8.1 THE BASIC PHYSICS OF FRICTION

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This natural phenomenon explains the resistance of one object to slide across another when they have common surfaces in contact.

It is primarily the result of surface roughness, material properties, and if the object is moving.

The graph of applied load versus friction helps illustrate the nature of friction. Notice that while the force is static, the force increases linearly up to the limit. After the object begins moving the force can be approximated with a constant value, using the dynamic coefficient of friction. Note that dynamic friction is shown to be lower that the maximum static friction.

The basic assumptions that we will use are,

1. the maximum friction force is proportional to the normal force

2. the maximum friction force is not proportional to the area of contact

3. the static friction force is always higher than the dynamic friction force

4. the kinetic friction force is independant of velocity

A couple of the major applications for friction calculations is the determination if an object will slip or tip. The following problem shows a typical application, ([Hibbeler, 1992], prob 8-8, pg. )

Consider the simple tip/slip problem below,

wm_demos/stat0005.wm

The general approach to slip-tip problem is,

1. Find the center of gravity for the object.

2. Determine which corner the object is most likely to tip (as if the corner is a pin joint). Sum the moments about the corner. If the sum of moments is equal to zero to block is about to tip. If not equal to zero look at the resulting moment to see if it will cause motion about the corner.

3. Find the component of the gravity and any other non-friction forces acting perpendiculr to the surface of contact. Find the components of applied forces acting parallel to the plane of contact.

4. Compare the actual parallel compoent to the maximum friction force possible. The the resultant is larger than the maximum the block will slip.

*************** More friction examples

8.1.1 Practice Problems

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1. We conduct an experiment using the 10 kg. block below on a slope that is being slowly tilted. The block tips over at 20°, and then stops moving, but then it starts to slip at 40°. What is the height `h', and the coefficient of friction?

8.1.2 References

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Beer, F.P., Johnson, E.R., Statics & Mechanics of Materials, McGraw-Hill, 1992.

Hibbeler, R.C., Engineering Mechanics: Statics and Dynamics, 6th edition, MacMillan Publishing Co., New York, USA, 1992.