Lecture 8

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STATICS: CE201

Chapter 8

Friction Notes are prepared based on: Engineering Mechanics, Statics by R. C. Hibbeler, 12E Pearson

Dr M. Touahmia & Dr M. Boukendakdji Civil Engineering Department, University of Hail (2012/2013) Chapter 8: Friction

8. Friction ______________________________________________________________________________________

Chapter Objectives: 

   

To introduce the concept of dry friction and show how to analyze the equilibrium of rigid bodies subjected to this force. Draw a FBD including friction. Present specific applications of frictional force analysis on wedges, screws, belts, and bearings. Investigate the concept of rolling resistance Solve problems involving friction.

Chapter 8: Friction

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APPLICATIONS ________________________________________________________________________________

In designing a brake system for a bicycle, car, or any other vehicle, it is important to understand the frictional forces involved.

For an applied force on the brake pads, how can we determine the magnitude and direction of the resulting friction force? Chapter 8: Friction

APPLICATIONS ________________________________________________________________________________________

The rope is used to tow the refrigerator. In order to move the refrigerator, is it best to pull up as shown, pull horizontally, or pull downwards on the rope?

What physical factors affect the answer to this question?

Chapter 8: Friction

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CHARACTERISTICS OF DRY FRICTION _________________________________________________________________________________________

Friction is defined as a force of resistance acting on a body which prevents or retards slipping of the body relative to a second body. Experiments show that frictional forces act tangent (parallel) to the contacting surface in a direction opposing the relative motion or tendency for motion. For the body shown in the figure to be in equilibrium, the following must be true: F = P, N = W, and W*x = P*h. Chapter 8: Friction

CHARACTERISTICS OF DRY FRICTION _________________________________________________________________________________________

To study the characteristics of the friction force F, let us assume that tipping does not occur (i.e., “h” is small or “a” is large). Then we gradually increase the magnitude of the force P. Typically, experiments show that the friction force F varies with P, as shown in the right figure above. Chapter 8: Friction

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CHARACTERISTICS OF DRY FRICTION ________________________________________________________________________________________

The maximum friction force is attained just before the block begins to move (a situation that is called “impending motion”). The value of the force is found using Fs = s N, where s is called the coefficient of static friction. The value of s depends on the two materials in contact. Once the block begins to move, the frictional force typically drops and is given by Fk = k N. The value of k (coefficient of kinetic friction) is less than s . Chapter 8: Friction

CHARACTERISTICS OF DRY FRICTION _________________________________________________________________________________________

It is also very important to note that the friction force may be less than the maximum friction force. So, just because the object is not moving, don’t assume the friction force is at its maximum of Fs = s N unless you are told or know motion is impending! Chapter 8: Friction

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DETERMING s EXPERIMENTALLY

_________________________________________________________________________________________

If the block just begins to slip, the maximum friction force is Fs = s N, where s is the coefficient of static friction.

Thus, when the block is on the verge of sliding, the normal force N and frictional force Fs combine to create a resultant Rs From the figure, tan s = ( Fs / N ) = (s N / N ) = s Chapter 8: Friction

DETERMING s EXPERIMENTALLY

_________________________________________________________________________________________

A block with weight w is placed on an inclined plane. The plane is slowly tilted until the block just begins to slip. The inclination, s, is noted. Analysis of the block just before it begins to move gives (using Fs = s N): +  Fy = = 0

N – W cos s

+  FX = S N – W sin s = 0 Using these two equations, we get s = (W

sin s ) / (W cos s ) = tan s This simple experiment allows us to find the S between two materials in contact. Chapter 8: Friction

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PROBLEMS INVOLVING DRY FRICTION _________________________________________________________________________________________

Steps for solving equilibrium problems involving dry friction: 1. Draw the necessary free body diagrams. Make sure that you show the friction force in the correct direction (it always opposes the motion or impending motion). 2.

Determine the number of unknowns. Do not assume F = S N unless the impending motion condition is given.

3.

Apply the equations of equilibrium and appropriate frictional equations to solve for the unknowns.

Chapter 8: Friction

IMPENDING TIPPING versus SLIPPING ________________________________________________________________________________________

For a given W and h of the box, how can we determine if the block will slide or tip first? In this case, we have four unknowns (F, N, x, and P) and only three E-of-E. Hence, we have to make an assumption to give us another equation (the friction equation!). Then we can solve for the unknowns using the three E-of-E. Finally, we need to check if our assumption was correct. Chapter 8: Friction

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IMPENDING TIPPING versus SLIPPING ________________________________________________________________________________________

Assume: Slipping occurs Known: F = s N Solve:

x, P, and N

Check:

0  x  b/2 Or

Assume: Tipping occurs Known: x = b/2 Solve:

P, N, and F

Check:

F  s N Chapter 8: Friction

EXAMPLE 1 _________________________________________________________________________________________

Given: A uniform ladder weighs 30 lb. The vertical wall is smooth (no friction). The floor is rough and s = 0.2. Find:

Whether it remains in this position when it is released.

