1. Forces

2. Forces A FORCE is a push or a pull exerted on an object
Forces cause objects to speed up, slow down, or change direction.
Force is measured in Newtons

3. Force Forces come in pairs A force is a vector
Forces have a magnitude and a direction
Magnitude: 5N
Direction: north (up)
5N, north (up)

4. Types of Forces There are two types of forces Contact forces
Forces at a distance

5. Contact Forces
Contact forces- types of forces that result when the two interacting objects are perceived to be physically contacting each other
Examples of contact forces:
Applied force
Friction force
Tension force
4. Normal force
5. Air resistance

6. Applied Force Identified as Fa
Applied to an object by a person or another object
Acts in the horizontal direction, parallel to the surface

7. Friction Force Identified as Ff
Acts opposite sliding motion between surfaces
Acts in the horizontal direction, parallel to the surface and opposite the direction of sliding

8. Tension Force Identified as FT
Pull exerted by a rope, string, or cable attached to a body and pulled tight
Acts away from the object and parallel to the string, rope or cable

9. Normal Force Identified as FN
Exerted by a surface on an object (presses two surfaces together)
Acts in an upward, vertical direction, perpendicular to the surface
Sometimes referred to as the support force

10. Air Resistance Force Identified as Fair
Type of frictional force that acts upon objects as they travel through the air
Acts opposite motion

11. Long Range Forces
Long Range Forces- 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
Examples of long range forces:
Gravitational force
Electrical force
Magnetic force

12. Gravitational Force
All objects on Earth experience a force of gravity that is directed downward towards the center of the earth.
Identified as FW
Referred to as the force of weight
Acts in the vertical direction

13. Balanced and Unbalanced Forces
Forces occur in pairs and they can be either balanced or unbalanced

14. Balanced Forces
Balanced forces do not cause change in motion, they are in equilibrium
They are equal in size and opposite in direction

15. Unbalanced Forces An unbalanced force always causes a change in motion
Forces are not in equilibrium
When unbalanced forces act in opposite directions you can find the net force
The net force is the sum of all the forces acting on an object

16. Net Force A net force shows magnitude and direction
The magnitude is the difference between forces acting on an object
The resulting motion of an object is in the direction of the largest force

17. Unbalanced Forces
3 N, right – 6 N, left = 3N, left

18. Unbalanced Forces
4 N, left – 10 N, right =
6N, right

19. Force Body Diagrams
Used to show all the forces acting on an object (can be 1,2,3, or 4 forces)
Can help you understand the motion of an object
Force vectors are represented by arrows
The length of the arrow represents the magnitude (size) of the force

20. Bellwork
Write two multiple choice questions. You must give 4 answer choices. Follow these rules:
One wrong-answer choice must be funny.
Only one answer choice may be right.
You may not use “all or the above” or “none of the above.”

21. How to Draw a Force Body Diagram
Step 1: Draw a sketch of the situation
Step 2: Identify the horizontal forces
Step 3: Identify the vertical forces
Step 4: Draw a box, place a dot in the middle
Step 5: Draw and label the horizontal forces

22. How to Draw a Force Body Diagram
Step 6: Draw and label the vertical forces
Step 7: Calculate the sum of all the forces

23. Example
A book is at rest on a tabletop. Diagram the forces acting on the book.

24. Newton’s 2nd Law

25. Review: Net Force The sum of all forces acting on an object
When net force is greater than zero, the object accelerates in the direction of the largest force
If the net force is zero the object is either
At rest OR
Moving with constant velocity

26. Physics of Football
What is Inertia?
What is an unbalanced force?
What does Newton’s First Law of Motion state?

27. Newton’s First Law
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Newton’s first law is sometimes called the Law of Inertia

28. Inertia
Inertia is a property of matter that tends to resist a change of motion
If an object is at rest, it wants to stay at rest
If an object is in motion, the object wants to stay in motion

30. Mass Mass is the amount of matter that makes up an object
Mass is a measure of an object’s inertia
The more mass an object has, the greater its inertia
The unit for measuring mass is the kilogram

31. Weight
Weight is the force that results from the pull of gravity on an object’s mass
The SI unit for weight is Newtons
Can be determined by using the equation

32. Example Mass = 59.7 kg Force on moon: 95.5 N
On Earth, a scale shows that you weigh 585 N. What is your mass? What would the scale read on the moon, where gravity is 1.67 m/s2?
Mass = 59.7 kg
Force on moon: 95.5 N

33. Bellwork Pick up today’s notes Work on Tuesday’s bellwork
Get out a calculator and complete the remaining practice problems from yesterday

35. Example 2: On Planet X a 50 kg barbell can be lifted by exerting a force of 180 N. What is the acceleration of gravity on Planet X?

36. Example 3: A 71. 1-kg man travels from an area where the gravity is 9
Example 3: A 71.1-kg man travels from an area where the gravity is 9.78 m/s2 to where it is 9.80 m/s2.
What is his change in mass?
What is his change in weight?

