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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 Start Rolling Factors involved in rolling resistance India

Friction Uses Friction and riding a bicycle India

Friction Uses Is walking aided by friction? USA

Coefficient of Friction Tables Friction table seems incorrect USA

Friction Uses Why does ice have a low coefficient friction? Israel

Friction Causes Writing a story about a world without friction USA

Friction Uses What things don't need friction? USA

Friction Equation What is resultant force in friction? India

Friction Uses Advantages of friction India

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First 10 letters

## Factors involved in rolling resistance

Topic: Friction Start Rolling

### Question

June 3, 2009

I was going thru' rolling resitance topic and found very useful. Thanks. Have one small doubt. I want to understand how rolling resitance value varies with speed, tyre size and road surface. I will appreciate if you can answer this.

Harshal - India

17847

The problem with calculating rolling resistance of a tyre is that there are too many variables and no simple equations for a solution.

The amount of resistance with speed depends on the type of rubber used and the inflation of the tyre. A soft tyre will increase in rolling resistance with speed more than a highly inflated tyre.

Likewise, road surface and texture can vary so much that it has to be considered on a case-by-case basis. For example, a cobblestone street would probably have less rolling resistance than a standard asphalt road.

Another factor is the weight of the automobile, which will deform the tyre and result in more resistance.

With this type of problem, experimental methods are often the only way to achieve numbers.

## Friction and riding a bicycle

Topic: Friction Uses

### Question

May 21, 2009

Do you want to increase or decrease friction when riding a bicycle? Possible answers are:
- Increase, because you don't want to go too fast
- Both, because you want to be able to stop but yet peddle easily
- Decrease, because you don't want to peddle too hard.

I think the answer is:Increase, because you don't want to go too fast

arun - India

17782

If there is too much friction on the bicycle, it will be difficult to peddle the bicycle forward. Thus, you want to decrease the friction.

But when you want to stop, you put on the brakes, which increases the friction.

## Is walking aided by friction?

Topic: Friction Uses

### Question

May 4, 2009

Is walking aided by sliding or static frictin?

Maya - USA

17695

You could not walk without static friction, because your foot would slide on the surface with each step you try to take. There is a small sliding friction factor in walking, but the biggest factor is from static friction.

## Ways to increase friction on icy roads

Topic: Friction Uses

### Question

May 3, 2009

what are the three ways to increase the friction on wet or icy roads?

17690

Many communities put sand on the roads when they are icy to increase the traction or friction. They may also put salt on the roads to melt the ice and increase the friction.

But also the tires on the cars can increase the traction or friction. Rain tires, snow tires and studded tires are used to increase the friction.

## Friction table seems incorrect

Topic: Coefficient of Friction Tables

### Question

April 13, 2009

When reading through your page on friction I noticed that the coefficients for aluminum-on-aluminum are 1.05 to 1.35 (Static) and 1.40 (Dynamic).

Just curious, how can this be? I always thought that the static coefficient has to be higher than the dynamic. How would this manifest itself physically ? Would the force required to keep an object moving be somehow greater than the force required to start it moving? This seems contradictory.

I'm working on an analysis of two aluminum mating parts, in which friction plays a prominent role. Any clarification you can give me will be very helpful. Thanks !

Dave - USA

17595

It certainly doesn't make sense that static friction should be less than dynamic or kinetic friction. Major engineering tables on the coefficient of friction seem to have the same values for aluminum-on-aluminum. My guess is that it is a typo that has been replicated everywhere from a single experiment. I'm going to delete that item from the table, since it is obviously incorrect.

A major problem in many coefficient of friction tables is that there is usually no indication concerning the environmental conditions of the experiment or who made the measurement. For example, the aluminum measurement could have been done under different conditions and even with different pieces of metal.

The best bet is to perform some simple experiments to determine the friction on your specific parts. Then you can be sure of the values.

## Why does ice have a low coefficient friction?

Topic: Friction Uses

### Question

April 7, 2009

hay, my question is why ice has low coefficient friction? is it because of the way the molecule organize?
thank you

menakhem (meni) - Israel

17574

The molecular structure of the ice crystals is such that there isn't much adhesion that creates friction. But also, the top, narrow layer of ice can easily turn to water, which is a good lubricant.

## Writing a story about a world without friction

Topic: Friction Causes

### Question

February 10, 2009

I am a junior in high school, and I have an assignment that I am having trouble with. I am to write a story or take the role of a news writer about a world without friction on the surface. There is still gravity and all other laws of physics apply, and all thre of Newton's laws of motion apply except no friction on the surface of the planet. I need to explain how things move, turn, and stop. If gravity holds us down but no friction - would we be slidding around the surface? Could we have transportation - how would anything work with no ground friction - no airplanes could take off. I just don't get this - HELP!!! Lennea

Lennea - USA

17245

You are on the right track. You could imagine that everything was covered with ice or something very slippery.

Obviously, life could not exist, because even the simplest things like eating require friction.

Without friction, something that was moving or rolling on the ground, would continue moving. If a marble would roll down the side of a bowl, it would roll up the other side and back down forever.

The only way something would change directions is if it collided with another object.

If this is to be a story, you might consider an astronaut landing on such a planet. Things would be flying or rolling all over the place. And the astronaut would not be able to get back into the spaceship.

I hope these ideas help. Best wishes in your story.

## What things don't need friction?

Topic: Friction Uses

### Question

November 19, 2008

what are the things that do not really needed friction?

errol - USA

16827

Anything that you would like to slide easily would work better without friction. That is why lubrication is used to reduce the friction and make the movement easier.

Although you need friction to start sliding a box on the ice, once it starts sliding it does not need friction.

## What is resultant force in friction?

Topic: Friction Equation

### Question

November 11, 2008

what is resultent force in friction

navjot - India

16777

The resultant force is what acts on a body. Friction acts in the opposite direction to the resultant force. Often the resultant force is a combination of forces.

Topic: Friction Uses

### Question

October 23, 2008

can u plzzz snd me sum advntags of friction.i m doing a project wrk on it...so kindly make it fast

nachi - India

16679

You cannot walk without friction. You cannot hold things without friction.

See the Reader Questions on this Friction page for more examples.

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## Summary

Observe friction in action

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