Plan:

a) Draw a FBD. b) Determine the unknowns. c) Make any necessary friction assumptions. d) Apply E-of-E (and friction equations, if appropriate ) to solve for the unknowns. e) Check assumptions, if required.

Chapter 8: Friction

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EXAMPLE 1 _________________________________________________________________________________________

NB FBD of the ladder

12 ft

12 ft

30 lb

FA 5 ft

5 ft

NA

There are three unknowns: NA, FA, NB.   FY = NA – 30 = 0 ;

so NA = 30 lb

+  MA = 30 ( 5 ) – NB( 24 ) = 0 ; +   FX = 6.25 – FA = 0 ;

so NB = 6.25 lb so FA = 6.25 lb Chapter 8: Friction

EXAMPLE 1 _________________________________________________________________________________________

NB

FBD of the ladder

12 ft

12 ft

30 lb

FA 5 ft

5 ft

NA

Now check the friction force to see if the ladder slides or stays. Fmax = s NA = 0.2 * 30 lb = 6 lb Since FA = 6.25 lb

 Ffriction max = 6 lb,

the pole will not remain stationary. It will move. Chapter 8: Friction

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GROUP PROBLEM SOLVING _________________________________________________________________________________________

Given: Refrigerator weight = 180 lb, s = 0.25 Find: The smallest magnitude of P that will cause impending motion (tipping or slipping) of the refrigerator. Plan: a) Draw a FBD of the refrigerator.

b) Determine the unknowns. c) Make friction assumptions, as necessary. d) Apply E-of-E (and friction equation as appropriate) to solve for the unknowns. e) Check assumptions, as required. Chapter 8: Friction

WEDGES AND FRICTIONAL FORCES ON FLAT BELTS ________________________________________________________________________________________

Today’s Objectives:

In-Class Activities:

Students will be able to:

• Check Homework, if any

a) Determine the forces on a wedge.

• Reading Quiz

b) Determine the tensions in a belt.

• Analysis of a Wedge

• Applications • Analysis of a Belt • Concept Quiz • Group Problem Solving • Attention Quiz

Chapter 8: Friction

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WEDGES AND FRICTIONAL FORCES ON FLAT BELTS ________________________________________________________________________________________

APPLICATIONS

Wedges are used to adjust the elevation or provide stability for heavy objects such as this large steel pipe. How can we determine the force required to pull the wedge out? When there are no applied forces on the wedge, will it stay in place (i.e., be self-locking) or will it come out on its own? Under what physical conditions will it come out? Chapter 8: Friction

WEDGES AND FRICTIONAL FORCES ON FLAT BELTS ________________________________________________________________________________________

APPLICATIONS

Belt drives are commonly used for transmitting the torque developed by a motor to a wheel attached to a pump, fan or blower.

How can we decide if the belts will function properly, i.e., without slipping or breaking? Chapter 8: Friction

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WEDGES AND FRICTIONAL FORCES ON FLAT BELTS ________________________________________________________________________________________

APPLICATIONS

In the design of a band brake, it is essential to analyze the frictional forces acting on the band (which acts like a belt).

How can you determine the tensions in the cable pulling on the band? Also from a design perspective, how are the belt tension, the applied force P and the torque M, related? Chapter 8: Friction

ANALYSIS OF A WEDGE ________________________________________________________________________________________

W

A wedge is a simple machine in which a small force P is used to lift a large weight W. To determine the force required to push the wedge in or out, it is necessary to draw FBDs of the wedge and the object on top of it. It is easier to start with a FBD of the wedge since you know the direction of its motion. Note that: a) the friction forces are always in the direction opposite to the motion, or impending motion, of the wedge; b) the friction forces are along the contacting surfaces; and, c) the normal forces are perpendicular to the contacting surfaces.

Chapter 8: Friction

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ANALYSIS OF A WEDGE ________________________________________________________________________________________

Next, a FBD of the object on top of the wedge is drawn. Please note that: a) at the contacting surfaces between the wedge and the object the forces are equal in magnitude and opposite in direction to those on the wedge; and, b) all other forces acting on the object should be shown. To determine the unknowns, we must apply EofE,  Fx = 0 and  Fy = 0, to the wedge and the object as well as the impending motion frictional equation, F = S N. Chapter 8: Friction

ANALYSIS OF A WEDGE ________________________________________________________________________________________

Now of the two FBDs, which one should we start analyzing first? We should start analyzing the FBD in which the number of unknowns are less than or equal to the number of EofE and frictional equations. Chapter 8: Friction

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ANALYSIS OF A WEDGE ________________________________________________________________________________________

W

NOTE: • If the object is to be lowered, then the wedge needs to be pulled out. • If the value of the force P needed to remove the wedge is positive, then the wedge is self-locking, i.e., it will not come out on its own.

