Friction

Friction:

Previous lesson:

Gravity
Center of Gravity

Things you'll need for this lesson:

(Optional) Materials of different roughness (jeans, silk, wood, & sandpaper of different grit). The pinewood derby kit with wheels, nails, and body.

Lesson

Sir Isaac Newton lived in a later time than Galileo, but he is credited with really isolating the theories of motion.

His first law of motion is something that is in motion will continue to do so unless affected by an outside force. In outer-space, there is no air. For the most part, there is absolutely nothing in space. So there is nothing to slow down a rocket ship.  So in outer-space, a rocket ship could blast off then turn off its engines and then travel forever in one direction at the same speed; that is, until it came close enough to something else to be affected by it, like gravity or friction.

What is friction?

Friction is a force. It is the force of resistance between two different materials sliding against each other. Let's say you want to go down a slide really fast.

You climb to the top of a giant playground slide that has four slides of exactly the same height and shape and steepness. One is made of cement and one is made of shiny steel.

Which one is going to be the fastest to slide down?

The steel one, of course.

And you have a choice of trousers that you can slide on.  You can wear your trousers made of sticky tape or jean? Which one would you wear to go faster? Jeans.

Certain materials slide easier.  They have lower "friction quotient". This friction quotient is a number that measures how easily things slide against another.

How does friction affect the Pinewood Derby car?

There are several things that can affect friction, but the biggest thing that causes friction in a Pinewood Derby is a material's roughness as it rubs against something else.

The car doesn't just run down the track, it is a bunch of different thing rubbing against each other. It is the wheels rolling on the track, the inside of the wheels rubbing against the axle pins, the wheels bouncing back and forth on the axle and with the outer wheel hub rubbing against the inside of the axle hub and the inside wheel cone rubbing against the car body, and the car pushing/rubbing against the air as it falls down the track.

So your Cub Scout is going to polish all of the surfaces that are places where things rub - the axle shaft/inside, wheel inside/outer-hub/tread/inner-hub, and the outer skin of the car (where air rubs against it).

Roughness = friction
It is easy to understand why a steel slides have less friction than a cement slide because the difference in roughness is obvious.  In the case of Pinewood Derby, every scout is given identical wood, nails, and wheels. So it is not a matter of having materials that have less friction, but how you treat the materials to have less friction.  

Here is an example, the axle right out of the box has a ring shanks (those line ridges in the middle of the shaft) and burr (that webbing of metal in between the head and the shaft. The ring shanks are purposely put on there so a nail won't come out once nail it.  Both obviously have to be removed (and I will talk about that more later). Scratches, dents, and bumps that you can see are big sources of friction.


Look at the side of the nail besides the ring shanks.  It looks smooth, right?

Yes, it is smooth, but it can be made smoother.  If you looked at the nail shaft under a microscope, you would see a bumpy surface with micro-cracks, small holes, and tiny bumps. These cracks, bumps, and holes slow your car down too.
















And the same thing happens with wood. Your block of wood looks like it has straight edges. But under a microscope, we see it has rough edges, each ridge acting like a rough surface to rub against you wheel hubs or catching the wind. These need to be sanded down.

Other forms of friction

And the car just doesn't glide down the track. It jiggles, crashes side to side, and rolls over really small unseen stuff in the track. This bumping about causes even more friction as downward energy motion is lost into other motions - side to side, up and down (with the down like pressing harder on the brakes).  The general rule is, the quieter the car, the less friction. 

What friction matters the most on a Pinewood Derby car?

In this order, this is what you should spend the most time on in terms of polishing.

Axles sides
Vibrational friction
Axle hubs
Inner wheel
Wood side
Air
Wheel surface

How do we reduce friction

We cannot eliminate all friction, but we try to remove as much friction as possible.  We will sand different parts of our car many different times, each time with a sandpaper with smaller and smaller rocks on it until the rocks are so small that we can't see them.