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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

Friction Uses Why is friction important? Philippines

Friction Uses Why do we need to increase and decrease friction? India

Friction Uses How can I determine the coefficient of friction? Nigeria

Friction Equation Sees problems in lesson USA

Friction Start Rolling Is the refenence point of the wheel through its center? Turkey

Coefficient of Friction Tables What types of friction aren't used much? USA

Friction Start Rolling How to determine friction in rolling motion? India

Friction Start Rolling Direction of friction seems wrong to me USA

Friction Uses Why is friction required when we are sleeping? Pakistan

Friction Uses Useful and harmful effect of friction in machines Sudan

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First 10 letters

## Why is friction important?

Topic: Friction Uses

### Question

October 2, 2008

why friction is very important to our lives? what's the connection on it to our lives? what's the significance of it? how friction come up to our lives?

jeanne - Philippines

16579

You cannot walk or pick up things without friction. You would also slip off your chair without friction. A car would not start or stop.

## Why do we need to increase and decrease friction?

Topic: Friction Uses

### Question

August 24, 2008

why do we need to increase and decrease friction?

natasha - India

16391

You increase friction to help stop moving things like in automobile brakes.

You decrease friction to allow things to move easier.

## How can I determine the coefficient of friction?

Topic: Friction Uses

### Question

July 6, 2008

How can I determine the coefficient of limiting friction

AKEEM - Nigeria

16182

Although you can get approximate values from charts such as on: http://www.school-for-champions.com/science/friction_coefficient.htm, you really need to measure the coefficient with an experiment.

Take a given weight and measure the force required to just to start to move it. Then find the coefficient from the equation fr = F/W. See: http://www.school-for-champions.com/science/friction_equation.htm

## Sees problems in lesson

Topic: Friction Equation

### Question

May 31, 2008

Your chapter on friction has two problems:

1) You say "force required to push the box along the floor is 70 pounds" -- "pounds" is not a measurement of force. "Newtons" is a measurement of force; "pounds" is a measurement of mass.

2) Question 2 asks what happens to the normal force as a function of incline. The problem is that, if the normal force is exerted beacuse of gravity, answer c is right, but if the normal force is exerted because of some independent pressure, answer b is right. To make the question unambiguous, you have to specify whether gravity is the source.

Jon - USA

16018

A slug is the unit of mass in the English system, while a pound is the unit of weight or force. Likewise, in the metric system, a kilogram is a unit of mass and the Newton is the unit of weight or force.

But you are correct about Question 2 being ambiguous. That was a careless mistake on my part. I added that the "...normal force, as affected by gravity" and corrected the page.

Thanks for pointing that out to me.

## Is the refenence point of the wheel through its center?

Topic: Friction Start Rolling

### Question

January 2, 2008

Firstly, I want to thank yo because of this very useful site. I have a question yet: You said that in an inclined plane the gravitational force will push the wheel and make it move but in this situation as we assume that the wheel is uniform and center of gravity of it is at the center of the wheel these force will
not create a torque because the perpendicular distance to the reference point (cog) is 0. In this case what makes a wheel move in an inclined plane? What is the force that creates torque? And what is the benefit of the ABS brake system? In this system wheels are prevented from being locked so they continue rolling motion
but isn't the friction in rolling motion less than sliding?

Ceyhun - Turkey

15044

A wheel can be considered to rotate about its point of contact on the ground. If you draw a picture of a wheel on an inclined plane, the force of gravity will be directed downward through the center of the wheel. But because of the incline, the pivot point is where the wheel touches the plane, and that is offset from the force of gravity. This produces a torque at that contact point, making the wheel roll forward.

Note that you can set a reference point where ever is convenient, as long as you are consistent in your analysis.

## What types of friction aren't used much?

Topic: Coefficient of Friction Tables

### Question

November 18, 2007

Hi. I have a science question about friction that I can't figure out. It is "What types of friction aren't used much? a. static b. Kinetic C. Fr and Fn or d. Deformation and molecular. Thanks so much for making this site, it has helped me on so much of my physical science that I don't understand.

Danielle - USA

14801

I'm not sure what Fr and Fn refer to. Sometimes Fr is short for the force of friction. They should be defined.

Deformation friction is used in all automobile tires, but molecular friction is not really used much.

## How to determine friction in rolling motion?

Topic: Friction Start Rolling

### Question

November 8, 2007

how should we exactly determine the of friction in rolling motion in different kinds of situations like accelerated motion or in constant velocity?

shruti - India

14738

Usually the wheel must be placed on a vehicle that has a minimum of axle friction. The force required to start, accelerate and keep the vehicle in constant motion can be easily measured. If the vehicle has four wheels, the value is divided by 4 to account for the friction from each wheel. The weight of the vehicle and wheel is used to determine the coefficient of rolling friction for each wheel.

Note that if the wheel is soft, such as a inflatable tire, the friction varies with the weight of the vehicle and inflation of the tire.

## Direction of friction seems wrong to me

Topic: Friction Start Rolling

### Question

October 21, 2007

In the Role of Friction in Starting Motion, the first part of the section on Pushing or pulling the wheel doesn't seem correct. When the wheel is pushed above the axle, there is a clockwise torque trying to rotate the wheel clockwise, just as the case when the engine is turning the axle clockwise. Shouldn't the static friction force be in the same direction in either case? The tire/road interface cannot sense if the torque is caused by the engine turning the axle or someone pushing on the tire. On the other hand, if the line of force pushing on the wheel is below the axle, then the static friction force would seem to be in the rearward direction to try to cause the tire to rotate backwards. Can you clarify?

Thanks

John - USA

14605

Instead of a wheel, suppose you had a square block. Pushing above the center of gravity would result in the block sliding and the friction force going in the opposite direction. If the friction was great enough, the block would not slide but would tip over. The same thing happens when you push on a wheel. The friction is preventing the wheel from sliding.

Also, it is really pivoting on the contact point of the ground. It isn't like having a torque applied to the axle.

When you push below the axle, again the friction is preventing sliding. But now there is another factor in that there is friction at the point of contact where you are pushing. The wheel really wants to rotate in the opposite direction. It can get complex then.

## Why is friction required when we are sleeping?

Topic: Friction Uses

### Question

October 18, 2007

why friction required when we are sleeping? why friction required when we are writting with pencil

monna - Pakistan

14571

If there was no friction between you and your bed, you could not turn over during your sleep. Also, you might even slide out of the bed.

The friction between the pencil lead and the paper causes some of the lead to rub off and mark the paper. Also, the lead is soft, so much rubs off onto the paper.

## Useful and harmful effect of friction in machines

Topic: Friction Uses

### Question

October 18, 2007

Ineed for many example of useful and harmful uses of friction in machine .

abdu - Sudan

14570

If there is no lubrication in a machine, the parts that rub together might overheat or wear out rapidly. But there are parts of a machine that need friction, such as devices that pick up other parts. Also, handles to control the machine must have friction so the worker's hand will not slip off.

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## Summary

Observe friction in action

## Resources and references

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