1. Forces
FORCEMAN

2. WHAT CAUSES THINGS TO MOVE?
FORCES!
What is a force?
A push or a pull that one body exerts on another.

3. WHAT CAUSES THINGS TO MOVE?
It gives energy to an object and causes it to:
start moving
stop moving
change its motion.

4. BALANCED FORCE

5. BALANCED FORCES Balanced forces are opposite and equal
act in opposite directions and cancel each other out
no motion occurs for the cart.

6. - There is ALWAYS a change in motion!
UNBALANCED FORCES
If the forces acting on a body are "unbalanced" this means that there is a "net" or "resultant" force.
The body will do one of the following:
Speed Up
Slow Down
Change the Direction of its Motion
In other words
- There is ALWAYS a change in motion!

8. NEWTONS SI unit of force is the Newton (N).
Defined as the amount of force that when acting on a 1 kg object produces an acceleration of 1m/s2.
Therefore, 1N = 1 kg•m s2

9. NEWTON’S FIRST LAW LAW OF INERTIA
An object at rest stays at rest and an object in motion stays in motion until acted on by a net external force.
In other words: Objects just keep doing what they are doing.
What do you mean by “Net Force”?
Net means: Final Sum, so net external force is the sum of all the forces acting on the object.

10. NEWTON’S FIRST LAW LAW OF INERTIA
Inertia - is the tendency of an object to resist any change in its motion.

11. NEWTON’S FIRST LAW

12. Why does the ladder continue to move?
Inertia!

13. Why does the person continue to move?
Inertia!
Newton’s First Law

14. TO DO Work on Vocabulary for Chapter three
Make sure you have the four FORCES vocabulary words from chapter two done since they are important to this chapter.

15. NEWTON’S FIRST LAW OF MOTION
Work with your lab partner to follow the directions and answer the questions for Newton’s First Law of Motion. You have ten minutes.

16. NEWTON’S SECOND LAW
The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass.
2nd Law Clip

17. NEWTON’S SECOND LAW OF MOTION
The force on an object equals the mass of the object times the acceleration of the object.
In other words, a force causes a mass to accelerate.
EQUATION: Force = mass x acceleration
EQUATION: F = ma
UNITS: kilogram x meter/second2 = Newton, N

18. NEWTON’S SECOND LAW OF MOTION

19. FORCE PROBLEMS
What force is needed to give an object with a mass of 25.0 kg an acceleration of 15.0 m/s2 ?
Formula
F = ma
Given:
m = 25.0 kg
a = 15.0 m/sec²
F = (25 kg)(15 m/sec²)
F = 375 kg•m/sec² = 375 N
Unknown:
F = ?

20. TO DO
The Forces: Newton’s Second Law handout is due tomorrow. Work on it now. Be sure to:
Write what you know and where you are going
Write the formula
Plug in your numbers into the formula and solve
Write the answer with the correct units.

21. NEWTON’S SECOND LAW OF MOTION
Work with your lab partner to follow the directions and answer the questions for Newton’s Second Law of Motion.

22. NEWTON’S THIRD LAW OF MOTION
LAW OF ACTION-REACTION
For every action force, there is an equal and opposite reaction force.
Every action has an equal and opposite reaction.

23. NEWTON’S THIRD LAW OF MOTION
Action-Reaction forces act on different objects

24. NEWTON’S LAWS OF MOTION
Revisiting Newton’s Laws

25. PRACTICE
Do the handout Newton’s Laws now

26. LAB
Obtain an index-card with a washer taped to it. Place a marble on the washer. Quickly slide the card across the table. What does the marble do? Justify your observation using Newton’s First Law.
Observations
Justification

27. LAB
Replace the marble in the washer and slowly start to slide the card across the table building up speed gradually. Suddenly stop the card. What happens to the marble? Be specific. Justify your observation using Newton’s First Law.
Observations
Justification

28. LAB
Go to the rotary platform. Place the index-card with marble on the washer as before near the edge of the rotary table. Start spinning the platform slowly picking up the speed until the marble moves off the washer. The marble moves in a (circle choice) circle straight line
Place an index card on top of a beaker, and put a penny on the center of the card. Flick the card with your finger in a horizontal direction. (You may have to practice several times before you get it right). What happens to the penny? Justify your observation.
Observations
Justification

29. LAB
Now set the index card on the beaker again, and this time place a paper penny on the card. Try the trick again using a small force. What happens? Justify your observation using Newton’s Second Law.
Observations
Justification

30. LAB
Stretch a string across a space of two chairs, lab desks, or stools. Make sure the space is 2-3 meters apart. Securely tape the ends of the strings to the two chairs/desks/stools. Blow up the balloon and hold the end closed. Do NOT tie it off. Tape the two paper strips around the belly of the balloon at one end of the string. Release the balloon and observe what happens. Explain your observations using Newton’s Third Law of Motion.
Observations
Justification

31. LAB
Do the Analysis and Conclusion questions
Lab is due Monday.

32. Forces that oppose motion

33. FRICTION
a force that opposes motion between two surfaces that are touching each other.
A force that brings an object to rest.
Acts between surfaces.

