1. CHAPTER 10
THE NATURE OF FORCE

2. Chapter Preview Questions
1. How do you know an object is in motion?
a. It has inertia.
b. It has mass.
c. It is changing position.
d. It has a reference point.

3. Chapter Preview Questions
1. How do you know an object is in motion?
a. It has inertia.
b. It has mass.
c. It is changing position.
d. It has a reference point.

4. Chapter Preview Questions
2. What is velocity?
a. speed
b. speed in a given direction
c. miles per hour
d. change in speed over time

5. Chapter Preview Questions
2. What is velocity?
a. speed
b. speed in a given direction
c. miles per hour
d. change in speed over time

6. Chapter Preview Questions
3. Which is the best definition of acceleration?
a. change in velocity
b. change in reference point
c. increase in speed
d. decrease in speed

7. Chapter Preview Questions
3. Which is the best definition of acceleration?
a. change in velocity
b. change in reference point
c. increase in speed
d. decrease in speed

8. Chapter Preview Questions
4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed?
a m/s
b. 0 m/s
c. 100 m/s
d. 32 m/s

9. Chapter Preview Questions
4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed?
a m/s
b. 0 m/s
c. 100 m/s
d. 32 m/s

10. What causes an object’s velocity to change?
You drop a tennis ball and a baseball at the same time. Why do the balls fall to the floor?

11. centri- Latin Word Origins Latin Origin Meaning Key Term Center
Centripetal force

12. com- Latin Word Origins Latin Origin Meaning Key Term Together, with
Compression

13. jacere Latin Word Origins Latin Origin Meaning Key Term Throw
Projectile

14. premere Latin Word Origins Latin Origin Meaning Key Term Press
Compression

15. pro- Latin Word Origins Latin Origin Meaning Key Term Forward, before
Projectile

16. tensus Latin Word Origins Latin Origin Meaning Key Term Stretch
Tension

17. Apply It!
Look at pro- and jacere and predict the meaning of projectile.
Revise your definition as needed as you read the chapter.
Look for words with these Latin origins as you read the chapter.
Sample: I see that pro- means “forward” and that jacere means “to throw.”
By combining the two I get the definition of projectile.

18. End of Chapter Preview

19. Section 1: The Nature of Force
How is a force described?
How do balanced and unbalanced forces affect an object’s velocity?

20. I. What is Force Force- a push or pull
Magnitude (strength) and direction
Newton- si unit
Net force- combined forces
Same direction + speed boat (60mph) + current downstream (5mph)- 65mph
Dif direction – boat (60) – current (5) = 55mph
Balanced force = 0 net force = same amt in opposite directions- NO MOVEMENT
Unbalanced force = MOVEMENT

21. Combining Force Vectors
The strength and direction of the individual forces determine the net force.

22. Unbalanced Forces
Unbalanced forces acting on an object result in a net force and cause a change in the object’s velocity.

23. Balanced Forces
Balanced forces acting on an object do not change the object’s velocity.

24. Click the SciLinks button for links on force.

25. End of Section 1: The Nature of Force

26. Section 2: Friction, Gravity, and Elastic Forces
What factors determine the strength of the friction force between two surfaces?
What factors affect the gravitational force between two objects?
Why do objects accelerate during free fall?
When is matter considered to be elastic?

27. II. Friction, Gravity and Elastic Forces
Friction- 2 surfaces exert pressure on e/o
Strength = types of surfaces and how much force they have on e/o
Rougher surface = more friction
More pushing force = more friction
Static- not moving- book on table- strongest
Sliding- slide over e/o- slide book across table-2nd
Rolling- rolls across surface- skateboard-4th
Fluid- solid object moves through liquid- surfing-3rd

28. Types of Friction

29. Gravity- force that pulls objects towards e/o
Gravity acts everywhere and on everything=Law of universal gravitation
Free Fall- force of gravity alone causes an obj to accelerate
Every second ojb falls its velocity increases by 9.8 m/s
Air Resistance= fluid friction of air acts on obj falling
Greater surface area = greater resistance
Projectile- an obj that is thrown will fall just as fast as obj dropped, will hit ground at same time.
Elastic forces- if it returns to its original shape after it is squeezed or streched
Compression- squeeze ex. sponge
Tension- stretch/pull, ex. rubberband

30. Gravity Between Objects
The force of gravity between objects increases with greater mass and decreases with greater distance.

31. Mass and Weight
The gravitational force exerted on a person or object at the surface of a planet is known as weight.

32. Free Fall
The graph shows how the speed of an object in free fall changes with time. Use the graph to answer the following questions.

33. Free Fall Interpreting Graphs:
What is the speed of the object at 1 second? At 3 seconds?
9.8 m/s; 29.4 m/s.

34. Free Fall Calculating:
Calculate the slope of the graph. What does this number represent?
The slope is 9.8. The speed increases by 9.8 m/s each second.

