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 box
by person
Force on floor by box
Force on person
by box
Force on box
by floor
Every force has an equal & opposite partner.
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 Friction
1. 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.
Large amount
of Friction
Reduced
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?