1. Forces
And Friction

2. Things move… but why?
What would cause something at rest to start moving? OR
What would cause something moving to change velocity (accelerate)?

3. THINK!
In the clip, what causes the tractor to start moving?

5. FORCE: action exerted on a body in order to change the body’s state of rest or motion.
A push or a pull
Cause acceleration or deceleration! (You can’t change your motion without a force)
Force is a vector (has magnitude and direction).
Units are newtons (N)
50 east N

6. NET FORCE: the combination of all forces acting on an object.
If net force is zero, object will NOT accelerate (it stay at rest or constant velocity).
Force of the bat’s legs holding it up
Force of gravity pulling down

7. BALANCED FORCES: More than one force acting on an object that
has a net force equal to zero
Forces are equal in size and opposite in direction
Objects affected by balanced forces either:
Do NOT MOVE or
Move at CONSTANT VELOCITY (don’t accelerate)
100 N east
100 N west
Net force = 0 N
100 N west
100 N east
Net force = 0 N

8. UNBALANCED FORCES: More
than one force acting on an object
that has a net force UNEQUAL to
zero
Have a net force that does NOT equal zero
Must be present to cause any change in an object’s state of motion or rest.
Causes the velocity to change (acceleration or deceleration)
west
east
Net force = 3 N east

9. THINK! Balanced or unbalanced? A parked car:
A car at a constant speed:
A car accelerating:

10. 0 N NO! YES! YES! (negative) Balanced or Unbalanced? Net Force =
100 N right
100 N left
NO!
Will the truck accelerate?
Balanced or Unbalanced?
Net Force =
40 N right
100 N right
60 N left
YES!
Will the truck accelerate?
Balanced or Unbalanced?
Net Force =
15 N south
south
Will the plane accelerate?
YES! (negative)
north

11. FRICTION

12. THINK! How do you know friction is at work in this clip?

14. Occurs between ALL surfaces
FRICTION: Force that opposes motion between two surfaces that are in contact.
Occurs between ALL surfaces
Molecular Adhesion
Surface roughness
Opposes the applied force.
Converts kinetic energy to heat energy
For motion to change, the force applied to an object must be greater than friction
100 N west
Net force = 0 N
100 N east

15. Net Force ≠ 0 N
Net Force = 0 N
Net Force = 0 N

16. TYPES OF FRICTION
STATIC FRICTION: resists the initiation of motion between two surfaces that are in contact and at rest.
Static friction is greater than kinetic friction.
KINETIC FRICTION: opposes the movement of two surfaces that are in contact and are in motion.
There are 3 types of kinetic friction you need to know:
SLIDING FRICTION: Two objects slide past each other
ROLLING FRICTION: Round object rolling over a flat surface
FLUID FRICTION: Force that resists the movement of a solid object through a fluid (gas or liquid).

18. THINK!
What are the 2 types of friction?
How are they different?

19. Sliding Friction
Rolling Friction
Fluid Friction

20. Drag is fluid friction that affects cars, airplanes, boats, etc.

21. THINK!
In each clip what type of friction do you see? How do you know?
Clip 1:
Clip 2:
Clip 3:

25. Friction can be helpful
Necessary to walk, hold an object, strike a match, etc.
Important for braking
Friction can be harmful
Causes excessive heating or wear of moving parts
Decreases fuel efficiency

26. WAYS TO REDUCE HARMFUL FRICTION
Replace sliding friction with rolling friction
Use lubricants
Use ball bearings
Make surfaces smoother

27. WAYS TO INCREASE HELPFUL FRICTION Make surfaces rougher or textured
Add salt/sand
Add tread to tires or shoes
Increase pressure on two objects
(Example: adding sandbags to a truck bed)

28. THINK! What is one way that friction can be good?
What is one way that friction can be bad?

29. Exit Ticket #25-28

30. 25. The combination of all of the forces acting on an object is called
a. total force
b. union of forces
c. combined force
d. net force

31. 26. An object that is stationary or moving at a constant velocity to the left has a net force of
a. 100 N left
b. 100 N right
c. 0 N
d. None of the above

32. 27. A tug-of-war that results in one team pulling the other across the line is an example of
a. action forces
b. reaction forces
c. balanced forces
d. unbalanced forces

33. c. It accelerates due to a force of 25 N east
28. Two friends are trying to push a box in opposite directions. One friend pushes with a force of 75 N east. The other friend pushes with a force of 100 N west. Does the box accelerate due to a force?
a. No, it does not accelerate
b. It accelerates due to a force of 15 N west
c. It accelerates due to a force of 25 N east
d. It accelerates due to a force of 25 N west