. In other words, gasses can only push on things, never pull on things. The lowest pressure achievable occurs when there are no atoms around, such as in outer space.*

Any time something seems to be pulled by suction, the actual cause must be explained by using pushing forces of gasses. Suction is better defined as a net pushing force in a particular direction due to the differences in two gas pressures.

This can best be understood by looking at several examples which can also be used in class discussion:

* Straws: When you “suck” on a straw, you actually expand the volume of gas on one side of the liquid (the side connected to your mouth). By expanding the volume of the gas inside the straw, you spread the atoms out, resulting in a lowering of pressure inside the straw. It is the outside air pressure that is also pushing on the liquid in the straw which pushes the liquid up the straw. You don’t pull the liquid into your mouth, you lower the internal pressure to allow the outside pressure to push the liquid up the straw.
* Vacuum Cleaners: Vacuum cleaners work by creating a lower pressure just inside the opening which touches the floor. By creating a low pressure inside the machine, higher air pressure in the room pushes its way into the vacuum cleaner, taking the dirt with it. Because there is no, or very little, atmosphere on the moon, you can’t create a lower or higher gas pressure inside and outside the machine, so you can’t “suck” any dirt up from the ground. Nothing happens when you turn on a vacuum cleaner on the moon. (You wouldn’t even hear it because some substance, usually air, is necessary to transmit sound waves.)

*Actually, there are some atoms in outer space, but they are so few and far between that the pressure is almost zero there.

Naive Conceptions

• Suction is a pulling force.
In almost any case where the word suction is used, it is referring to some pulling force. For example, sucking a drink up a straw, using a vacuum to suck up dirt, astronauts being sucked out into space if a hole in their ship occurs. In every one of these cases the cause of the suction is a difference in gas pressure between two regions of space. Gas under higher pressure will push its way to an area of lower pressure. A straw works because the air pressure inside your mouth is less than the air pressure outside which pushes the liquid up the straw. A vacuum cleaner creates a region of low pressure inside so that air under higher pressure will push its way into the vacuum cleaner. Air in a spaceship is under high pressure (compared to outside the ship) and pushes its way out of the hole in the ship. There is no such thing as a pulling force of suction - only pushing forces.