1. Forces, Free Body Diagrams, and Newton’s Laws of Motion
(Oh my!)

2. Forces
Force is:
a push or pull upon an object resulting from the object's interaction with another object.
any action that has the ability to change an object’s motion.
a vector quantity.
measured in Newtons.
***Remember, you only have to write the underlined portion in your notes.

3. Forces
All forces (interactions) between objects can be placed into two broad categories:
Contact forces
Forces resulting from “action-at-a-distance”

4. Forces Contact forces are those types of forces
that result when the two interacting objects are perceived to be physically contacting each other.
Action-at-a-distance forces are those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation.

5. Examples of Forces Contact Forces Action-at-a-Distance Forces
Frictional Force
Tension Force
Normal Force
Air Resistance Force
Applied Force
Spring Force
Action-at-a-Distance Forces
Gravitational Force
Electrical Force
Magnetic Force

6. Important Forces to Remember…
Normal force is the supporting force and is always perpendicular to the supporting surface.
Friction is the force exerted by a surface as an object moves across it or makes an effort to move across it.

7. We draw them!!!!
#physics

8. Free Body Diagrams This book is being pushed across the
Go over the forces shown. Reiterate normal force.
This book is being pushed across the
top of a table.
Here is a free body diagram of the
scenario shown to the left.

9. Free Body Diagrams Free Body Diagrams (FBDs)
are tools used to analyze physical situations
Free Body Diagrams show all the forces acting on a single object
The net force is the sum of all forces acting on an object

10. Free Body Diagrams The object itself may be drawn as a circle or box
Draw and label all the external forces on the object
The only “rule” for drawing free-body diagrams is to depict all the forces that exist for that object in the given situation.

11. Free Body Diagrams
A circus elephant falls off a tight rope. Neglecting air resistance, draw a free body diagram for the falling elephant.

12. Free Body Diagrams
Fgrav
Remember that we ignored air resistance.

13. Free Body Diagrams
A circus elephant falls off a tight rope. Including air resistance, draw a free body diagram for the falling elephant.

14. Free Body Diagrams
Fair
Fgrav
This time we included air resistance.

15. Free Body Diagrams
Draw an FBD of the scenario below. Assume the dog is moving right. Include air resistance.

16. Free Body Diagrams
Fair

17. Free Body Diagrams
Free Body Diagrams are useful to quickly see if there are unbalanced forces on an object.
If an object is in equilibrium, the net force is zero.
*An object can still be in motion while in equilibrium
If the forces are unbalanced, then acceleration occurs.

18. Free Body Diagrams
Math Time!!!

19. Let’s calculate the magnitude and directions of this FBD!
Free Body Diagrams
Let’s calculate the magnitude and directions of this FBD!
Assign positive and negative directions in both the horizontal and the vertical.
Can assign a positive and negative direction.
Vertically
Up is +
Down is –
Horizontally
Right is +
Left is –

20. Let’s calculate the magnitude and directions of this FBD! Yay!
Free Body Diagrams
Let’s calculate the magnitude and directions of this FBD! Yay!

21. Let’s calculate the magnitude and directions of this FBD! Woohoo!
Free Body Diagrams
Let’s calculate the magnitude and directions of this FBD! Woohoo!
Pythagorean theorem.

23. Free Body Diagrams
Suppose three people are trying to keep an injured polar bear in one place. Each person has a long rope attached to the bear. Two people pull on the bear with forces of 100 N each . What force must the third person apply to balance the other two? The bear will not move if the net force is zero.
Pythagorean theorem. Remind students that they can move vectors to form a triangle.

25. Newton’s Laws of Motion
Isaac Newton-
developed the laws of motion
discovered that gravity is universal

26. And now for the first one…
Suppose you want to move a box from one side of the room to the other. What would you do?

27. Newton’s 1st LOM

28. You would push or pull it across the room.
Newton’s 1st LOM
You would push or pull it across the room.
In physics terms, you would apply a force to the box.

29. Newton’s 1st LOM

30. Forces have the ability to :
Newton’s 1st LOM
Forces have the ability to :
Increase the speed of an object
Decrease the speed of an object
Change the direction in which an object is moving

31. Newton’s 1st LOM
Anytime there is a change in motion, a force must exist, even if you cannot immediately recognize the force.

