1. OBJECTIVES Sec II Describe forces & explain how forces act on objects
Determine the net force when more than one force is acting on an object
Compare balanced and unbalanced forces
Describe ways that unbalanced forces cause changes in motion

2. II. What is a force? A. Force= a push or a pull
Has both size and direction
Can change the acceleration (speed or direction)of an object
newton (N) = units force is measured in
Remember: 1 N = 100g

3. B. Forces Acting on Objects
All forces act on an object
You cannot push or pull on nothing
Applying a force does NOT mean that there has to be motion
Pushing against the wall
Sitting in a chair
Forces can be unseen
Static electricity
Gravity
Magnetism

4. C. Determining Net Force
Net force= combination of all forces acting on an object
Forces in the same direction will be added together to figure out the net force
Forces in the opposite direction will be subtracted from each other to get the net force

5. Balanced forces = zero
When the forces are equal in opposite directions
There is no movement
Sitting on a chair

6. Unbalanced forces produce a change in motion
Change in motion could be speed or direction
Fnet does NOT equal 0 N
Cause an unmoving object to move

7. Surface of any object is rough (even if it feels smooth)
III. Friction: A force that opposes motion between 2 surfaces that are in contact A. The source of friction
Surface of any object is rough (even if it feels smooth)
When 2 surfaces come into contact with each other they will “stick together”
Microscopic hills and crevices “stick & slip”
2 factors that effect the amount of friction:
The force pushing the surfaces together
the more weight the more force pushing down the more friction
How rough the surface is
rougher equals more friction
because of more “hills &
valleys” to cause the
“stick & slip”

8. B. Types of Friction: https://youtu.be/VUfqjSeeZng
Kinetic friction: friction between moving surfaces
Sliding friction is usually greater than rolling friction (depending on the surface)
Breaks on a car or bike
Writing with a pencil
Rolling friction is used in transportation

9. Static means= not moving
Static friction: force applied to an object but does not cause the object to move
Static means= not moving
Force of the static friction balances the applied force
Leaning an object against the wall and it doesn’t fall
Static friction can be overcome with a large enough force
As soon as movement occurs static friction disappears & kinetic friction immediately occurs

10. https://youtu. be/AX_lCOjLCTo (phone book friction) https://youtu
(phone book friction) C Friction: Helpful or Harmful???
Without friction it would be hard to walk, drive a car, or write on paper
Friction wears down engine parts, can cause “rug” burns, & leads to erosion of land

11. To reduce friction lubricants are used
Substances that are applied to surfaces to reduce friction
Motor oil in an engine
Air on an air hockey table
Sanding wood

12. Making the surface rough also increases friction
To increase friction you could increase the force used to push against something
Making the surface rough also increases friction
Rosin
Batting gloves
Sand/salt on icy roads