BELT ANALYSIS ________________________________________________________________________________________

Consider a flat belt passing over a fixed curved surface with the total angle of contact equal to  radians. If the belt slips or is just about to slip, then T2 must be larger than T1 and the motion resisting friction forces. Hence, T2 must be greater than T1. Detailed analysis (please refer to your textbook) shows that T2 = T1 e   where  is the coefficient of static friction between the belt and the surface. Be sure to use radians when using this formula!! Chapter 8: Friction

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EXAMPLE 2 ________________________________________________________________________________________

Given: The crate weighs 300 lb and S at all contacting surfaces is 0.3. Assume the wedges have negligible weight. Find: The smallest force P needed to pull out the wedge. Plan: 1. Draw a FBD of the crate. Why do the crate first? 2. Draw a FBD of the wedge. 3. Apply the E-of-E to the crate. 4. Apply the E-of-E to wedge.

Chapter 8: Friction

EXAMPLE 2 ________________________________________________________________________________________

NC FC=0.3NC

300 lb

FB=0.3NB

P

NB

15º

FD=0.3ND FC=0.3NC

15º

NC FBD of Crate

ND

FBD of Wedge

The FBDs of crate and wedge are shown in the figures. Applying the EofE to the crate, we get +  FX =

NB – 0.3NC = 0

+  FY = NC – 300 + 0.3 NB = 0

Solving the above two equations, we get NB = 82.57 lb = 82.6 lb,

NC = 275.3 lb = 275 lb Chapter 8: Friction

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EXAMPLE 2 ________________________________________________________________________________________

NC FB=0.3NB

FC=0.3NC

300 lb

P

NB = 82.6 lb

15º

FD=0.3ND

FC=0.3NC

15º

NC = 275 lb FBD of Crate

ND

FBD of Wedge

Applying the E-of-E to the wedge, we get +  FY = ND cos 15 + 0.3 ND sin 15 – 275.2= 0; ND = 263.7 lb = 264 lb +  FX = 0.3(263.7) + 0.3(263.7)cos 15 – 0.3(263.7)cos 15 – P = 0; P = 90.7 lb

CONCEPT QUIZ _________________________________________________________________________________________

1. Determine the direction of the friction force on object B at the contact point between A and B. A) 

B) 

C)

D)

2. The boy (hanging) in the picture weighs 100 lb and the woman weighs 150 lb. The coefficient of static friction between her shoes and the ground is 0.6. The boy will ______ ? A) Be lifted up

B) Slide down

C) Not be lifted up D) Not slide down Chapter 8: Friction

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GROUP PROBLEM SOLVING _________________________________________________________________________________________

Given: Blocks A and B weigh 50 lb and 30 lb, respectively. Find: The smallest weight of cylinder D which will cause the loss of static equilibrium. Chapter 8: Friction

GROUP PROBLEM SOLVING _________________________________________________________________________________________

Plan: 1. Consider two cases: a) both blocks slide together, and, b) block B slides over the block A. 2. For each case, draw a FBD of the block(s). 3. For each case, apply the EofE to find the force needed to cause sliding. 4. Choose the smaller P value from the two cases.

5. Use belt friction theory to find the weight of block D. Chapter 8: Friction

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GROUP PROBLEM SOLVING _________________________________________________________________________________________

Case a (both blocks sliding together):

P

B

 +  FY = N – 80 = 0 N = 80 lb

A

+  FX = 0.4 (80) – P = 0

30 lb

50 lb

F=0.4 N N

P = 32 lb

Chapter 8: Friction

GROUP PROBLEM SOLVING _________________________________________________________________________________________

Case b (block B slides over A):

 +  Fy = N cos 20 + 0.6 N sin 20 – 30 = 0 N = 26.20 lb  +  Fx = – P + 0.6 ( 26.2 ) cos 20 – 26.2 sin 20 = 0 P = 5.812 lb Case b has the lowest P (case a was 32 lb) and thus will occur first. Next, using a frictional force analysis of belt, we get: W D = P e   = 5.812 e 0.5 ( 0.5  ) = 12.7 lb A Block D weighing 12.7 lb will cause the block B to slide over the block A. Chapter 8: Friction

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