1. Today’s special Test results Banana experiment Notes I
HW I due next time!

2. The wind pushes against the man and his umbrella
The wind pushes against the man and his umbrella. The push from the wind is a force.

3. What Is a Force?
How do forces affect the motion of an object?
A force is a push or a pull that acts on an object.
A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

4. What Is a Force?
Measuring Force
You may have measured forces using a spring scale. The stretch of the spring in the scale depends on the amount of weight (a type of force) acting on it.

5. What Is a Force?
Units of Force
One newton (N) is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2).
1 N = 1 kg•m/s2

6. What Is a Force?
Representing Force
Arrows can represent a force. The lengths of the arrows show relative amounts of force.

7. Combining Forces
How do forces affect the motion of an object?
When the forces on an object are balanced, the net force is zero, and there is no change in the object’s motion.
When an unbalanced force acts on an object, the object accelerates.

8. Combining Forces
The net force is the overall force acting on an object after all the forces are combined.
Forces in the same direction add together.
Forces in opposite directions subtract from one another.

9. Combining Forces
Balanced Forces
Balanced forces are forces that combine to produce a net force of zero.
An unlimited number of individual forces can act on an object to produce a net force of zero.

10. Combining Forces
The two groups pull with equal forces in opposite directions. The forces combine to make a net force of zero.

11. Combining Forces
Unbalanced Forces
An unbalanced force is a force that results when the net force acting on an object is not equal to zero.

12. Forces can add together or subtract from one another.
Combining Forces
Forces can add together or subtract from one another.
Adding forces

13. Forces can add together or subtract from one another.
Combining Forces
Forces can add together or subtract from one another.
Adding forces
Subtracting forces

14. Forces can add together or subtract from one another.
Combining Forces
Forces can add together or subtract from one another.
Adding forces
Subtracting forces
Equal and opposite forces

15. Friction
What are the four main types of friction?
Friction is a force that opposes the motion of objects that touch as they move past each other.
There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.

16. Friction
Static Friction
Static friction is the friction force that acts on objects that are not moving.
Static friction always acts in the direction opposite to that of the applied force.

17. Friction
Sliding Friction
Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface.
Sliding friction is less than static friction.

18. Friction
Static friction–the potted tree does not move.
Sliding friction–when the tree moves, sliding friction acts to oppose the direction of motion.

19. Friction
Rolling Friction
Rolling friction is the friction force that acts on rolling objects.
The force of rolling friction is about 100 to 1000 times less than the force of static or sliding friction.

20. Friction
Ball bearings in these wheels greatly reduce friction by replacing sliding friction with rolling friction.

21. Friction
Fluid Friction
Fluid friction opposes the motion of an object through a fluid.
Fluid friction acting on an object moving through the air is known as air resistance.
Fluid friction increases as the speed of the object moving through the fluid increases.

22. Gravity
In what direction does Earth’s gravity act?
Earth’s gravity acts downward toward the center of Earth.

23. Gravity
How do gravity and air resistance affect a falling object?
Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

24. Gravity is a force that acts between any two masses.
Gravity is an attractive force.
Gravity can act over large distances.

25. Gravity
Earth exerts an attractive, downward force on this boulder. The supporting rock exerts an upward force on the boulder. The forces are balanced.

26. Gravity
Falling Objects
Both gravity and air resistance affect the motion of a falling object.
As objects fall to the ground, they accelerate and gain speed.
Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

27. Gravity
This flying squirrel takes advantage of air resistance to slow its fall and increase the distance covered in the jump.

28. Projectile Motion
Why does a projectile follow a curved path?
The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

29. Projectile Motion
A thrown ball follows a curved path.
Projectile motion is the motion of a falling object (projectile) after it is given an initial forward velocity.
Air resistance and gravity are the only forces acting on a projectile.

30. Projectile Motion
Their masses are different, but the blue and green balls fall at the same rate.

31. Projectile Motion
Their masses are different, but the blue and green balls fall at the same rate.
The yellow ball is a projectile, following a curved path.

