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Understanding Friction: Types, Forces, and Applications

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tasya
10 diapositives · 236 Views · Jul 29, 2014

Friction is the force that opposes the motion of objects in contact, arising from microscopic surface irregularities. There are two main types of friction: static friction, which acts on objects at rest when a force is applied, and kinetic friction, which comes into play when objects are sliding or rolling. The coefficient of friction varies with material and surface conditions and is crucial for understanding motion. Examples illustrate how to calculate frictional forces in real-world scenarios, emphasizing the practical applications of friction in physics.

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  1. Applications involving Friction Chapter 4 Sec 8

  2. What is Friction? • Friction is a FORCE that opposes or impedes the motion of an object. • Friction is caused by microscopic bumps between solid objects in contact. • Friction can exist from sliding objects or rolling objects (though rolling friction is less than sliding). • Friction depends on the types of materials AND on the normal force (or weight)of the object.

  3. Types of Friction • Friction between solid objects sliding against each other (or rolling) is called Kinetic Friction. (Greek for moving) • Friction between solid objects can exist parallel to the surface of them even when they are not moving. This is Static Friction. • Each type of material changes the amount of Friction present, so we have a coefficient of friction, μk or μs, for kinetic and static coefficients. • Friction depends very little on surface area.

  4. Friction Coefficients • The friction force, Ffr, is always perpendicular to the normal force, FN. • In calculating kinetic friction: • μ depends on whether the object is wet or dry, what type of finish is on them, but NOT on speed of the objects sliding.

  5. Static Friction • Static friction is a force that exists between objects that are in contact, but NOT moving when a force is applied. • Eventually with a hard enough push, the object will move and kinetic friction takes over as you exceed the MAXIMUM static friction. • Fmax = μsFN and since static friction can vary from 0 to max, we write • μs is generally more than μk as its harder to start an object than keep it moving.

  6. Example 1 • A 10.0kg box rests on the floor. μs = 0.4 and μk =0.3 • Determine the force of friction, Ffr, acting on the box if the horizontal applied force, FA = 20N. If FA = 40N. Draw a free body diagram and label all forces.

  7. Solution for Example 1 • In the vertical direction there is no motion, so the net force, Σfy = ma = 0 yields FN – mg = 0. • In all cases, the normal force, FN = mg = (10.0kg)(9.80m/s/s)= 98 N. • The force of static friction will oppose any applied force up to the maximum of • Ffr = μsFN = (0.40)(98N)=39N • If FA = 20 N, the box won’t move so Ffr = -20N to balance the applied force.

  8. FN Continued FA Ff Fg • If FA = 40N, which is more than the maximum static friction force, the box will accelerate and we have kinetic friction, Ffr = μkFN • Ffr = (0.30)(98N) = 29 N. • ΣFx= FA + Ffr = 40 N + (-29N) = 11N, so the box will accelerate at a = Fnet / mass. • a = 11N / 10.0 kg = 1.1 m/s/s in a direction of the applied force. (positive horizontal).

  9. More Examples • Look at the Example 4-16 on page 99. (two boxes on a pulley) • Now look at the Example 4-17 on page 100. (skier on a hill) • Recall how to obtain components of weight when not perpendicular to the surface? • FN is always perpendicular to the surface and Ffr is always parallel to the surface here.

  10. Your turn to Practice • Please do Chapter 4 Review p 107 #s 38 and 39 • Please do Chapter 4 Review p 108 #s 40, 42, 43, 44, 46.