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23.2 THE BASIC PHYSICS OF FRICTION
À pThis 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,
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,
The general approach to slip-tip problem is,
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 perpendicular to the surface of contact. Find the components of applied forces acting parallel to the plane of contact.
4. Compare the actual parallel component to the maximum friction force possible. The the resultant is larger than the maximum the block will slip.
******************* Solve using Mathcad ********************
23.2.1 Practice Problems
(|23.2.2 References
(|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.
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