Friction – that annoying, yet necessary force!!

• Friction is caused when two objects rub or slide against one another

• Friction is a force, and it pushes in the opposite direction of motion

• Friction is what slows objects down– A soccer ball kicked across a field– A skier slowing down at the bottom of a run

Two types

• Static Friction– This is the friction holding two objects in place

as they are static (staying in one place)

• Kinetic Friction– This is the friction created from kinetics

(motion)

Static examples

• When you prepare to sled down a hill, static friction is what keeps you and the sled at the top of the hill

• You need a push to get started – this push overcomes static friction and you move

Kinetic Examples

• Once you start sliding down the hill, you still have friction between your sled and the snow

• This friction determines how fast and far you will slide

What Changes Friction?

• Think about the sled – how can you make it go faster?– You can get a bigger push at the start, but this

typically won’t have an impact on the speed by the time you reach ½ way down the hill

– You can make the sled faster by waxing the bottom (skiers do this all the time!!)

Waxing?

• So what does waxing do to make skis and sleds faster?

• It makes the surface smoother & “slipperier”

• If the two objects can slide past each other easier, there is less friction

Other places we decrease friction

• We put motor oil into our car engines to decrease the friction of the pistons moving in the cylinders

• We seek better, slipperier, synthetic oils to decrease the friction in the engine

• We put grease into the bearing of bicycles so the tires can spin better – decreasing the friction inside the axle of the tire

Surfaces?

• If surfaces are rough, the peaks of the two surfaces slide between each other and get caught creating friction

• Can you skate on a rough pond?• As the surfaces get smoother, there are fewer peaks

to catch on each other, and there is less friction• This is why it doesn’t really matter how good your

tread is on the car tires when the road is black ice! The smooth ice gives no rough peaks for the tires to grab!!

Measuring Friction in the lab

• We can determine friction in two ways– We can pull an object and measure how hard it

is to keep it moving.– We can put the object on a board and lift an end

until the object finally starts to slide down the board

Mousetrap Car?!?

• Friction is the killer of the mousetrap car!!• We will measure the amount of static

friction – friction preventing the car from starting to move – by putting the car on a board and finding out how high we need to lift the board to get the car to roll

• The higher we have to lift, the more friction in the car

• We will measure kinetic friction by pulling the mousetrap car across the table and measuring how hard we have to pull to keep it moving.

So why is Friction Necessary?

• How well can you walk across a smooth ice skating rink?

• How well can you stop?

• What if you were to try to ride a bike and turn figure eights?

• All of these things need friction to enable us to alter a inertia – our speed and direction!!