1. Newton’s Laws

2. Force  push or a pull Contact Force  physical contact
Field Forces  No contact: gravity, magnetic force, etc.

3. Inertia

4. Inertia Newton’s 1st Law
An object wants to keep on doing what it is already doing
In order to change, it needs a net force not equal to zero
= Sum of all forces

5. Newton’s 2nd Law: Acceleration
When you do have a net force…
object will accelerate in direction of the net force
This acceleration is proportional to the net force and inversely proportional to its mass

6. Exploring 2nd Law Put the two together: mass units: kg
acceleration units: m/s2
This makes force units:
1 kg•m/s2 is called a Newton (N)

7. Mass is not Weight Weight is a common force
Weight is the force exerted by gravity on an objects mass
Weight can change if gravity changes

8. Mass: 70 kg
Weight: 700 Newtons
(157 pounds)
Mass: 70 kg
Weight: 112 Newtons
(25 pounds)
Mass is Same!!

9. MASS
WEIGHT
•Mass is a measure of how much matter makes up an object
Depends on object itself – NEVER CHANGES
Mass Is a measure of inertia
•Mass is measured in: grams, Kilograms
• Weight is a Force
• Depends on object’s location
Weight is measured in: Newtons (N) or pounds (lbs)

10. Mass vs. Weight
You can convert mass into weight using Newton’s Second Law
On Earth, a = 9.8m/s2

11. Example 1: An object has a mass of 10.0 kg, find its weight

12. Example 2: A 2500 kg car is pushed with a 250 N force, what is the acceleration acting on the car due to the force? If the force is doubled, what will happen to the acceleration?

13. Example 3: An artillery shell has a mass of 55 kg
Example 3: An artillery shell has a mass of 55 kg. The projectile is fired from the piece and has a velocity of 770 m/s when it leaves the barrel. The gun barrel is 1.5 m long. Assuming the force and therefore the acceleration is constant while the projectile is in the barrel, what is the force that acted on the projectile?

14. Equilibrium vs. Static Equilibrium
If an object is in equilibrium it is not acceleration (no change in motion)
If an object is in Static equilibrium it is at rest (not moving)
An object is in equilibrium if the sum of the forces acting on the object is zero

15. Free Body Diagrams Identify all forces acting upon an object
Show direction and relative size of forces
Vectors originate from the center of mass

16. Only # 3 1 2 3 NONE! b/c it’s possible that they are all moving
Which boxes are in equilibrium? Which boxes are definitely in static equilibrium?
NONE! b/c it’s possible that they are all moving 1 2 3

17. Free Body Diagrams
Scenario 1:
10 kg block going upwards
10 kg

18. Y Free Body Diagrams Scenario 2:
10 kg block pulled across a frictionless floor by a string Y
Does it fall through the floor?
Just like projectiles, we treat x and y separately
10 kg
Normal Force
• perpendicular () to the surface
• equal to the force acting  on the opposite side of surface

19. X Free Body Diagrams Scenario 2:
10 kg block pulled across a frictionless floor by a string X
3 Forces
10 kg
net force only in x direction

20. Free Body Diagrams
A 1000kg car moving right using its brakes to slow down on a level freeway. Consider friction.
Scenario 3: Y
Does it fall through the floor?
Just like projectiles, we treat x and y separately
10 kg

21. Free Body Diagrams
A 1000kg car moving right, using its brakes to slow down on a level freeway. Consider friction.
Scenario 3: X
1000 kg

22. What’s up with friction?!
f does not actually act on the COM, but at the surface. X
1000 kg
Note: f is a resistive force.
it always acts against the motion

23. Free Body Diagrams
A 1000kg car moving right, using its brakes to slow down on a level freeway. Consider friction.
Scenario 3: X
4 Forces
1000 kg
net force only in x direction

24. Free Body Diagrams Scenario 4: 10 kg
10 kg block sliding up a ramp at 30° with friction
Scenario 4:
10 kg

25. Free Body Diagrams 10 kg block sliding up a ramp at 30° with friction
Scenario 4:
There are 3 forces
10 kg

26. Free Body Diagrams
10 kg block sliding up a ramp at 30° with friction
Scenario 4:
There are 3 forces y x
10 kg
Choose x and y to line up with the direction of motion x y
Weight has both an x and y component

27. Free Body Diagrams x y
The angle between the weight and Fy is the same as the angle between the ramp and the ground

28. Free Body Diagrams Scenario 4:
10 kg block on a frictionless ramp at 30°
The y forces cancel
(block does not accelerate in the y direction)
10 kg
the only Net Force is:

29. Practice!
Drawing a FBD for # 5 from HW 3

30. HW 3: #5
A 34.5 kg block rests on the ramp as shown in the drawing. What is the tension in the line that is connected to the top of ramp?

31. Language Objective
Students will be able to explain to each other what force causes a ball to accelerate down a hill

32. F1,… F2 Fair fk fs Weight / gravity Normal Tension Direction of push
Opposite to motion fk fs
Opposite to motion
Weight / gravity
Straight down
Normal
90° to surface
Tension

33. Practice!
Complete the Free Body Exercises in your HW packet

34. 2nd Law: Example
The Batman, with a mass of 70-kg, rappels down a rope from his bat-copter with a downward acceleration of 3.5 m/s2. What vertical force does the rope exert on Batman?

