If you slap your hands together and rub them back and forth, you’ll feel your skin start to warm. Why is that? If you answered friction, you’re right. But what is friction really? Would your hands feel warmer if they were larger? Or rougher? Or darker? What sorts of things affect the amount of friction between two surfaces?If you read a textbook from twenty years ago, you’ll find some things have changed about how we think about friction. Engineers used to only look at surface roughness… but then they took a look on the molecular level and saw a few things that made us update our way of thinking about treads.
Friction is a very complicated interaction that uses ideas from both the electromagnetic field as well as the chemistry field to fully explain exactly what it is and how it works. From ice skates to moving furniture, you encounter friction everyday. We’re going to use rubber bands, shoes, ramps, and more to experiment with these ideas on our own.

## Scientific Concepts:

• Friction is the force between two objects in contact with one another.
• Friction is dependent on the materials that are in contact with one another. How much pressure is put on the materials. Whether the materials are wet or dry. Whether they are hot or cold - in other words, it’s quite complicated!
• Static friction is the friction between two objects that are not moving.
• Kinetic friction is the friction between two objects where at least one of them is moving.
• Friction happens due to the electromagnetic forces between two objects.
• Friction is not necessarily due to the roughness of the objects but rather to chemical bonds “sticking and slipping” over one another.

## Select a Lesson

 Simple HovercraftHovercraft transport people and their stuff across ice, grass, swamp, water, and land. Also known as the Air Cushioned Vehicle (ACV), these machines use air to greatly reduce the sliding friction between the bottom of the vehicle (the skirt) and the ground. Tracking your TreadsNow let’s talk about the other ever present force on this Earth, and that’s friction. Friction is the force between one object rubbing against another object. Friction is what makes things slow down. What a Drag!There’s a couple of misconceptions that I’d like to make sure get cleared up here a bit. I don’t want to go into too much detail but I want to make sure to mention these as they may be important as you go deeper into your physics education. Stick & SlipFriction is everywhere! Imagine what the world would be like without friction! Everything you do, from catching baseballs to eating hamburgers, to putting on shoes, friction is a part of it. Exponential FrictionFind a smooth, cylindrical support column, such as those used to support open-air roofs for breezeways and outdoor hallways (check your local public school or local church). BearingsStand on a cookie sheet or cutting board which is placed on the floor (find a smooth floor with no carpet). Ask someone to gently push you across the floor. Notice how much friction they feel as they try to push you. HovercraftHovercraft transport people and their stuff across ice, grass, swamp, water, and land. Also known as the Air Cushioned Vehicle (ACV), these machines use air to greatly reduce the sliding friction between the bottom of the vehicle (the skirt) and the ground.