What Causes Friction? Even though a surface may look smooth, if you magnify the surface, you’ll see it isn’t smooth at all. Everything is made of molecules. The molecules come together to make bumps. Some surfaces have molecules that leave large bumps and some leave smaller bumps, but all surfaces have bumps. Microwelds occur when two bumpy surfaces are rubbed up against each other and they stick together.

Friction is the force that opposes the motion between two surfaces that touch. The surface of any object is rough. Even an object that feels smooth is covered with tiny hills and valleys. The contact between the hills of valleys of two surfaces causes them to stick, resulting in friction. What Causes Friction?

Different Types of Forces Friction is just one of the forces we encounter daily. Here are a few of those forces and their symbols: Gravitational Force ( Gravity ) – F g Applied Force - F app Normal Force ( acts perpendicular to surface ) – F n Frictional Force ( Friction ) – F fr Air Resistance – F air Net Force – F net

All objects on earth are under the influence of friction, the force that opposes movement.

Helpful and Harmful Friction allows us to “push off” when we walk, and allows brakes to stop our cars. Friction is sometimes called “traction”. Friction can cause wear on surfaces in an engine, or rip a hole in our jeans when we fall.

Types of Friction Friction – force that opposes the motion of objects that touch as they move past each other Contact force – “touching” 4 Main Types of Friction 1. Static Friction 2. Sliding Friction 3. Rolling Friction 4. Fluid Friction

Friction Your skateboard slows down because of friction. Friction opposes motion between two surfaces that are touching each other. The amount on two surfaces depends on two things: 1. Kind of surface 2. Amount of force(s) pressing the surfaces together

Static Friction – friction force that acts on objects that are not moving. It acts in direction opposite to applied force, and makes it harder to start things moving.

Types of Friction Sliding Friction – force that opposes the direction of motion of an object as it slides over a surface Friction

Types of Friction Rolling Friction – friction force that acts on rolling objects

Ball bearings supply rolling friction

Types of Friction Fluid Friction – force that opposes motion of an object through a fluid such as water or air Gases and liquids are fluids. Skydivers experience air resistance as they fall

Static vs. Sliding Friction Also

Types of Friction For a given set of materials: Static Friction is greater than Sliding Friction Sliding Friction is greater than Rolling Friction

To reduce the amount of friction, apply a LUBRICANT between the two surfaces. Motor oil, wax, and grease are examples. How to Reduce Friction

Other ways to reduce friction Smooth the surfaces (polish) Replace sliding friction with rolling friction (wheels, ball bearings) Pick it up!

Summary of Friction Magnitude of sliding friction depends on Surface Type (texture) and Normal Force acting on sliding object Normal force equals weight of object (which depends on object’s mass) Surface area does not have significant effect on sliding friction For a given set of materials (and not counting fluid friction), rolling friction is the least friction