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by Ron Kurtus

A total of 120 comments and questions have been sent in. They are listed according to date.

## List of next 10 letters

### Country

Coefficient of Friction Values for Clean Surfaces Coefficient of friction between fabric and metal KSA

Rolling Friction Frictional losses in a rollercoaster UK

Friction Equation Is friction coefficient independent of normal force? South Africa

Coefficient of Friction Values for Clean Surfaces Coefficient of friction for asphalt and concrete Tunisia

Resistive Force of Friction Friction problem India

Friction Start Rolling Friction at the start of rolling motion India

Friction Uses When is friction needed? USA

Friction Uses What happens if the lubricant is too thin and runny? Australia

Friction Causes Classical versus morder theories of frction India

Coefficient of Friction Tables Coefficent of friction between Brass and Nickle India

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First 10 letters

## Coefficient of friction between fabric and metal

Topic: Coefficient of Friction Values for Clean Surfaces

### Question

November 15, 2012

hi
i need to know about slid friction between fabric and metal.
THANK YOU

HASSAN - KSA

23039

Unfortunately, I do not have numbers for the coefficient of friction between fabric and metal.

The coefficient of friction between fabric and metal varies so much according to what the fabric is and the conditions that there really isn't much published about the situation.

Most of the time, an experiment is necessary to measure the force required to slide the materials together in order to get an approximation of the friction. I know that this is not satisfying, but with the lack of data, it usually is the only way to do it.

## Frictional losses in a rollercoaster

Topic: Rolling Friction

### Question

September 10, 2012

Hello Sir. I was wondering if you could tell me where the frictional losses occur in a rollercoaster (in a fair amount of depth - I already know 'between the rollercoaster and the air, as well as the track') and the sorts of quantitative proportions.

Quentin - UK

22892

A major area of loss in a roller coaster is due to air resistance. There are no simple equations to determine that resistance, plus it varies with the number of people and such.

As far as the frictional losses go, the wheel or axle bearings provide one source and the rolling friction is another source of resistance.

The amount of friction caused by the bearings depends on the type and the lubrication used.

The rolling friction of the steel wheels on a steel track is very small. However, if the wheels slide due to resistance of the bearings, you have sliding friction, which can be a factor.

There is some data on rolling friction tracks, but the other forms usually need to be found experimentally.

## Is friction coefficient independent of normal force?

Topic: Friction Equation

### Question

May 8, 2012

Is the friction coefficient independent to the normal force?

sinazo - South Africa

22639

In general, coefficient of friction is considered independent of the normal force.

However, in reality, it is a function of the differences in materials, their hardness, their smoothness, and whether there is a lubricant between them. And the normal force can affect those items.

For example, an automobile tire would have a higher coefficient of friction for a heavier vehicle.

## Coefficient of friction for asphalt and concrete

Topic: Coefficient of Friction Values for Clean Surfaces

### Question

May 6, 2012

I'm doing a research project, I need the coefficient of friction asphalt-concrete but I can't find it.
there is only rubber-asphalt and rubber-concrete is it correct to estimate the coefficient of friction asphalt-concrete from this two values?

Rahma - Tunisia

22636

Since there are so many combinations of materials, charts and lists seldom cover most of them.

The best way to determine the coefficient of friction between asphalt and concrete is to perform an experiment with samples of the two experiments. Weigh the top item, and then measure the force to drag it across the bottom material, using a spring scale. Then use the F = fr*N equation to find fr.

You can get a very good approximation from such a simple experiment.

## Friction problem

Topic: Resistive Force of Friction

### Question

April 16, 2012

Ques.> THE FRICTIONAL FORCE OFFERED BY A FLOOR ON A BOX IS ONE FIFTH OF THE WEIGHT OF THE BOX , IF THE BOX HAS A MASS OF 20KG.FIND THE WORKDONEBY THE FRICTIONAL FORCE ON THE BOX AS THE BOX SLIDES THROUGH 50cm. (answer is -19.6J)

Ques.> AN EXTERNAL COMBUSTION ENGINE HAS A POWER OF 6 KILO-WATT. AT WHAT SPEED IT CAN RAISE A LOAD OF 120KG. (ANSWER IS 5m/s)

SIR,
PLEASE GIVE ME THE SOLUTIONS OF THESE TWO QUESTIONS

THANKYOU....

ZAID - India

22590

Fictional resistive force = weight/5
Mass = 20kg
Weight = 20 * 9.8 = 196
Force = 39.2
Work = Force times distance
50cm = 0.5m
Work = 39.2 * 0.5m = 19.6

## Friction at the start of rolling motion

Topic: Friction Start Rolling

### Question

October 25, 2011

Is it really the static friction at the start of rolling motion? Also after the start too, in case of pure rolling motion, the point of contact of the wheel always remains stationary. This implies that while pure rolling also (accelerated/decelerated)at any time, there is static friction between the wheel and the surface.

Is there any formula by which one can calculate how much torque is required by the motor to start the acceleration?

- India

22114

When you push a wheel or apply a torque to it, static friction prevents the wheel from simply sliding due to the force. Once the wheel is rolling, the friction at the point of contact is somewhat like a "sticky" force caused by molecular attraction between the wheel and the other surface. This is still considered static friction, although it is slightly different that the friction holding the wheel in place.

Torque is the force on the outer surface times the radius of the wheel: T = FR.

The force is a combination of the static friction holding the wheel in place plus the inertia required to move the mass. If there was no friction, the torque required to start a heavy iron wheel would be much greater than that required to start a lighter weight wheel. In such a case, the torque without friction would be T = maR, since F = ma.

## When is friction needed?

Topic: Friction Uses

### Question

October 3, 2011

when is friction needed? and when is friction not needed please two paragraps

jenny - USA

22038

Friction is needed whenever you don't want things to slip, like when you walk or write with a pencil. It is not needed if you want things to move without resistance.

## What happens if the lubricant is too thin and runny?

Topic: Friction Uses

### Question

September 18, 2011

hi i was just wondering
if lubricants are used in engines to help sliding parts with less friction what would happen if the lubricant was too runny?
and what would hapen if it was "gluggy"?

Ra - Australia

21997

If the lubricant was too thin or runny, it might not be able to keep the parts sufficiently separated. If the lubricant was too thick, it might create enough resistance that the parts do not move properly.

An example is with the oil used in automobiles. Years ago, a thin oil was used in the winter, because the cold weather made it thicker. A thick oil was used in the summer, because the hot weather made it thinner. In both cases. the viscosity of the oil was just about right. Modern oils do not have that problem.

## Classical versus morder theories of frction

Topic: Friction Causes

### Question

August 26, 2011

In the given article it is said that there are classic as well as
modern theories on friction.Now the classic theory is abandoned.
But still in the n.c.e.r.t class8 textbook,we are given the classic theory only.Is this fare?

Sudhakaran.S - India

21923

The classical theory of friction can be applied to many situations, and that is why it is still studied. It has not been abandoned, because it explains many practical cases.

Then you start to consider more complex cases of friction, like those concerning soft or elastic objects or sticky materials, you need to consider the more complex modern explanations.

## Coefficent of friction between Brass and Nickle

Topic: Coefficient of Friction Tables

### Question

June 30, 2011

Sir, What must be coefficent of friction value between Brass(CuZn37) And Nickle Plated brass(CuZn39Pb3)

Sunil - India

21778

The coefficient of friction for many such combinations of materials is often found experimentally. I don't know of any tables for those two metals.

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## Summary

Observe friction in action

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