1. ES2501: Statics/Unit 24-1: Friction Related Problems
Introduction
Friction – force of resistance against motion or impending motion on
and tangent the contact surface of two bodies.
Classification of friction:
against relative
motion
Fluid friction
Friction
Kinetic friction
Dry friction
Static friction
Theories of friction:
Several theories exist.
For examples:
against impending
relative motion
studied in Statics
Impact between two rough
surfaces causes friction
bonding force between two surfaces
causes friction

2. ES2501: Statics/Unit 24-2: Friction Related Problems
Law of Dry Friction
Friction exists on the contact surface If B moves or has impending motion relative to A B A
Case 1: In Motion
Kinetic friction
Case 2: Impending Motion
static friction
Direction of motion relative to A
Direction of impending motion trlstivr to A B B
Normal pressure
of A on B
Kinetic
Friction
of A on B
Normal Pressure
of A on B
Static Friction
of A on B
static friction
coefficient
Kinetic friction
coefficient
Magnitude:
Magnitude:
Direction: Opposite to the direction
of the impeding motion
Direction: Opposite to the
direction of motion
Maximum static friction --- a limit .
Not necessarily the true static friction

3. ES2501: Statics/Unit 24-3: Friction Related Problems
Angle of Static Friction
Static friction can be characterized by either the static friction coefficient of the angle of static friction
Direction of the impeding motion
Maximum static Friction
Normal Pressure
Resultant of friction and pressure
Materials of contact
Metal on ice
Wood on wood
Leather on wood
Leather on metal
Aluminum on Aluminum

4. ES2501: Statics/Unit 24-4: Friction Related Problems
Example 1:
Find the friction force when F=5N and 10N, respectively.
FBD:
Equilibrium equations:
Case 1:
Case 2:
Note:
gives the limit but not true static friction;
gives true kinetic friction,
satisfied
Not-satisfied
Static friction
Kinetic friction in motion

5. ES2501: Statics/Unit 24-5: Friction Related Problems
Example 2: Three concrete pipes of weight W and radius r are stacked on the ground. Determine the minimum static friction coefficient such that the pile doesn’t collapse. A B C
FBD and Equilibrium equations: A C B

6. ES2501: Statics/Unit 24-6: Friction Related Problems
Example 3: A wedge of an angle is used to split a log. Find the splitting force when the wedge is hit by P and the force to keep the wedge in the position in the log.
FBD and Equilibrium equations: A
Load
Friction
To advance
normal force
Wedge angle
Splitting force generated by P
Force required to keep wedge in position

7. ES2501: Statics/Unit 24-7: Friction Related Problems
Example 3: Friction on flat belts
Proof:
Small T1 can hold large T2