1. What is a force? How does friction affect motion?
Motion, Forces and Friction
What is a force?
How does friction affect motion?

2. Motion
A change in position, also known as an object’s displacement

3. Speed (rate) Rate of change in position How fast or how slow
Measured in m/s (meters per second)

4. Velocity
Speed with a direction
Vector

6. Constant speed A speed that does not vary
Graph – a straight line, no turns

7. Acceleration Rate of change in velocity Change in speed over time
Change in direction

8. Acceleration Units for acceleration – m/sec2
Acceleration refers to increasing speed, decreasing speed (deceleration, negative acceleration), or changing direction

9. Calculating Acceleration
Acceleration = Final speed – Initial speed
Time
A plane travels from 0 m/s to 24 m/s in 3 sec. What is its acceleration?
Acceleration = 8 m/s2

10. Calculating Acceleration, alt.
Force = mass x acceleration
Acceleration =
A force of 6 N is applied to an object that has a mass of 3 kg. What is the acceleration?
Acceleration = 2 m/s2

11. What is a Force? A force is a push or a pull.
All forces have two properties:
Direction and Size

12. A Newton (N) is the unit that describes the size (magnitude) of a force.

13. The student is pushing down on the chair, but the chair does not move.
The floor is balancing the force by pushing on the chair, which in turn pushes on the student.

14. What is friction?
Friction is a force that opposes motion between two surfaces that touch each other.
A force between two surfaces that touch and are sliding, or trying to slide across one another, for example when you try to push a toy car along the floor.

15. Friction always works in the direction opposite from the direction the object is moving, or trying to move. It always slows a moving object down.
Moving
Friction

16. The surface of any object is rough.
Even an object that feels smooth is covered with tiny hills and valleys.
The contact between the hills and valleys of two surfaces causes them to stick, resulting in friction.

17. Friction converts kinetic (motion) energy into thermal (heat) energy.

18. Principles of Friction
Friction acts parallel to the surfaces that are in contact.
The direction that friction acts is OPPOSITE the direction of the motion (or intended motion).
Fnormal
Ffriction(k)
Fapplied
Fweight

19. What Affects Friction? The amount of friction depends on:
Roughness of the surfaces
Force pushing the surfaces together

20. What Creates More Friction?
Rough Surfaces
Rough surfaces have more microscopic hills & valleys.
The rougher the surface, the greater the friction will be.
Greater Force (caused by increased mass)
The amount of force exerted by the object increases the amount of friction
Increase the amount of force exerted by the object by increasing the mass of the object

21. What Does Not Affect Friction?
Surface Area
Changing the amounts of the surfaces that touch does NOT change the amount of friction
4 N
4 N

22. What Does Not Affect Friction?
5 m/s
20 m/s
2 N
2 N
Velocity
Friction forces are independent of speed or velocity
The force of kinetic friction is the same at 5 m/s as it is for 20 m/s.

23. Types of Friction There are two types of friction:
Static Friction – the frictional force present just before motion begins.
Kinetic Friction – the frictional force present with motion
Note: Static Friction is usually higher than Kinetic Friction

24. Static Friction
Static friction occurs when force applied to an object does not cause the object to move.
In order to move something, you must first overcome the force of static friction between the object and the surface on which it is resting.

25. Kinetic friction occurs when force is applied to an object and the object moves.
Examples:
Sliding Friction: pushing an object across a surface
Rolling Friction: between wheels and a surface
Fluid Friction: opposes the motion of objects traveling through a fluid (air or water), also called drag force or air resistance

26. Reducing Friction
Reduce the contact area by using rollers/ball-bearings/wheels
Change the surfaces of the materials that are touching by using lubrication like motor oil, wax, or grease
Roll objects instead of sliding them
Create a cushion of air
Eg. Like a hovercraft or air hockey table

27. Friction Can Be Harmful or Helpful
Harmful friction
Holes in socks or knees of jeans
Wind & water cause erosion
Helpful friction
Friction between pencil & paper
Without friction, you would just slip & fall when you tried to walk

28. Useful Friction
Friction can be a useful force because it prevents our shoes slipping on the pavement when we walk and stops car tires skidding on the road.

29. Ice causes very little friction, which is why it is easy to slip over on an icy day. But this is a good thing for ice skating and sledding.

30. Recall the Sliding Penny
4/18/2008
Recall the Sliding Penny
Why do things not continue to move at constant velocity?
Friction
If the sliding penny slows down, what’s the force responsible?
Lecture 8

31. How could I keep it moving at a constant velocity? Reduce the friction
4/18/2008
How could I keep it moving at a constant velocity?
Reduce the friction
Do I need to apply a force to keep it moving? Why?
No, inertia will keep it moving once the initial force was applied.
Lecture 8

32. How is friction measured?
Measures of friction are based on the type of materials that are in contact.
Each material has what is known as a coefficient of friction. The coefficient is a measure of how easily one object moves in relationship to another.
When you have a high coefficient of friction, you have a lot of friction between the materials.
Concrete on concrete has a very high coefficient of friction.
A material such as Teflon (on most things) has a very low coefficient. Teflon is used on surfaces where we don't want things to stick; such as pots and pans.

33. Standard Friction Equation
Friction = coefficient of friction x normal force.
Normal force comes into play any time two objects are in direct contact with one another, and always acts perpendicular to the object that applies the force.
The simplest example of the Normal force can be seen in the situation of an object on a horizontal surface.
If the object is on a horizontal surface, then Normal force = Weight

34. Friction and the Normal Force
The force required to overcome static or kinetic friction is proportional to the normal force, n.
fs = msn
fk = mkn

35. μ μk μs = Symbol for the Greek letter mu, used in
the formula to represent the coefficient. μk
= coefficient of kinetic friction μs
= coefficient of static friction FN
= the normal force

36. Sample Coefficients of Friction
4/18/2008
Sample Coefficients of Friction
Material on Material
s = static friction
k = kinetic friction
steel / steel
0.6
0.4
add grease to steel
0.1
0.05
metal / ice
0.022
0.02
brake lining / iron
0.3
tire / dry pavement
0.9
0.8
tire / wet pavement
0.7
Spring 2008
Lecture 8

37. Fk = µ kFN Fk = µ kFN Fk = (0.45)(10.0)(9.8 m/s2) Fk = 44 N
The coefficient of kinetic friction between a block and the level surface it slides on is 0.45.
If the mass of the block is 10.0 kg, what is the minimum force needed to keep the block moving with uniform motion?
Fk = µ kFN
Fk = µ kFN
Fk = (0.45)(10.0)(9.8 m/s2)
Fk = 44 N
The applied force need only balance the kinetic frictional force in order to maintain uniform motion.

38. A student pulls a 5.00 kg object and discovers that she
needs to exert 30.0 N of force before the object moves.
What is the coefficient of static friction between the
object and the surface on which it rests?
Since the applied force is equal to 30 N just before moving, the friction force must be equal in size (30 N).
Therefore:  = 30 N ÷(5 kg)(9.8 m/s2)
 = 0.61