34. WHAT CAUSES FRICTION?
Caused by microscopic, electrostatic interactions between contacting surfaces

35. THREE TYPES Static (kinetic) Friction Sliding Friction
Rolling Friction

36. THREE TYPES OF FRICTION
Static Friction: The frictional force that prevents two surfaces from sliding past each other.
Sliding Friction: The force that opposes the motion of two surfaces sliding past each other.
Rolling Friction: The frictional force between a rolling object and the surface it rolls on.

37. FRICTION
To move one surface over the other, a force must be applied to break the microwelds.

38. STATIC FRICTION
Suppose you have filled a cardboard box with books and want to move it.
It’s too heavy to lift, so you start pushing on it, but it doesn’t budge.
If the box doesn’t move, then it has zero acceleration.

39. STATIC FRICTION
force required to overcome inertia of a stationary object.

40. STATIC FRICTION CHANGES TO SLIDING FRICTION

41. SLIDING FRICTION You ask a friend to help you move the box.
Together you are able to supply enough force to break the microwelds between the floor and the bottom of the box.
Pushing together, the box moves. Together you and your friend have exerted enough force to break the microwelds between the floor and the bottom of the box.

42. SLIDING FRICTION
force required to keep an object sliding at a constant speed
Depends on
Mass
Surface type

43. ROLLING FRICTION
As a wheel rolls over a surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed.

44. ROLLING FRICTION
force required to keep an object rolling at a constant speed
Opposes motion less than sliding.

45. FRICTION Lubricants are used to help sliding friction
Friction can be helpful
Tire treads
Brakes
walking

46. IMPORTANT TO REMEMBER
The amount of force required to overcome static friction is always greater than the force required to overcome sliding or rolling friction.

47. FRICTION
FRICTION

48. AIR RESISTANCE
a friction-like force that opposes the motion of objects that move through the air
The amount of air resistance on an object depends on the speed, size, and shape of the object.
Air resistance, not the object’s mass, is why feathers, leaves, and pieces of paper fall more slowly than pennies, acorns, and apples

49. AIR RESISTANCE
Air Resistance and Terminal Velocity

50. TERMINAL VELOCITY

51. FREE FALL
A free falling object is an object that is falling under the sole influence of gravity.
Any object that is being acted upon only by the force of gravity is said to be in a state of free fall.
There are two important motion characteristics that are true of free-falling objects:
Free-falling objects do not encounter air resistance.
All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s2

52. FREE FALL AND TERMINAL VELOCITY
The opposing force of air resistance increases with speed.
Objects accelerate towards the Earth until the force of gravity is cancelled or balanced by this air resistance.
This is the highest speed an object can reach.
Different objects have different terminal velocities.

53. TO DO Backside of Newton’s Three Laws is Friction handout. Do it now.
Vocabulary Quiz is MONDAY.

54. FRICTION LAB Objectives:
To explain the relationship between mass, surface type, and friction.
To be able to control variables in an experiment

55. FRICTION LAB Materials: Wooden block Masses spring scale
friction board and/or various surfaces

56. FRICTION LAB Part A Attach your spring scale to the wooden block.
Use your spring scale to slowly drag the wooden block across the lab desk.
Record the data in Newtons, N, in Part A data table. Record the force right before the block moves (static) and then right after it moves (sliding).
Add a 50g mass to the block and slowly drag the wooden block across the lab desk. Record the data in Newtons, N, in Part A data table.
Remove 50g mass and add 100g mass. Repeat step 4.
Remove 100g mass and add 200g mass. Repeat step 5.

57. FRICTION LAB Part A Data Static Friction (N) Sliding Friction (N)
Static Friction (N)
Sliding Friction (N)
Block
50g
100g
200g

58. FRICTION LAB

59. FRICTION LAB Part B Attach your spring scale to the wooden block.
Place the wooden block on six straw halves on the lab desk.
Place the 100g mass on top of the block and slowly drag the wooden block across the lab desk. Record the data in Newtons, N, in Part B data table. Be sure to record the static force and the rolling force in Newtons, N.
Repeat step 2 with a 200g mass.