35. 2nd Law: Example
The Batman, with a mass of 70-kg, rappels down a rope from his bat-copter with a downward acceleration of 3.5 m/s2. What vertical force does the rope exert on Batman?
Start all force questions with a diagram showing the forces
Pick the positive direction
(make direction of motion positive)
Identify all givens with symbols

36. 2nd Law: Example
The Batman, with a mass of 70-kg, rappels down a rope from his bat-copter with a downward acceleration of 3.5 m/s2. What vertical force does the rope exert on Batman?
Given:
Want:

37. 2nd Law: Example
Calculations:

38. Practice!
Try # 7 from HW 1

39. Air Resistance Most falling objects don’t accelerate at 9.8 m/s2!
Because air resistance makes objects accelerate less 9.8 m/s2!
Two things determine the magnitude of air resistance
surface area (shape)
speed

40. Terminal Velocity
Terminal velocity is the fastest an object can fall due to air resistance!
When FAir resistance = W an object is falling at terminal velocity and not accelerating

41. Terminal velocity When FAir resistance = W the object’s a = zero
10 kg block in freefall traveling at terminal velocity
Scenario 1:
10 kg

42. Time to practice
Start your homework
Turn to pg. 405

43. Atwood Machine Video

44. Newton’s 3rd Law Equal and Opposite
For every force that one object exerts on a second object, the second object exerts an equal (in size) and opposite (in direction) reaction force on the first object.

45. Newton’s 3rd Law Subtleties
Forces only come in pairs (you cannot create a single force)
Does action cancel out reaction?
NO! Each force acts on a different object.
Each force has different effect on their object!

46. Action Fore: A bat applies a force to a baseball
Action Fore: A bat applies a force to a baseball. Action force effect: Reaction force: Reaction force effect:
The baseball accelerations a LOT
The baseball applies a force to a bat
The baseball bat (de)accelerates a little

47. When I am zooming down the highway I crush small bugs that hit my car Which exerts the larger force? a. the bug on my car b. My car on the bug
Neither! The forces are equal (but opposite)

48. How does a rocket work in space if there is no air to push it?
Lets watch WALL-E
What happens when the pod reaches “cruising speed”?
How does WALL-E move in space when there is no air or ground for him to push against?

49. Time to practice
Check Yourself #11-15

50. Answers
11. A 12. B 13. B 14. B 15. A

51. 2. Does the moon put a gravitational pull on the earth?
1. Why does the moon orbit the earth?
2. Does the moon put a gravitational pull on the earth?
3. How does that force compare to the force the earth puts on the moon?

52. Friction
Friction is everywhere there is motion on a surface or within a fluid
Friction is a resistive force (a force that opposes motion)
Friction evidence?
Heat
Noise (air resistance)

53. Friction
Let’s examine sandpaper
40 grit
400 grit

54. Friction
Even “smooth” sandpaper is rough
40 grit
400 grit

55. Friction Slide the surfaces of two pieces together
Atoms at the peaks bond with atoms at the peaks of the other
It requires force to break these bonds
opposing force is friction

56. Perpendicular Force Rub your hands together
What happens to friction if you press harder?
So, f

57. Nature of Surfaces Rub your hands together again
What would happen to friction if you added some oil to your hands?
What would happen to friction if you added some dry glue?
This condition of a surface is given as a quantity known as the coefficient of friction

58. Force of Friction Friction between solid surfaces depends on:
The perpendicular (Normal) force between the surfaces in contact
The nature of the surfaces in contact
together, these create a net frictional force

59. Nature of Surfaces Each situation has two coefficients of friction
μs = static friction
Surfaces μs μk
steel on steel
0.74
0.57
glass on glass
0.94
0.40
tire on dry road
1.0
0.80
tire on icy road
0.30
0.015
bone joints
0.010
0.0030
when an object is being pushed, but hasn’t moved yet
μk = kinetic friction
the friction once the object starts moving

60. Nature of Surfaces μs is always bigger This means …μk
steel on steel
0.74
0.57
glass on glass
0.94
0.40
tire on dry road
1.0
0.80
tire on icy road
0.30
0.015
bone joints
0.010
0.0030
It takes more force to make an object start moving than it takes to keep an object moving

61. Frictional Force
Friction is the product of the normal force and the coefficient of friction
when μ = 0, surface contact is frictionless

62. Friction Example
A 500kg car is going 30 m/s. How long will it take to come to a stop on a dry road compared to an icy one?
Givens
m = 500 kg
vo= 30 m/s
vf = 0
Unknown
t = ?
Equation
adry = ?
aice = ?

63. Friction Example
A 500kg car is going 30 m/s. How long will it take to come to a stop on a dry road compared to an icy one?
Draw a FBD!
Identify the + direction
Write the Net Force Eqs

64. Friction Example
A 500kg car is going 30 m/s. How long will it take to come to a stop on a dry road compared to an icy one?
4. Solve for the net acceleration

65. Friction Example
A 500kg car is going 30 m/s. How long will it take to come to a stop on a dry road compared to an icy one?
The difference in friction is why it takes so long to stop on ice

66. Friction Example
A 500kg car is going 30 m/s. How long will it take to come to a stop on a dry road compared to an icy one?
4. Solve for the net acceleration for each road

67. Friction Example
A 500kg car is going 30 m/s. How long will it take to come to a stop on a dry road compared to an icy one?
4. Solve for the net acceleration for each road

68. Friction Example
A 500kg car is going 30 m/s. How long will it take to come to a stop on a dry road compared to an icy one?
Givens
m = 500 kg
vo= 30 m/s
vf = 0
Unknown
t = ?
Equation
adry = m/s2
aice = m/s2

69. Solving Force Problems
Draw a FBD
Identify direction of movement
Write the expression for Fnet = F1+F2-F3…
Set Fnet expression equal to mtotanet
F1+F2-F3…= mtotanet