Lesson 20: Friction

Friction is a force that always exists between any two surfaces in contact with each other.

• There is no such thing as a perfectly frictionless environment.
• Even in deep space, bits of micrometeorites will hit a moving object, causing some friction (although it is incredibly small).

One of the problems that NASA would need to solve before sending astronauts on a long journey (like Mars) is protection from the microdust and micrometeorites in space. One of the most serious problems is that as the spacecraft travels through space at high speeds, the front will be damaged the most. Most plans have some kind of ablative shield that would cover the front of the craft. Ablative is just a what you call any material that you expect to wear away because of some form of damage, while it protects whatever is underneath.

There are two kinds of friction, based on how the two surfaces are moving relative to each other:

1. Static friction
The friction that exists between two surfaces that are not moving relative to each other.
2. Kinetic friction
The friction that exists between two surfaces that are moving relative to each other.

In any situation, the static friction is greater than the kinetic friction.

• Have you ever tried to push a really big object? Did you notice that you were pushing harder, and harder, and HARDER, until suddenly it was like glue that was holding it to the floor snapped? Then, it felt easier to push the object than it did just to get it started.
• When it was still, you were trying to overcome the static friction (bigger force).
• When it finally started to move, you were now pushing against the kinetic friction (smaller force).

Watch a video of me explaining the difference between static and kinetic friction by clicking here. Requires Windows Media Player 9 or later and a broadband connection (dial-up connection not recommended).

Nobody is exactly sure why friction acts the way it does…

• Some physicists’ theories on friction involve the idea of the minute (tiny) imperfections in the surfaces grinding against each other.
• Imagine two pieces of sandpaper rubbing past each other… they’d have a difficult time!
• Now remember that any surface, no matter how smooth it might appear to the naked eye, has tiny bumps.
• These bumps on any surface will grind past other bumps on the other surface and cause friction.
• There is also the hypothesis that there are small electrostatic attractions between atoms of the two surfaces, pulling on each other.
• Think of the electrons in one of the surfaces being attracted to the protons in the other surface.
• As you hold one object against another, billions of these attractions between the electrons and protons of the two objects cause them to stick to each other somewhat.
• This pulling on each other could also be a source of friction.
One of Wu-Li's
relatives!

Some people think that my tarantula can climb the walls of her tank because of some sort of "stickiness" on her feet. Actually, she's using friction more than anything else. A tarantula's feet are covered with thousands of microscopic hairs. When she touches her feet to the glass, these hairs jam into the micro-cracks in the surface of the glass and hook on. This is why you'll often see her tap one of her feet against the glass a few times before it takes hold.

Friction always acts in the direction opposite to the motion of the object.

• Just look at the direction the object is traveling. The direction of the force due to friction will be exactly 180° opposite.
• Friction is also proportional to the normal force, which is how we'll be able to calculate it.

Ff α FN

The actual formula for friction is…

Ff = μ FN

Ff = force due to friction (Newtons)

FN = normal force (Newtons)

μ = Greek letter “mu”, coefficient of friction between two surfaces (no units)
μs is static, μk is kinetic

Obviously, some surfaces have less friction than others…

• A rubber hockey puck against ice has less friction than a car tire on an asphalt road.
• As mentioned above, there are also two measurements of friction (static & kinetic) for any combination of surfaces.
• "Empirical" evidence means that you actually have to perform the experiment each time to get results. There is no shortcut, regular pattern, or formula that you can use to get the results.
• When we measure the coefficient of friction (μ), the smaller the number, the less the friction between the two surfaces.
• By gathering empirical evidence of different combinations of surfaces physicists have been able to come up with values to use for coefficients of friction.
• You are not expected to memorize this table…
Surfaces
μs
μk
steel on steel
0.74
0.57
aluminum on steel
0.61
0.47
copper on steel
0.53
0.36
rubber on concrete
1.0
0.8
wood on wood
0.25 - 0.5 *
0.2
glass on glass
0.94
0.4
waxed wood on wet snow
0.14
0.1
waxed wood on dry snow
-
0.04
metal on metal (lubricated)
0.15
0.06
ice on ice
0.1
0.03
teflon on teflon
0.04
0.04
synovial joints in humans
0.01
0.003

* depends on type of wood

Example 1: A 12kg piece of wood is placed on top of another piece of wood. There is 35N of static friction measured between them. Determine the coefficient of static friction between the two pieces of wood.

As long as the surface is completely horizontal, we can say FN = Fg.

First calculate FN ...

FN = Fg = mg

= (12kg) (9.81m/s2)

FN = 1.2e2 N

Then use this answer to calculate Ff ...

Ff = μs FN

μs = Ff / FN

= (35N) / (1.2e2 N)

μs = 0.30

Example 2: I have a steel box (mass of 10 kg) sitting on a steel workbench. I try to push the box out of the way…

a) Sketch a free body diagram of the box.

A free body diagram is a drawing that shows all of the forces acting on an object. You draw these forces as vector arrows, and label each one.

b) I push against the box with a force of 25 N. Determine if anything will happen.

Well, let’s calculate the MAXIMUM force due to static friction. First we figure out the normal force...

FN = Fg = mg

= (10 kg) (9.81 m/s2)

FN = 98 N

And then use that to calculate the maximum static friction. We can get the values for μs and μk from the table above...

Ff = μs FN

= 0.74 (98 N)

Ff = 73 N

So, does this mean that when I push with Fa = 25 N, the friction will push back with 73 N?

• No. That wouldn't make sense, since that would mean that if you gently pushed the box, it would actually start to accelerate back towards you!
• The force due to static friction can go up to a maximum of 73 N, but can also be less.
• It will be equal to whatever the Fa is, up to the maximum calculated here.

Ff = Fa = 25 N (they just point in opposite directions!)

FNET = Zero

With no net force acting on it, the box will not start to move.

c) Determine if I push with a force of 73 N if anything will happen.

This exactly equals the maximum static frictional force between these two surfaces.

Ff = Fa = 73 N (but in opposite directions!)

FNET = Zero

With no net force acting on it, the box will not start to move.

d) If I push with a force of 100 N , determine if anything will happen.

This applied force is greater than the static friction, so it will start to move… but remember that we will now be using kinetic friction!

 Ff = μk FN = 0.57 (98 N) Ff = 56 N FNET = FN + Ff = 100 + -56 FNET = 44 N FNET = ma a = FNET / m = 44 / 10 a = 4.4 m/s2

The box will accelerate at 4.4 m/s2.