1. Forces

2. Force: (a push or a pull) An action exerted on an object
in order to change the state of rest or motion of an object.
Measured in Newtons (N)

3. 2 Categories of Forces Contact Forces: two interacting
objects are physically in
contact with each other.
At-a-distance Forces:
two interacting objects are
not in physical contact but
are able to exert a push or pull

4. Force of Gravity The Earth pulls you downward
Any object near Earth is under it’s pull

5. Force of Tension (FT)
Contact force acting through a string, chain, etc…

6. Friction Friction Without friction
When surfaces slide over one another friction acts
Direction of friction always opposing motion
Without friction
Tires of a car would not be able to push against the ground and move car forward
You would not be able to grip objects
You would slip and fall when you tried to walk

7. Force of Friction From sliding on a surface
Always goes in the opposite direction of motion (or the opposite direction an object would move)

8. Normal Force
Why doesn’t the book fall to the floor?

9. Normal Force Surface Force that acts perpendicular to object
Consider the following: kg mass on a table
We know from Newton’s 3rd law the table has to….
Push upward with the same force
How does the block know?
What if it didn’t push hard enough?
What if it pushed too hard?

10. Normal Force (FN)
Reactive Force

11. Types of Forces WRITE THESE DOWN!!! Fg = Force of Gravity
FA = Force Applied
FT = Force of Tension
Ff = Force of Friction
FN = Normal Force (“Support”)

12. Free Body Diagrams Identifies all of the Forces acting on an object
Each Diagram has a Vector Arrow:
Size of arrow: Shows the magnitude of the force
Direction of arrow: Shows direction of force

13. Example: Book on Table FN Draw a circle around your object
Fg force on book by the Earth
Support force on book by table Fg

14. Example: Continued
1) Push Block
2) Block Moving
3) Book stops

15. A baseball is in free fall traveling toward a person
A baseball is in free fall traveling toward a person. Neglect air resistance.

16. of all forces acting upon an object.
Free Body Diagrams
Diagrams used to show the:
magnitude (how much) and
direction
of all forces acting upon an object.

17. A cat is at rest on a table top
A cat is at rest on a table top. The free-body diagram for this situation is shown below:

18. A rightward force is applied to a Piano in order to move it across a desk at constant velocity. Consider frictional forces.

19. Force: Net Force: An action exerted on an object (a push or a pull)
in order to change the state of rest or motion of an object.
Measured in Newtons (N)
Net Force:
Combination of all the forces acting on an object. (add them up)

20. Balanced Forces
If all of the pushes and pulls are even on an object, then the object “keeps on doing what it was doing”
Tug-o-war
Two teams that are evenly matched play tug-o-war
NOTHING HAPPENS

21. Balanced Forces Mechanical Equilibrium: when net force is zero
Does not cause a change
in object’s motion

22. Unbalanced Forces Unbalanced forces cause objects to change velocities
Tug-o-war
Two teams that are unevenly matched play tug-o-war
One team will win

23. Unbalanced Force
What happens when an object has an unbalanced force acted upon it?
Unbalance forces cause acceleration!!

24. If the forces are… Equal – Unequal – Nothing happens
Ex: A book sitting on a shelf
Fg = 25 N
FN = 25 N
Unequal –
The object changes what it is doing
Ex:
FA = 25 N right
FA = 100 N left

25. A rightward force is applied to a plate in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.

26. A rightward force is applied to a box in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance

27. A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration.