1. Resistance in Mechanical Systems
Unit 4 – Subunit 1

2. When an object rests on a floor, the weight of the object is a force that pushes down on the floor.
At the same time, the floor pushes back up on the object to counter the objects force.
The object is in vertical equilibrium

3. Vertical Equilibrium Equilibrium is when two forces are in balance.
Vertical equilibrium is when the weight of an object is countered by an opposing force.

4. The force that pushes back (or up) is called the Normal Force (N) because it is perpendicular to the surfaces of contact.
Examples of normal forces are:
floor
hand
lift
thrust

5. Can an object in equilibrium be moving?
Remember
When an object rests on a floor, the weight of the object is a force that pushes down on the floor.
At the same time, the floor pushes back up on the object to counter its force.
The object is in vertical equilibrium
Question:
Can an object in equilibrium be moving?
Yes – think of being in an elevator.
It also can be moving horizontally > > > > >

6. Friction
Friction is a force that opposes the force that causes motion.
Friction is caused by the roughness of a surface

7. The two types of friction are :
Static – stationary friction
this is the initial friction on a non moving object with a force applied.
Kinetic – moving friction
This is the friction that is generated as the object moves.

8. The amount of friction depends upon:
Nature of the surfaces in contact
Force that presses the two surfaces together
Friction does not depend upon:
The rate at which the objects are moving (unless its stationary vs moving)
Size of the contact area

9. The Coefficient of Sliding Friction
MEW
tomorrow

10. The amount of friction depends upon:
Remember:
The amount of friction depends upon:
Nature of the surfaces in contact
Force that presses the two surfaces together
The ratio of the frictional force to the normal force is called the “Coefficient of Friction”

11. μ
Coefficient of Friction “μ“
Pronounced Mew & spelled mu
μ represents the measure of the frictional characteristic between the surfaces of materials
The higher the value of μ, the more frictional force is present when sliding occurs (or movement)

12. The formula for the coefficient of friction:
μ = f / N
Where f = the frictional force generated
N = the Normal force (= to weight)

13. (in order of increasing resistance)
The three μ’s
(in order of increasing resistance)
μs = coefficient of static friction
μk = coefficient of kinetic friction
μr = coefficient of rolling friction

14. See example 4-A and 4-B on page 11
Complete problems 1,2 and 6,7,8 on pages 22 & 23. These problems deal with mechanical resistance.
Tomorrow:
Effects of lubricants
Rolling friction
Drag in fluid systems

15. Lubricants in Mechanical Systems
Lubricants protect sliding surfaces by providing a thin layer of fluid between them.
The lubricant between two surfaces does three things:
Reduces friction
Reduces wear
Reduces heat

16. Drag
Drag is a mechanical resistance produced when an object moves through a fluid.
A fluid can be either a liquid or a gas

17. The formula for Drag Resistance is: Rd = F/v Where:
Rd = drag resistance
F = drag force
V = speed (velocity)

18. Assuming constant velocity, an airplane will be in both horizontal and vertical equilibrium as it moves through the air

19. Streamlining
Streamlining reduces Drag
Streamlining reduces turbulence

20. Complete problems 2, 3,4, 8 & 9 - pg 22/23
Complete “lets Review” on top of page 22 (units)
Complete “Student Exercises” on page 17