1. WEDGES AND FRICTIONAL FORCES ON FLAT BELTS
Today’s Objectives:
Students will be able to:
a) Determine the forces on a wedge.
b) Determine the tensions in a belt.
In-Class Activities:
Check Homework, if any
Reading Quiz
Applications
Analysis of a Wedge
Analysis of a Belt
Concept Quiz
Group Problem Solving
Attention Quiz
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

2. 1. A wedge allows a ______ force P to lift a _________ weight W.
READING QUIZ
1. A wedge allows a ______ force P to lift
a _________ weight W.
A) (large, large) B) (small, small)
C) (small, large) D) (large, small) W
2. Considering friction forces and the indicated motion of the belt, how are belt tensions T1 and T2 related?
A) T1 > T B) T1 = T2
C) T1 < T D) T1 = T2 e
Answers:
1. C
2. C
All quizzes (Reading, Attention, Concept):
The questions are numbered.One space is left between the numeral and question text. Do not use automatic bullets as this is not compatible with ’97 version of PowerPoint
Answer choices are listed by upper case alphabets A, B, C, D, enclosed by right parenthesis. Single space is Left between the Alphabet and the Answer choice. The choices have hanging indentation aligned with the starting of the question text.
For multiple fill up the blanks,answers are enclosed in parenthesis. (see #1 above)
Figures/Pictures are positioned on the right side of the slide.
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

3. How can we determine the force required to pull the wedge out?
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?
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

4. APPLICATIONS (continued)
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?
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

5. APPLICATIONS (continued)
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?
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

6. ANALYSIS OF A WEDGE
A wedge is a simple machine in which a small force P is used to lift a large weight W. 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.
As noted earlier, important text is given a blue color and underlined.
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

7. ANALYSIS OF A WEDGE (continued)
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.

8. ANALYSIS OF A WEDGE (continued)
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.

9. ANALYSIS OF A WEDGE (continued)
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. W

10. 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!!

11. Find: The smallest force P needed to pull out the wedge.
EXAMPLE
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.
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

12. Solving the above two equations, we get
EXAMPLE (continued)
The FBDs of crate and wedge are shown in the figures. Applying the EofE to the crate, we get NB
300 lb
FB=0.3NB
FC=0.3NC NC
15º
FD=0.3ND ND P
FBD of Crate
FBD of Wedge
Known forces are indicated by blue color arrow, Unknown forces indicated by yellow color.
The thickness of arrow lines are 2 1/4 pt.
Bolden the Decimal Point
Symbols used in the equations are from the Symbols library.
do not use sentence case on ‘cos’ and ‘sin’, they are to be in lower case.
+  FX = NB – 0.3NC = 0
+  FY = NC – NB = 0
Solving the above two equations, we get
NB = lb = 82.6 lb, NC = lb = 275 lb
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

13. Applying the E-of-E to the wedge, we get
EXAMPLE (continued)
NB = 82.6 lb
300 lb
FB=0.3NB
FC=0.3NC
NC = 275 lb
15º NC
FD=0.3ND ND P
FBD of Crate
FBD of Wedge
Applying the E-of-E to the wedge, we get
+  FY = ND cos 15 ND sin 15 – = 0;
ND = lb = 264 lb
+  FX = 0.3(263.7) + 0.3(263.7)cos 15 – 0.3(263.7)cos 15 – P = 0;
P = lb
Known forces are indicated by blue color arrow, Unknown forces indicated by yellow color.
The thickness of arrow lines are 2 1/4 pt.
Bolden the Decimal Point
Symbols used in the equations are from the Symbols library.
do not use sentence case on ‘cos’ and ‘sin’, they are to be in lower case.
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

14. A) Be lifted up B) Slide down C) Not be lifted up D) Not slide down
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 The boy will ______ ?
A) Be lifted up B) Slide down
C) Not be lifted up D) Not slide down
Answers:
1. C
2. C The maximum force she can apply before she slides is 150 (0.6) = 90 lb. This will not be enough to lift the boy up as she needs to apply more than 100 lb to lift the boy up. Without knowing the coefficient of friction between the cliff and the rope, we can not say whether the boy will slide down or not.
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

15. 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.
Plan:

16. GROUP PROBLEM SOLVING (continued)
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.

17. GROUP PROBLEM SOLVING (continued)
Case a (both blocks sliding together):
+  FY = N – 80 = 0
N = 80 lb
+  FX = 0.4 (80) – P = 0
P = lb P B A N
F=0.4 N
30 lb
50 lb

18. GROUP PROBLEM SOLVING (continued)
 +  Fy = N cos 20 N sin 20 – 30 = N = lb
 +  Fx = – P ( 26.2 ) cos 20 – 26.2 sin 20 = P = lb
20º
30 lb
0.6 N P N
Case b (block B slides over A):
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
WD = P e   = e 0.5 ( 0.5  ) = lb
A Block D weighing 12.7 lb will cause the block B to slide over the block A.

19. A) The wedge B) The block
ATTENTION QUIZ
1. When determining the force P needed to lift the block of weight W, it is easier to draw a FBD of ______ first.
A) The wedge B) The block
C) The horizontal ground D) The vertical wall W
2. In the analysis of frictional forces on a flat belt, T2 = T1 e  . In this equation,  equals ______ .
A) Angle of contact in degrees B) Angle of contact in radians
C) Coefficient of static friction D) Coefficient of kinetic friction
Answers:
1.A
2.B
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5

20. End of the Lecture Let Learning Continue
In the last slide add the ‘home’ action button
Statics:The Next Generation (2nd Ed.) Mehta, Danielson, & Berg Lecture Notes for Sections 8.3,8.5