Why is it so difficult to slide a brick across a carpet? And why is it easy if you place that brick inside a shoebox? They weigh about the same. Why is it that it's easy to slip in the bathtub but not on concrete? They look just as smooth. Why don't you have a blanket made of sandpaper? Well, that one's easy to answer. It would hurt. Everything in the world slides a little differently. Why? Maybe we are looking at this through the wrong eyes. I am going to lay out two different surfaces: a piece of sandpaper and a piece of plastic. Let's shrink down to the size of fleas and see if we can get a different look.
BZEEEEOOoooooooo . . . DING! Hey, you look different . . . Oh, right! We are small now! Whoa. Look at that. Mountains! Tunnels! Caves! You might think that I brought you to an alien planet, but this is what things look like when you're up close.
Microscopic means something is so small that you can only see it through a microscope. Up close, the sandpaper has all sorts of parts sticking out. This is why it's really hard to rub it against things, even wood. I have an idea. Let's climb to the top of that--Whoa, look out!
And now for a closer look, let's use this microscope.
GcG, Public domain, via Wikimedia Commons
A piece of wood the size of a planet is crashing down on us! Fast! Hide in this cave! . . . Safe. Watch as the wood rubs up against the sandpaper. All of the mountains and bumps drag against the wood. They hold on and make it hard for the wood to move.
Rough means something has lots of bumps on it and is not smooth. You might not be able to see if something is rough or not, but you can feel it. All of these mountains and tunnels will be a scratchy feeling on the tip of your finger.
Sandpaper up close. Looks like things are going to get a little rough around here.
Thomas Wydra, Public domain, via Wikimedia Commons
Ggggkkkkrrrgggkkkkk! Watch and listen to how the wood grinds up against the sandpaper landscape.
Friction is the force that slows two things down when they rub together. That is why there is only a little bit of a slowdown when an ice skate meets ice. The ice does not drag on the skate, so the skater can glide across it. There's more slowing down when sandpaper rubs on wood, though. Someone should hire a sandpaper landscaper. Phew. The wood is gone. Maybe we should get out of here.
With these skates, I can almost fly across the ice.
We pass over the many mountains and hills and come to . . . flat land. We made it to the plastic! Sure, there are still a few bumps. But this is a lot different than the sandpaper.
Material is what something is made of. We have walked across two different kinds of material, rough and smooth. One makes a lot of friction because all of its hills rub up against anything that touches it, stopping both things from moving. This smooth, plastic does not have as many parts sticking up to rub up against things. Yes, it is more boring here. One is too dangerous. One is no fun. Ah, well. Let's grow you back again.
Plastic wrap may be smooth but it ALWAYS sticks together.
Things look a lot different up close. There are hills! There are caves! There are things to catch and stop the things that come up against it. If you look at something close enough, you can see how much friction it has. This is not so easy to see when you look with your eyes. With a swipe of your finger, though, you can get a good idea. Now every time you rub something that's rough to the touch, think of that alien place. And the other students I have shrunk that might be down there.
References: Electronics Teacher. "Causes of Friction" Electronics Teacher, 2010. <
http://www.electronicsteacher.com/succeed-in-physical-science/friction/causes-of-friction.php>
Satori Steel. "What is Friction and What Does It Have to do with Rubber?" Satori Steel, 2013. <
http://www.satoriseal.com/technical/technical_articles/what_is_friction.htm>