Friction- The Big Rub

Student learning outcomes: students will

1. explain that friction is a force that opposes motion.

2.describe what factors determine the strength of friction between 2 surfaces.

3. provide examples of how friction both helps and hinders things that we do, such as sports & playing musical instruments.

 

Friction

• Sometimes it helps, sometimes it hurts, but no matter which way you cut it, friction is all around you!

• So what is friction exactly?

Like Gravity, Friction is a Force

Force on person by box

Force on floor by box Force on box by floor

Every force has an equal & opposite partner.

Force on box by person

Friction is a Force that Opposes Motion

• Friction a force that opposes motion– Caused by rough

surfaces of all materials

– Even air can cause friction- called air resistance

– Eventually friction will cause an object to come to a stop.

Friction• No matter which direction something moves

in, friction pulls it the other way.

• Move something left, friction pulls right. Move something up, friction pulls down.

• Friction is actually a force that appears whenever two things rub against each other or come in contact with each other. Although two objects might look smooth, microscopically, they're very rough and jagged.

Friction’s Strength depends on 2 things

1. The types of surfaces involved (are the smooth, rough, textured?)

• Rough surfaces produce more friction

2. Force with which the 2 surfaces push together.

• The harder 2 things are pushed together, the MORE friction there is, like rubbing your hands really hard together.

4 Types of Friction1. Sliding resists motion when solid objects

grind (slide) over each other• puck and ice

2. Rolling resists the motion of rolling objects.• skateboards eventually roll to a stop OR a ball

rolling on the grass

3. Fluid liquids or gases resist the motion of a solid

• wind resistance (air resistance)• oil a squeaky hinge• pushes a surfer

4. Static resists the motion just as an object begins to move.

– Because of static friction, you must use extra force to start the motion of an object.

– - once object starts moving, there’s no static friction.

Sliding Friction

• Is when solid surfaces slide over each other.

Rolling Friction

• Is when an object rolls over a surface.

• The force needed to overcome rolling friction is a lot LESS than the force needed to overcome sliding friction.

Reduced Friction

Large amount of Friction

Rolling Friction examples

Fluid Friction

• Is when a solid object moves through a fluid (which remember is a liquid or a gas)

• Ex. Friction between you & the air when riding a bike.

Static Friction

Friction, Friend or Foe• Not all friction is bad, it depends on the

situation.• Friction enables us to walk. Without it, we’d slip

& slide all over the place!• Friction allows things such as cars, bikes,

skateboards to go.• At times, we want to increase friction, like

getting traction on sand or salt with our tires.• Friction CAN wear out engine parts & reduce

efficiency• Musicians both use AND reduce friction at

times, like playing the violin.

Controlling Friction

• Friction CAN be increased or decreased

• Ball bearings, wax, liquids such as oil, polish surfaces, roll objects rather than sliding them.

• Discussion Questions 1.Explain how surface type influences the amount of friction there is.

• 2.Discuss the relationship between the size and mass of an object and the amount of friction that is present.

• 3.Analyze how friction can be both a positive and negative aspect in our everyday lives. Use examples to support your statements.

• 4.Sports such as soccer involve running, stopping, jumping, and kicking. Discuss how friction helps players.

• 5.Describe a situation in which using wheels would reduce friction between a moving object and the surface over which it travels.

• 6.Hypothesize what your life would be like if there were no friction. Which actions would be more difficult? Which would be easier?