32. Newton’s 1st LOM
Newton’s first law states that an object will continue indefinitely in its current state of motion, speed, and direction, unless acted upon by a net force.

33. Newton’s 1st LOM
Generally speaking, objects tend to keep doing what they are doing; if they are moving they keep moving, if they are at rest, they stay at rest.
Demos -

34. Newton’s 1st LOM
Inertia is a term used to measure the ability of an object to resist a change in its state of motion.

35. Newton’s 1st LOM
Because inertia is a key idea in Newton’s first law, the first law is sometimes referred to as the law of inertia.

36. The amount of inertia an object has depends on its mass.
Newton’s 1st LOM
The amount of inertia an object has depends on its mass.
More mass=more inertia Less mass=less inertia

37. Newton’s 1st LOM
An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop.

38. Newton’s 1st LOM
Which one would you rather have to push to the gas station? Why?

39. Newton’s 2nd LOM
Newton’s 2nd law states: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

40. Newton’s 2nd LOM
If you apply more force to an object, it accelerates at a higher rate.
If the same force is applied to an object with greater mass, the object accelerates at a lower rate because mass adds inertia.

41. Newton’s 2nd LOM
Math Time!!!

42. F = ma Newton’s 2nd LOM (Force = mass x acceleration)
Force is measured in Newtons (N)
Mass is measured in Kg
Acceleration is measured in m/s2
***Remember that acceleration is speeding up, slowing down, or turning

43. Newton’s 2nd LOM
One Newton of force is needed to give a mass of one kilogram an acceleration of one meter per second squared
1 N = 1 kg * m/s2
So, if you see units of kg*m/s2, you know that they are the equivalent of Newtons (N)

44. Force = mass x acceleration Find the missing variable
Net Force (N)
Mass
(kg)
Acceleration (m/s2)
1. 10 2
(A)
2. 20
(B)
3. 20 4
(C)
4. (D) 5
5. 10
(E) 10

45. Newton’s Second Law of Motion
Force is equal to ________ x ___________.
Acceleration is directly proportional to ________.
Acceleration is inversely proportional to_______.

46. And now, the 3rd one…
Newton’s 3rd law:
For every action, there is an equal and opposite reaction

47. Newton’s 3rd LOM Forces always occur in action-reaction pairs
One force is called the action force. The other force is called the reaction force

48. Newton’s 3rd LOM
The action and reaction forces are equal in strength and opposite in direction
Forces always come in pairs

49. Newton’s Third Law of Motion
When you ride a skateboard, strictly speaking, what is the force that moves the skateboard?

50. Newton’s Third Law of Motion
All forces come in
pairs. When you push
on the ground (action),
the ground pushes back (reaction)
on your foot.

51. Newton’s Third Law of Motion
Why doesn’t
your foot make the ground move?
Earth is too massive 51

52. Newton’s Third Law of Motion
Forces still come in pairs even when objects are NOT moving.

53. Newton’s Third Law of Motion
When your book is on the table what forces are acting upon it?

54. Newton’s 3rd LOM Fnorm Fgrav
Even though there is no movement, there are still forces acting on the book.

55. What is the net force on the object and in what direction would it move?

56. Newton’s 3rd LOM
Remember to assign a positive and negative direction in both the vertical and horizontal.
Vertically
Up is +
Down is –
+150
-50
+100
Horizontally
Right is +
Left is –
+100
-100
Fnet is 100 N moving in the upward direction.

57. Newton’s 3rd LOM
If you push with equal and opposite forces on opposite sides of an object at rest, what will happen to the object?

58. Newton’s 3rd LOM
It won’t move!

59. Newton’s Third Law of Motion
Equilibrium - When the net force on an object is zero.
The object can be moving in a constant speed or motionless

60. This gymnast is not moving so the net force must be zero.
If the weight of the gymnast is 700 N, then each rope must pull upward with a force of 350 N in order to make the net force zero.

61. A woman is walking two dogs on a leash
A woman is walking two dogs on a leash. If each dog pulls with a force of 80 Newtons, how much force does the woman have to exert to keep the dogs from moving?
160 N