37. Second Law Simulation Forces Simulation Independent variable:
Control Variable:

38. Acceleration Vs. Force Graphs

39. Newton’s 2nd Law
Newton’s 2nd Law helps is identify how an object’s motion changes when a net force is applied to the object
When forces are unbalanced, the object accelerates
Acceleration is dependent on two variables
Net force
The mass of an object

40. Newton’s 2nd Law http://www.nbclearn.com/nfl Causes a change in motion
A change in motion means a change in velocity
A change in velocity is called an acceleration
Newton’s 2nd law describes motion when unbalanced forces act on an object
If forces are unbalanced, the net force is greater than zero.

41. Newton’s 2nd Law Summary
The acceleration of an object depends directly on the net force acting on the object, and inversely on the mass of the object.
As the force acting on an object is increased, the acceleration of the object is increased.
As the mass of an object is increased, the acceleration of the object is decreased
The acceleration is always in the direction of the net force

42. Example 1
A net force of 24 N is applied to an object whose mass is 14 kg. What is the object’s acceleration?

43. Example 2
Thomas, the 72 kg star forward on Buford High School’s basketball team, collides with Killer, a guard for the other team, and is brought to a stop with an acceleration of –20 m/s2. What force does Killer exert on Thomas?

44. Bellwork Work on Wednesday’s bellwork
Get out calculator and practice from yesterday
Complete the practice problems from yesterday
Complete today’s assignment
Read October Sky CH. 8-9 OR work on research for Science Fair project

45. Bellwork Complete Thursday’s bellwork Work on practice from yesterday
Tomorrow:
Quiz over Newton’s Laws/Forces and October Sky
Be sure to bring your logbook and October Sky

46. Practice problems

47. Diagram: Balanced Vs. Unbalanced Forces

48. Bellwork Pick up today’s notes and bellwork Work on today’s bellwork
Do not play with the materials

49. Friction
Question of the day Once a skateboard has been pushed, what are the forces acting on it?

50. Predict
On which material will the toy walk fastest, your lab table or the sandpaper?
Design a mini lab to test your prediction?
Was your hypothesis correct?
Why or why not?

51. Friction Notes Types of Friction STATIC FRICTION
A force that opposes motion
Types of Friction
Amount of friction depends on:
KINETIC FRICTION
STATIC FRICTION
1) Force (weight) pushing surfaces together
Friction b/w MOVING surfaces
Applying a force & NO movement
2)Roughness of the surface
Rougher the surface, greater the friction
SLIDING
ROLLING
Examples
Examples
Increasing force/weight, increases the friction

52. Harmful friction
Causes

53. Coefficient of Friction
The friction between surfaces is a direct result of the surface and the force pressing surfaces together, the normal force
On a flat surface, when vertical forces are balanced, the normal force is equal to the force of weight

54. Coefficient of Friction
The coefficient of friction refers to the “stickiness” of two surfaces
The closer to 1, the greater the friction
The closer to 0, the less the friction

55. Coeffieicent of Friction
Is symbolized with greek letter mu.
Mu is calculated using the ratio between the frictional force and the normal force
Equation

56. Example
A hockey puck has a coefficient of kinetic friction of μk = If the puck feels a normal force (FN) of 5 N, what is the frictional force that acts on the puck?

57. A block has a weight of 5 N. An applied horizontal force of 1
A block has a weight of 5 N. An applied horizontal force of 1.5N is required to cause the block to slide with constant speed on a smooth concrete floor. What does “constant speed” tell you about the shoe?
Draw a force diagram for the block.
What is the coefficient of sliding friction?
What is the mass of the block?
If the coefficient of friction were 0.10, what would be the acceleration of the block?

58. Bellwork Answer Tuesday’s bellwork questions
Define each of the terms (not already defined) on the side board on your vocab organizer for this unit
Get out a calculator

59. Terms to Know
Terminal velocity
Drag
Air friction
Aerodynamics

60. Demo- Drag

61. What is Drag?
Drag is the force exerted by a fluid on the object moving through the fluid
Air is a fluid
Drag is dependent on the motion of the object, the properties of the object, and the properties of the fluid.
Drag is a resistance to motion, so it acts opposite of motion (air friction)

62. Drag and Aerodynamics
Aerodynamics is the study of forces and the resulting motion of objects through the air.
Drag also effects objects not in free fall
Air exerts force in all directions

63. Terminal Velocity
Terminal Velocity is the constant velocity that is reached when the drag force equals the force of gravity

64. Newton’s 3rd law

65. Forces act in force pairs
For every action, there is an equal and opposite reaction.

66. Baseball pushes glove to the left

67. Man pushes car

68. Foot pushes floor

69. Bowling ball pushes pin leftward

70. Bus hits bug

71. Newton’s Third Law Problem:
How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?
Aren’t these “balanced forces” resulting in no acceleration?
NO!!!