35. Free Fall
Predicting:
Use the slope that you calculated in Step 2 to predict the object’s speed at 6 seconds.
58.8 m/s

36. Free Fall Drawing Conclusions:
The graph has a constant slope. What does the slope tell you about the object’s motion?
The object’s speed increases at a constant rate.

37. Air Resistance
Falling objects with a greater surface area experience more air resistance.

38. Click the SciLinks button for links on friction.

39. Click the Video button to watch a movie about free fall.

40. End of Section 2: Friction, Gravity, and Elastic Forces

41. Section 3 and 4: Newton’s First and Second and Third Laws
What is Newton’s first law of motion?
What is Newton’s second law of motion? What is Newton’s third law of motion?
How can you calculate the momentum of an object?
What is the law of conservation of momentum?

42. III. Newtons Laws
1st law- obj at rest will stay at rest (book on table wont move until force acts on it, pull or push), obj. in motion will stay in motion unless force acts on it (train will stop with rolling and sliding friction)
Inertia- is the tendency of an obj to resist change in motion. MORE MASS = MORE INERTIA
2nd Law- A= net force (mass x acceleration)/mass
3rd Law- obj. exerts force on another, then the 2nd obj exerts a = force in the opposite direction on the 1st obj

43. Calculating Force
A speedboat pulls a 55-kg water-skier. The skier to accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2

44. Calculating Force
A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration.
Plan and Solve
What quantity are you trying to calculate?
The net force (Fnet) = __
What formula contains the given quantities and the unknown quantity?
a = Fnet/m or Fnet = m x a
Perform the calculation.
Fnet = m x a = 55 kg x 2.0 m/s2
F = 110 kg • m/s2
F = 110 N
cccc

45. Calculating Force
A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration.
Look Back and Check
Does your answer make sense?
A net force of 110 N is required. This does not include the force that overcomes friction.

46. Calculating Force Practice Problem
What is the net force on a 1,000-kg object accelerating at 3 m/s2?
3,000 N (1,000 kg x 3 m/s2)

47. Calculating Force Practice Problem
What net force is needed to accelerate a 25-kg cart at 14 m/s2?
350 N (25 kg x 14 m/s2)

48. Click the PHSchool.com button for an activity about Newton’s laws.
More on Newton’s Laws
Click the PHSchool.com button for an activity about Newton’s laws.

49. Newton’s Third Law of Motion
Newton’s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

50. Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s, or a 4.0-kg sledgehammer swung at 0.9 m/s?
What information are you given?
Mass of smaller sledgehammer = 3.0 kg
Velocity of smaller sledgehammer = 1.5 m/s
Mass of larger sledgehammer = 4.0 kg
Velocity of larger sledgehammer = 0.9 m/s

51. Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
What quantities are you trying to calculate?
The momentum of each sledgehammer = __
What formula contains the given quantities and the unknown quantity?
Momentum = Mass x Velocity
Perform the calculation.
Smaller sledgehammer = 3.0 km x 1.5 m/s = 4.5 kg•m/s
Larger sledgehammer = 4.0 km x 0.9 m/s = 3.6 kg•m/s

52. Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Look Back and Check
Does your answer make sense?
It is possible for the 3.0-kg hammer to have more momentum than the 4.0-kg one because it has a greater velocity.

53. Calculating Momentum
A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is 0.14 kg. Which has greater momentum?
Golf ball: kg x 16 m/s = 0.72 kg•m/s
Baseball: 0.14 kg x 7 m/s = 0.98 kg•m/s
The baseball has greater momentum.

54. Calculating Momentum
What is the momentum of a bird with a mass of kg flying at 15 m/s?
0.27 kg•m/s (0.018 kg x 15 m/s = 0.27 kg•m/s)

55. Conservation of Momentum-
In the absence of friction, momentum is conserved when two train cars collide.

56. Momentum Activity
Click the Active Art button to open a browser window and access Active Art about momentum.

57. End of Section 3 and 4: Newton’s Laws

58. Section 5: Rockets and Satellites
How does a rocket lift off the ground?
What keeps a satellite in orbit?

59. IV. Rockets and Satellites
Satellite- obj that orbits another obj
Planets
Rockets- lift off with = and opposite force pushing off the ground
Artificial satellites orbit around Earth continuously fall toward Earth, but because Earth is curved, they travel around it- falling projectile
Centripetal force- force that causes obj to move in circular path
Centripetal = “center-seeking”

60. What Is a Satellite?
A projectile follows a curved path. The horizontal and vertical motions act independently.

61. What Is a Satellite?
The faster a projectile is thrown, the father it travels before it hits the ground. A projectile with enough velocity moves in a circular orbit.

62. What Is a Satellite?
Depending on their uses, artificial satellites orbit at different heights.

63. End of Section 5: Rockets and Satellites

64. QuickTake Quiz
Click to start quiz.