0.7 3.8 friction  (Page 5/10)

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But the atomic-scale view promises to explain far more than the simpler features of friction. The mechanism for how heat is generated is now being determined. In other words, why do surfaces get warmer when rubbed? Essentially, atoms are linked with one another to form lattices. When surfaces rub, the surface atoms adhere and cause atomic lattices to vibrate—essentially creating sound waves that penetrate the material. The sound waves diminish with distance and their energy is converted into heat. Chemical reactions that are related to frictional wear can also occur between atoms and molecules on the surfaces. [link] shows how the tip of a probe drawn across another material is deformed by atomic-scale friction. The force needed to drag the tip can be measured and is found to be related to shear stress, which will be discussed later in this chapter. The variation in shear stress is remarkable (more than a factor of ${\text{10}}^{\text{12}}$ ) and difficult to predict theoretically, but shear stress is yielding a fundamental understanding of a large-scale phenomenon known since ancient times—friction.

Section summary

• Friction is a contact force between systems that opposes the motion or attempted motion between them. Simple friction is proportional to the normal force $N$ pushing the systems together. (A normal force is always perpendicular to the contact surface between systems.) Friction depends on both of the materials involved. The magnitude of static friction ${f}_{\text{s}}$ between systems stationary relative to one another is given by
${f}_{\text{s}}\le {\mu }_{\text{s}}N,$
where ${\mu }_{\text{s}}$ is the coefficient of static friction, which depends on both of the materials.
• The kinetic friction force ${f}_{\text{k}}$ between systems moving relative to one another is given by
${f}_{\text{k}}={\mu }_{\text{k}}N,$
where ${\mu }_{\text{k}}$ is the coefficient of kinetic friction, which also depends on both materials.

Conceptual questions

The glue on a piece of tape can exert forces. Can these forces be a type of simple friction? Explain, considering especially that tape can stick to vertical walls and even to ceilings.

Problems&Exercises

A physics major is cooking breakfast when he notices that the frictional force between his steel spatula and his Teflon frying pan is only 0.200 N. Knowing the coefficient of kinetic friction between the two materials, he quickly calculates the normal force. What is it?

$5.00 N$

Suppose you have a 120-kg wooden crate resting on a wood floor. (a) What maximum force can you exert horizontally on the crate without moving it? (b) If you continue to exert this force once the crate starts to slip, what will the magnitude of its acceleration then be?

(a) 588 N

(b) $1\text{.}\text{96 m}{\text{/s}}^{2}$

Show that the acceleration of any object down an incline where friction behaves simply (that is, where ${f}_{\text{k}}={\mu }_{\text{k}}N$ ) is $a=g\left(\phantom{\rule{0.25em}{0ex}}\text{sin}\phantom{\rule{0.25em}{0ex}}\theta -{\mu }_{\text{k}}\text{cos}\phantom{\rule{0.25em}{0ex}}\theta \right).$ Note that the acceleration is independent of mass and reduces to the expression found in the previous problem when friction becomes negligibly small $\left({\mu }_{\text{k}}=0\right).$

Calculate the deceleration of a snow boarder going up a $5.0º$ , slope assuming the coefficient of friction for waxed wood on wet snow. The result of [link] may be useful, but be careful to consider the fact that the snow boarder is going uphill. Explicitly show how you follow the steps in Problem-Solving Strategies .

$1\text{.}\text{83}\phantom{\rule{0.25em}{0ex}}{\text{m/s}}^{2}$

(a) Calculate the acceleration of a skier heading down a $\text{10}\text{.}0º$ slope, assuming the coefficient of friction for waxed wood on wet snow. (b) Find the angle of the slope down which this skier could coast at a constant velocity. You can neglect air resistance in both parts, and you will find the result of [link] to be useful. Explicitly show how you follow the steps in the Problem-Solving Strategies .

Calculate the maximum acceleration of a car that is heading up a $\text{4º}$ slope (one that makes an angle of $\text{4º}$ with the horizontal) under the following road conditions. Assume that only half the weight of the car is supported by the two drive wheels and that the coefficient of static friction is involved—that is, the tires are not allowed to slip during the acceleration. (Ignore rolling.) (a) On dry concrete. (b) On wet concrete. (c) On ice, assuming that ${\text{μ}}_{\text{s}}=\phantom{\rule{0.25em}{0ex}}\text{0.100}$ , the same as for shoes on ice.

(a) $4\text{.}{\text{20 m/s}}^{2}$

(b) $2\text{.}{\text{74 m/s}}^{2}$

(c) $\text{–0}\text{.}{\text{195 m/s}}^{2}$

Repeat [link] for a car with four-wheel drive.

A contestant in a winter sporting event pushes a 45.0-kg block of ice across a frozen lake as shown in [link] (a). (a) Calculate the minimum force $F$ he must exert to get the block moving. (b) What is the magnitude of its acceleration once it starts to move, if that force is maintained?

(a) $\text{51}\text{.}0 N$

(b) $0\text{.}\text{720 m}{\text{/s}}^{2}$

Repeat [link] with the contestant pulling the block of ice with a rope over his shoulder at the same angle above the horizontal as shown in [link] (b).

what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Mostly, they use nano carbon for electronics and for materials to be strengthened.
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is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
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Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
for screen printed electrodes ?
SUYASH
What is lattice structure?
of graphene you mean?
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or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
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China
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types of nano material
I start with an easy one. carbon nanotubes woven into a long filament like a string
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many many of nanotubes
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what is the function of carbon nanotubes?
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I'm interested in nanotube
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what is nanomaterials​ and their applications of sensors.
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preparation of nanomaterial
how did you get the value of 2000N.What calculations are needed to arrive at it
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