Science Buddies: "Ask an Expert"

hi i did a hovercraft using a cd, a bottle lid and a balloon and it work. for my project i put: when two objects rub against each other they cause friction. Friction can slow down or stop them all together, but what happens when air is introduced bewteen the two objects? my hypothesis was: it will move easy with air between the two surfaces. I tested in floor, carpet , water, grass and concrete. water started sinking, grass did not move and concrete hardly move, this project is for 5th grade i am wondering if i should add more stuff to make it apropiate for the grade level maybe i can also introduced the amount of air in the balloon affects how long the hovercraft hovers? please help i am not even sure i have a good hypothesis

Hello,
I think that it would be a great idea to add the different surfaces and balloon sizes you tried to your data. Your hypothesis seems fine according to the question you provided, but if you add the extra variable of different surfaces you should indicate that in the question and add your prediction of which surface will work best to the hypothesis. To present your data, you should make a table with a row for each surface and a column for each balloon size. It is important to show that you tested every balloon size on every surface so your results are clear and reliable. To make the project grade-appropriate, you might want to measure the volume of the air in the balloon and record it as suggested in the "Make It Your Own" section of the project.
I wish you luck as you continue your project!

so my question should read like:when two objects rub against each other they cause friction. Friction can slow down or stop them all together, but what happens when air is introduced bewteen the two objects? and what if more or less air is introduced? my hypothesis : it will move easy with air between the two surfaces and hoovers longers with more air?
also 5 surfces and 3 balloons will that means 15 trials? and to measure the balloon i google the formula for volume of a sphere but still do not understand it do you think this is really necesary?

Hello:
While it might not be "necessary", it would be good to know the volume of air in the balloon. You can use math (the formula) to get the volume of air, or you can use displacement (like Archimedes). (You can find a lot of ideas about how to do this by searching "water displacement volume measurement.")

Your hypothesis is:

it will move easy with air between the two surfaces and hovers longers with more air.

You are only measuring the amount of time the vehicle hovers. You might want to find a way to measure the friction between the two surfaces since you talk about friction in your hypothesis.

You will want way more than 15 trials. You want around 5 per combination to make your data more believable.

Keith