How can I test relative friction at home? My 7 year old is anxious to test whether or not banana peels are slippery. He has been waiting for 2 years for the science fair to come back. We have the idea to test different types of banana peels (fresh, day old, squished, green, frozen etc) but we are unsure how to authentically see which ones are more slippery. Is there a way to test for friction (or lack there of) in our kitchen?
 A: This is only a supplementary answer to BowlOfRed's good one.
Maybe it is not yet suitable for a 7 year old, but a special point could be made by showing if certain states of banana peel (let's call them 'fermented'...) probably do not experience simple Coulomb friction ($F_F=\mu F_N$) but actually fluid friction/viscosity ($F_F=\gamma v$), which would explain why they are so slippery. As fluid friction is velocity dependent, it shows no stiction (fluid friction is also what makes ice slippery under pressure, e.g. with skates!).
One could try to show that a banana peel in that state allows objects to slide even with the smallest drag forces (or at the smallest angles of the inclined plane, see BowlOfReds answer). Maybe this also depends on the weight of the objects, because the starch/water mixture in the banana pulp probably liquefies under pressure (similar to what ice does). So, maybe, lighter objects still stick at rather large angles, but heavier objects slide at almost any angle.
A: The easiest think I can think of is a block or plank and an adjustable ramp.
You could set the ramp at different angles (or just set the end of the ramp at different heights), put your material on the ramp, then place the block on top of the tested material.
For elementary school, you don't need the trig and calculations of the coefficient of friction.  Instead you can just compare at what height different products cause the "standard block" to slide.  Slippery substance, block slides at a lower height/angle.
I'm thinking of something like a smallish rectangle of plywood with something heavy on top.  But just about anything that is able to be repeatably used and can sit on top of the tested product should be good.