60. FRICTION LAB
PART B
Static Friction (N)
Rolling Friction (N)
100g
200g

61. FRICTION LAB Part C Attach your spring scale to the wooden block.
Place the wooden block on the plain portion of the friction board.
Place the 100g mass on top and slowly drag the wooden block across the plain portion. Record the data in Newtons, N, in Part C data table. Be sure to record the static force and the sliding force in Newtons, N.
Place the 100g mass on the cork portion of the friction board and repeat step 2.
Repeat step two with the rubber portion and then the sandpaper portion of the friction block.

62. FRICTION LAB PART C 100g Mass Static Friction (N) Sliding Friction (N)
Plain
Cork
Rubber
Sandpaper

63. FRICTION LAB Answer the Analysis and Conclusion questions
Lab is due Wednesday

64. GRAVITY
The force of gravity is an attractive force between all objects in the universe.
The amount of GRAVITATIONAL FORCE between objects depends upon their:
Masses (greater mass means more gravity)
Distance apart (greater distance means less gravity)
Gravitational force is only observable when masses are large – like those of planets.

65. GRAVITY
a friction-like force called air resistance opposes the motion of objects that move through the air.

66. THE LAW OF UNIVERSAL GRAVITATION
Isaac Newton formulated the law of universal gravitation, which he published in 1687.

67. FREE FALL

68. Feather and hammer on the moon
GRAVITY
*All objects fall toward the earth at the same rate of acceleration, regardless of their masses!
All objects accelerate at 9.8m/s2 on earth
Acceleration is due to the force of gravity between objects and the earth
Feather and hammer on the moon

69. WEIGHT AND MASS
Weight is a measure of the force of gravity on an object.
Weight = mass x acceleration due to gravity (9.8m/s2)
Unit: Newton, N
REMEMBER:
Mass never changes – it is the amount of matter in an object.
Weight changes with gravity – it is a measure of the force of gravity.

70. WEIGHT AND MASS Weight and mass are not the same.
Weight is a force and mass is a measure of the amount of matter an object contains.
Weight and mass are related. Weight increases as mass increases.

71. WEIGHT AND MASS
The table shows how various weights on Earth would be different on the Moon and some of the planets.

72. WEIGHTLESSNESS
Weight is the force of gravity between the earth and a body on its surface.
Weight ≠ mass!
Weight is measured in Newtons, N.
Weightlessness is lacking acceleration due to gravity.

73. GRAVITY AND WEIGHTLESSNESS
Gravity visualized
Weightlessness Visualized

74. CALCULATING WEIGHT Ex. Mass = 100kg Acceleration = 9.81 m/s2
When the mass of an object and the acceleration due to gravity are known, the weight of an object can be calculated.
Ex. Mass = 100kg Acceleration = 9.81 m/s2
Weight = (mass, m)(accel. due to gravity, g)
W = (m)(g) = (100kg)(9.81m/s2)
Weight = 981 N

75. CALCULATING WEIGHT W = (4.52kg)(9.8 m/s²) W = 44.296 kg•m/s² = 44.3 N
What is your weight if your mass is 4.52 kilograms?
Known: Unknown:
m = 4.52 kg W = ?
a = g = 9.8 m/s²
Formula: W = mg
W = (4.52kg)(9.8 m/s²)
W = kg•m/s² = 44.3 N

76. TO DO
Do Forces-Friction-Gravity handout
Due tomorrow for a grade.

77. PROJECTILE MOTION
Thrown objects do not travel in a straight line. They tend to curve downward.
Anything that is thrown or shot through the air is a projectile.
Because of the Earth’s gravitational pull and the object’s own inertia, projectiles follow a curved path.

78. PROJECTILE MOTION
Projectiles follow a curved path because of the Earth’s gravitational pull.
They have 2 types of motion that are independent of each other:
1. Horizontal motion
2. Vertical motion

79. PROJECTILE MOTION Which will hit the ground first?
A ball that is dropped straight down or a ball that is “shot” out horizontally?
* An object launched horizontally will land on the ground at the same time as an object simply dropped from the same height!

80. CIRCULAR MOTION
An object moving in a circular motion at the same speed is accelerating toward the center because its direction is constantly changing.
Centripetal acceleration is acceleration toward the center of a curved path.
'Centripetal' comes from the Latin word for 'center seeking'

81. only centripetal is the real force
CIRCULAR MOTION
Centrifugal 'force' is really a function of the inertia of the object being pushed into a circle. It is not really a force at all, it is simply the tendency of an object to go in a straight line.
only centripetal is the real force

82. CIRCULAR MOTION
Anything that moves in a circle is doing so because a centripetal force is accelerating it toward the center.

83. CIRCULAR MOTION
Centripetal force is the force that causes a moving object to move in a curved path
Without a centripetal force, an object in motion continues along a straight-line path.
With a centripetal force, an object in motion will be accelerated and change its direction.

85. ROLLER COASTER PHYSICS