32. Assessment Questions
If an object is at rest, which of the following statements must be true?
There are no forces acting on the object.
There is no friction acting on the object.
The forces acting on the object are unbalanced.
The net force acting on the object is zero.

33. Assessment Questions
If an object is at rest, which of the following statements must be true?
There are no forces acting on the object.
There is no friction acting on the object.
The forces acting on the object are unbalanced.
The net force acting on the object is zero. ANS: D

34. Which of the following is not a type of friction?
Assessment Questions
Which of the following is not a type of friction?
static friction
sliding friction
fluid friction
pull friction

35. Which of the following is not a type of friction?
Assessment Questions
Which of the following is not a type of friction?
static friction
sliding friction
fluid friction
pull friction
ANS: D

36. In which direction does Earth’s gravitational force act?
Assessment Questions
In which direction does Earth’s gravitational force act?
opposite the direction of motion
downward toward the center of Earth
upward away from the center of Earth
in the direction of motion

37. In which direction does Earth’s gravitational force act?
Assessment Questions
In which direction does Earth’s gravitational force act?
opposite the direction of motion
downward toward the center of Earth
upward away from the center of Earth
in the direction of motion ANS: B

38. Assessment Questions
A ball thrown into the air follows a projectile course due to the initial velocity and the
force of gravity.
effect of air resistance.
motion of Earth beneath it.
mass of the ball.

39. Assessment Questions
A ball thrown into the air follows a projectile course due to the initial velocity and the
force of gravity.
effect of air resistance.
motion of Earth beneath it.
mass of the ball. ANS: A

40. Assessment Questions
The SI unit for force is 1 kg•m/s2, also called one kepler. True False

41. Assessment Questions
The SI unit for force is 1 kg•m/s2, also called one kepler. True False
ANS: F, newton

42. Galileo’s work helped correct misconceptions about force and motion that had been widely held since Aristotle’s time.

43. Aristotle, Galileo, and Newton
It took about 2000 years to develop the modern understanding of the relationships between force and motion.

44. Aristotle, Galileo, and Newton
Aristotle made scientific discoveries through careful observation and logical reasoning.
Aristotle incorrectly proposed that force is required to keep an object moving at constant speed.

45. Aristotle, Galileo, and Newton
Galileo Galilei studied how gravity produces constant acceleration.
He rolled balls down wooden ramps.
He concluded that moving objects not subjected to friction or any other force would continue to move indefinitely.

46. Aristotle, Galileo, and Newton
Newton built on the work of scientists such as Galileo.
Newton first defined mass and force.
He then introduced his laws of motion.

47. Aristotle, Galileo, and Newton
Isaac Newton published his work on force and motion in the book entitled Principia.

48. Newton’s First Law of Motion
How does Newton’s first law relate change in motion to a zero net force?
According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.

49. Newton’s First Law of Motion
Unless an unbalanced force acts, an object at rest remains at rest.
Unless an unbalanced force acts, an object in motion remains in motion with the same speed and direction.
Inertia is the tendency of an object to resist a change in its motion.

50. Newton’s First Law of Motion
This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops.

51. Newton’s First Law of Motion
This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops.

52. Newton’s First Law of Motion
This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops.

53. Newton’s First Law of Motion
This crash sequence illustrates inertia. The test dummy continues its forward motion as the car slows and stops.

54. Newton’s Second Law of Motion
How does Newton’s second law relate force, mass, and acceleration?
According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object’s mass.

55. Newton’s Second Law of Motion
The acceleration of an object is directly proportional to the net force acting on it.
The acceleration of an object also depends upon its mass.
Mass is a measure of the inertia of an object.

56. Newton’s Second Law of Motion
The acceleration of an object is always in the same direction as the net force.
When a net force acts in the direction opposite to the object’s motion, the force produces a deceleration.

57. Newton’s Second Law of Motion
An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

58. Newton’s Second Law of Motion
Read and Understand
What information are you given?

59. Newton’s Second Law of Motion
Read and Understand
What information are you given?

60. Newton’s Second Law of Motion
Plan and Solve
What unknown are you trying to calculate?
What formula contains the given quantities and the unknown?

