I am in class 8 i have to make a model on pressure and its applcation. can anyone suggest anything?

Question

it would be better if its a working model and not to tough for my class

Answer 1

An air pressure bottle rocket.  You need a 2 quart soda bottle, a cork to fit into the top a bicycle pump with ball inflating needle and a plastic or styrofoam cup.  The cup is the platform to hold the bottle upside down.

Fill the bottle about 1/3 of the way, maybe a little more.  Force the cork in as tight as you can.  Push the pump needle through the cork, until you can see the tip come out of the cork inside of the bottle.  Attach the pump nozzle to the needle.

Now turn the bottle upside down and rest it of the cup.  You will have to cut a slit in the cup so the pump hose can get through.  Point the bottom of the bottle skyward and slightly away from you and start pumping.  Eventually the air pressure will force the bottle to break free of the cork and take off.  It is a splashy effective air powered rocket.  

The tighter you can get the cork in the better the take off,  I have gotten them to go nearly a football field length.

You can ad wings to it and shape the flight quite a bit by experimentation.

Be careful to always aim the bottle a bit away from you and other spectators.  Although it is very light at take off it can do you some damage.