Lesson 20: Friction

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

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.

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…

One of Wu-Li's

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.

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…

steel on steel
aluminum on steel
copper on steel
rubber on concrete
wood on wood
0.25 - 0.5 *
glass on glass
waxed wood on wet snow
waxed wood on dry snow
metal on metal (lubricated)
ice on ice
teflon on teflon
synovial joints in humans

* 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?

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.