Physics - Friction

What is the Normal Force (F N)? The normal force is not... ...a force that's just like the other forces. ...an ordinary force. ...an action-reaction force pair of an object's weight The common common meanings meanings of the word "normal" do not apply. apply. In the case ca se of the Normal Force what is actually being be ing described is geometry. geometry.

If two lines are at right angles to each other, we call them perpendicular .

If two surfaces are at right angles to each other, we call them orthogonal .

If a surface and a line are at right angles to each other, we say the line is normal. So any line that is at a right angle to the surface is called a normal line. If there is a force coming from the surface and at a right angle to the surface then we call it a Normal line. Any force coming from the surface and acting at a right angle to the surface is called the Normal Force.

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When an object is sitting on a level surface then the normal force is always equal and opposite of the weight of the object.

On an inclined surface notice that... The angle of the incline and the angle of the weight away from the normal is the same sa me size.. The weight continues to act straight down toward the center of the earth. The normal force is equal and opposite to the perpendicular force.

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The Force of Friction Friction is a force that is created whenever two surfaces move or try to move across each other. Friction always opposes the motion or attempted at tempted motion of one surface across another surface. Friction is dependant on the texture of both surfaces. Friction is also dependant on the amount of contact force pushing the two surfaces together (norma (no rmall force).

In this sim simulation ulation you see a block sitting sitting on a level level table. You can place p lace an applied force on the object by pressing the "More "More Force" button. butto n. Each time time you you press the button butto n the applied force will will increase. increase. As you you use u se this sim simulation ulation there t here are several several things you should shou ld notice. no tice. On a level surface, the normal no rmal force force (F N) is always always equal and opposite opp osite to the th e weight weight (only (onl y on a lev l evel el surface). surface). The force of static friction ( f s) cancels out the applied force right up to and including when static friction reaches its maximum ( f smax). For applied ap plied forces greater greater than the maximum maximum force of static friction friction the block starts to slip and then the value for friction becomes kinetic kinetic friction friction ( f k ) and the box is then under und er a net force so it accelerates to the th e right. right.

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The force force of friction friction depends upon both b oth surfaces in contact and the norma no rmall force. A mathematica mathematicall relationship can be created here. In this first example, a block of wood is shown sliding sliding across a wooden table. (notice the cause of this sliding sliding is not shown) shown) Notice that the th e force of kinetic friction (f k  k)  is equal to 40% of  the normal force (FN). Another way of writing this relationship would be Now as we compare the first simulation to the next, we find that if the weight of the block is doubled, the normal force doubles, and the force of friction becomes doubled. Once again we find that the force of kinetic friction (f k  k)  is equal to 40% of the normal force (F N). Another way of writing this relationship would be Since this value is true for any weight of wood on wood, we say this value represents the coefficient of friction. friction. (It's just just the percentage of the normal force that can be friction.) The formula for the coefficient of kinetic friction is

As we compare the simulation of wood on wood to wood on asphalt, we find that the amount of  friction on the block increased for for the same amount of weight. weight. Notice that the force of  kinetic friction (f k  k)  is equal to 60% of the normal force (FN) or we could say Another coefficient can be used to describe the relationship between the maximum force of  static friction and the normal force. It's called called the coefficient of static friction and its formula looks like

The maximum

force of static friction is used because static friction has a whole range from zero newtons up to the maximum force of static friction.

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The coefficients for static static and kinetic friction are listed listed in some reference tables. tab les. The coefficient of static friction is usually a little bit bit higher than coefficient of kinetic friction for the same two surfaces. When coefficients are listed listed they must be given given for one surface surface on another anothe r surface (ie wood-on-asphalt). The higher higher the coefficient, the greate greaterr the force of friction. The table below below lists lists the coefficients for a few common surfaces surfaces used in physics. They are arranged from "sticky" to slippery slippery..

surface-on-surface hook velcro-on-fuzzy velcro

> 6 .0

>5.9

avg tire-on-dry pavement

0.9

0.8

grooved tire-on-wet pavement

0.8

0.7

glass-on-glass

0.9

0.4

metal-on-metal (dry)

0 .6

0.4

smooth tire-on-wet pavement

0 .5

0.4

metal-on-metal (lubricated)

0.1

0.05

steel-on-ice

0 .1

0.05

steel-on-Teflon

0.05

0.05

You should keep in mind that it isn't possible to give accurate values for the coefficient of frictions due to changing changing surface surface smoothness. For example, not all pieces of metal have the same surface smoothness. Some Some that are highly polished polished may be more more slippery slippery than others that are pitted pitte d or scratched. These values are just just meant to give give you the approximate values.

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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 most serious problems is that as the spacecraft travels through 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  not moving relative to each other. The friction that exists between two surfaces that are  not

friction ion 2. Kinetic frict

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 (bigger force). When it finally started to move, you were now pushing against the kinetic friction (smaller force).

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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.

Some people think that my tarantula can climb the walls of her tank because of some sort of "stickin "s tickiness" ess" 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

One of Wu-Li's relatives!

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

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is static, µk is kinetic

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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. When we measure the coefficient of friction (µ), "Empirical" evidence means that you

the smaller the number, the less the friction

actually have to perform the

between the two surfaces.

experiment each time to get results.

By gathering empirical evidence of different

There is no shortcut, regular pattern,

combinations combinations of surfaces physicists physicists have been

or formula that you can use to get the results.

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

0.04

0.04

synovial joints in humans

0.01

0.003 * depends on type of wood

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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. First calculate F   N  ... As long as the surface is completely

FN = Fg = mg

horizontal, we can say FN = Fg.

= (12kg) (9.81m/s 2) FN = 1.2e2 N Then use this answer to calculate F  f ...

Ff  = µs FN µs

= Ff  / FN = (35N) / (1.2e2 N) µs

= 0.30

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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 2

= (10 kg) (9.81 m/s ) FN = 98 N And then use that to calculate the maximum static friction. We can get the values for µs

and µk from the the table above...

Ff  = µs FN = 0.74 (98 N) Ff  = 73 N So, does this mean that when I push with F a = 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, 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  not start to move. With no net force acting on it, the box will  not

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c) Determine if I push with a force of 73 N if anything will happen. This exactly equals the maximum static frictional frictional force between these 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!

FNET = ma ma Ff  = µk FN

FNET = FN + Ff  a = FNET / m

= 0.57 (98 N)

= 100 + -56 = 44 / 10

Ff  = 56 N

FNET = 44 N a = 4.4 m/s 2

2

The box will accelerate at 4.4 m/s .

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