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Answers to Readers' Questions on Physical Science:

Friction Equation

The following 4 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Making a mobile home for Haiti earthquake
• Friction is much less when object is on its edge
• What is resultant force in friction?
• Sees problems in lesson

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Making a mobile home for Haiti earthquake

Question

March 27, 2010

We design a mobile home(Single wide)for Haiti. We have to take in consideration earthquake.The mobile home will be resting on polished concrete pads (2' x 2') on the ground,to be located at each corner of the structure .The purpose of doing this is,that when an earthquake horizontal force acts against the structure.the pads on the ground will slide under the structure ,resulting in a minimal shear force(moment force).Resulting in no damage to the structure. The weight of the structure is 36,000.lbs.,the coeficient of kinetic friction of steel over concrete is .35.The horizontal displacement of an earthquake it is suppose to be less than 12 in.Want to know if we are right. Eng. Amador

Demetrio - USA

19447

Answer

Many structures in earthquake zones are placed on rubber pads, which allow some give in the case of a tremor. See the articles:

http://illumin.usc.edu/article.php?articleID=127
http://www.patentstorm.us/patents/4864785/description.html

However, considering having the home slide on a steel against concrete base may be a solution, assuming the horizontal ground motion is around 12".

Since I am sure there are many structural engineers versed in earthquake protection doing work in Haiti at this time, it would be good to get a professional's advise, especially if you are investing money in a project.

Best wishes on your work on the mobile home.

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Friction is much less when object is on its edge

Question

February 19, 2010

Dear Sir,

I have understood that the friction between two surfaces depends on the normal force and the coefficient of friction It is independent of area of contact. So a rectangular box will need the same force to slide along the longer or shorter sides on a marble floor. But if the box is tilted on the edge or a corner and pushed, much less force is needed meaning that the friction has reduced. ( Here only the area of contact is reduced - but reduced drastically) Why do we experience this as such? Can you please give me a scientific explanation? Thanks in advance.

Regards,

Anand - India

19253

Answer

Friction is due to a certain percentage of points on both surfaces being close to each other at an atomic level. In general, sliding friction is independent of the area of contact.

However, this is really an approximation. If you would measure the friction as you decrease the contact area, the friction would remain close to being constant. But there would be a point where the friction sudden decreased dramatically, such as when the contact area became close to being an edge.

The rule that friction does not vary with area is useful in common situations, but there are a number of situations where it fails.

Likewise, if you increase the normal force, there is a force where the friction would be much greater than expected.

The friction laws are a simplification of a highly complex physical phenomenon.

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What is resultant force in friction?

Question

November 11, 2008

what is resultent force in friction

navjot - India

16777

Answer

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.

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Sees problems in lesson

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

Answer

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.

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