61. Newton’s Second Law of Motion
Plan and Solve
What unknown are you trying to calculate?
What formula contains the given quantities and the unknown?

62. Newton’s Second Law of Motion
Plan and Solve
Replace each variable with its known value and solve.

63. Newton’s Second Law of Motion
Plan and Solve
Replace each variable with its known value and solve.

64. Newton’s Second Law of Motion
Look Back and Check
Is your answer reasonable?

65. Newton’s Second Law of Motion
Look Back and Check
Is your answer reasonable?
Powerful sports cars can accelerate at 6 m/s2 or more.
Thus, a smaller acceleration of 4 m/s2 seems reasonable.

66. Newton’s Second Law of Motion
1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

67. Newton’s Second Law of Motion
1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?
Answer: a = F/m = 60.0 N/40.0 kg = 1.50 m/s2

68. Newton’s Second Law of Motion
2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?

69. Newton’s Second Law of Motion
2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?
Answer: a = F/m = 10,000 N/5000 kg = 2 m/s2

70. Newton’s Second Law of Motion
3. An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.)

71. Newton’s Second Law of Motion
3. An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.)
Answer: a = F/m
F = m/a = 1200 kg × 3.0 m/s2 = 3600 N

72. Newton’s Second Law of Motion
4. A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2. What is the mass of the boy and the wheelchair? (Hint: Solve Newton’s second law for mass.)

73. Newton’s Second Law of Motion
4. A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2. What is the mass of the boy and the wheelchair? (Hint: Solve Newton’s second law for mass.)
Answer: a = F/m
m = F/a = 25 N/0.50 m/s2 = 50 kg

74. Newton’s Second Law of Motion
Acceleration depends directly on force and inversely on mass. The same force causes the single cart to accelerate eight times faster than the chain of eight carts.
Force
Acceleration
Force
Acceleration

75. Weight and Mass
How are weight and mass related?
Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.

76. Mass and weight are related but are not the same.
Weight and Mass
Mass and weight are related but are not the same.
Mass is the measure of the amount of material an object contains.
Weight is the force of gravity acting on an object.
Weight is the product mass and acceleration due to gravity.

77. Weight and Mass
W = mg is a different form of Newton’s Second Law, F = ma.
The value of g in the formula is 9.8 m/s2.

78. Weight and Mass
If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?
Weight = Mass × Acceleration due to gravity
= 112 kg × 9.8 m/s2
= 1100 kg•m/s2 × 1100 N

79. Weight and Mass
On the moon, the acceleration due to gravity is only about one sixth that on Earth.
The astronaut weighs only about one sixth as much on the moon as on Earth.
The mass of the astronaut is the same on the moon and on Earth.

80. Weight and Mass Astronaut on Earth Mass = 88.0 kg, Weight = 863 N
Astronaut on Moon Mass = 88.0 kg, Weight = 141 N

81. Assessment Questions What is inertia?
the force of gravity acting on an object
forces of friction slowing an object’s motion
the mass of an object
the tendency of an object to resist change in its motion

82. Assessment Questions What is inertia?
the force of gravity acting on an object
forces of friction slowing an object’s motion
the mass of an object
the tendency of an object to resist change in its motion ANS: D

83. Assessment Questions
A 3600-N force causes a car to accelerate at a rate of 4 m/s2. What is the mass of the car?
600 kg
900 kg
14,400 kg
1200 kg

84. Assessment Questions
A 3600-N force causes a car to accelerate at a rate of 4 m/s2. What is the mass of the car?
600 kg
900 kg
14,400 kg
1200 kg ANS: B

85. Assessment Questions
How would your mass and weight change if you were on the moon’s surface?
They wouldn’t change.
Your mass would remain constant, and your weight would increase.
Your mass and weight would decrease.
Your mass would remain constant, and your weight would decrease.

86. Assessment Questions
How would your mass and weight change if you were on the moon’s surface?
They wouldn’t change.
Your mass would remain constant, and your weight would increase.
Your mass and weight would decrease.
Your mass would remain constant, and your weight would decrease. ANS: D