1. Chapter 12
Forces and Motion

2. 12.1 Forces
Forces and Motion
12.1 Forces
What is a Force?
A force is a push or 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.

3. Units of Force Force is measured in newtons (N).
12.1 Forces
Forces and Motion
Units of Force
Force is measured in newtons (N).
One newton is the force that causes a kilogram mass to accelerate at a rate of 1 m/s/s.
A newton is equal to 1 kg-m per second per second (kg∙m/s2).

4. 12.1 Forces
Forces and Motion
Representing Forces
The direction and strength of a force can be represented by an arrow.
The direction of the arrow represents the direction of the force.
The length of the arrow represents the magnitude of the force.

5. 12.1 Forces
Forces and Motion
Combining Forces
Force arrows can be combined to show the result of how forces combine.
Forces in the same direction add together and forces in opposite directions subtract from one another.
The net force is the overall force acting on an object after all the forces are combined.

6. 12.1 Forces
Forces and Motion

7. 12.1 Forces
Forces and Motion

8. 12.1 Forces
Forces and Motion

9. 12.1 Forces
Forces and Motion

10. 12.1 Forces
Forces and Motion

11. 12.1 Forces
Forces and Motion

12. 12.1 Forces
Forces and Motion
Balanced Forces
Balanced forces are forces that combine to produce a net force of zero.
When forces on an object are balanced, the net force is zero and there is no change in the object’s motion.
An unlimited number of individual forces can act on an object and still produce a net force of zero.

13. 12.1 Forces
Forces and Motion
Unbalanced Forces
An unbalanced force is a force that results when the net force acting on an object is not equal to zero.
When an unbalanced force acts on an object, the object accelerates.

14. 12.1 Forces
Forces and Motion
Friction
All moving objects are subject to friction, a force that opposes the motion of objects that touch as they move past each other.
Friction acts at the surface where objects are in contact.

15. There are four main types of friction: static friction
12.1 Forces
Forces and Motion
There are four main types of friction:
static friction
sliding (kinetic) friction
rolling friction
fluid friction (drag)

16. 12.1 Forces
Forces and Motion
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. 12.1 Forces
Forces and Motion

18. 12.1 Forces
Forces and Motion
Sliding Friction
Once an object is moving, static friction no longer acts on it.
Instead, a smaller friction force called sliding (kinetic) friction acts on the object.
Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface.

19. 12.1 Forces
Forces and Motion
Because sliding friction is less than static friction, less force is needed to keep an object moving than to start it moving.

20. Rolling Friction Rolling friction acts on rolling objects.
12.1 Forces
Forces and Motion
Rolling Friction
Rolling friction acts on rolling objects.
For a given set of materials, the force of rolling friction is about 100 to 1000 times less than force of static or sliding friction.

21. 12.1 Forces
Forces and Motion
Fluid Friction
The force of fluid friction opposes the motion of an object through a fluid.
Fluid friction increases as the speed of the object moving through the fluid increases.
Fluid friction acting on an object moving through the air is known as air resistance.
At higher speeds, air resistance can become a significant force.

22. Gravity Gravity is a force that acts between any two masses.
12.1 Forces
Forces and Motion
Gravity
Gravity is a force that acts between any two masses.
Gravity is an attractive force, that is, it pulls objects together.
Gravity is a field force in that it does not require objects to be in contact for it to act on them.
Earth’s gravity acts downward toward the center of Earth.

23. 12.1 Forces
Forces and Motion
Falling Objects
Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.
As objects fall to the ground, they accelerate.

24. At this point, the forces acting on the object are balanced.
Forces and Motion
If an object falls for a long time, the upward force of air resistance becomes equal to the downward force of gravity.
At this point, the forces acting on the object are balanced.
Acceleration is zero and the object continues falling at a constant velocity.

25. 12.1 Forces
Forces and Motion
Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

26. 12.1 Forces
Forces and Motion

27. 12.1 Forces
Forces and Motion
Projectile Motion
Projectile motion is the motion of a falling object after it is given an initial forward velocity.
Air resistance and gravity are the only forces acting on a projectile.

28. Figure 9 shows the motion of two balls released at the same time.
12.1 Forces
Forces and Motion
Figure 9 shows the motion of two balls released at the same time.
Figure 9A shows that balls of different masses fall at the same rate.
Figure 9B shows a ball given an initial horizontal velocity falls vertically at the same rate as a ball with zero horizontal velocity.

29. 12.1 Forces
Forces and Motion
The combination of an initial forward velocity and the downward force of gravity causes the ball to follow a curved path.
The two balls fall with the same vertical acceleration and strike the ground at the same time.

30. 12.1 Forces
Forces and Motion

31. 12.1 Forces
Forces and Motion

32. 12.2 Newton’s First and Second Laws of Motion
Forces and Motion
12.2 Newton’s First and Second Laws of Motion

33. 12.2 Newton's First and Second Laws of Motion
Forces and Motion
Aristotle
Aristotle incorrectly proposed that force is required to keep an object moving at constant speed.

34. 12.2 Newton's First and Second Laws of Motion
Forces and Motion
Galileo
Galileo concluded that moving objects not subjected to friction or any other force would continue to move indefinitely.

35. Newton’s First Law of Motion
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Newton’s First Law of Motion
Newton summarized his study of force and motion in several laws of motion.
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.

36. Unless an unbalanced force acts on it,
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Unless an unbalanced force acts on it,
an object at rest remains at rest
an object in motion remains in motion with the same speed and direction.
Newton’s first law is sometimes called the law of inertia.
Inertia is the tendency of an object to resist a change in its motion.

37. Newton’s Second Law of Motion
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Newton’s Second Law of Motion
An unbalanced, or net force causes an object’s velocity to change.

38. 12.2 Newton's First and Second Laws of Motion
Forces and Motion
The acceleration is directly proportional to the net force acting on the object.
Acceleration also depends upon the mass of the object (inversely proportional)
Mass is a measure of inertia of an object and depends on the amount of matter the object contains.

39. The equation is a=F/m or more familiarly F=ma.
12.2 Newton's First and Second Laws of Motion
Forces and Motion
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.
The equation is a=F/m or more familiarly F=ma.
The acceleration of an object is always in the same direction as the net force.

40. 12.2 Newton's First and Second Laws of Motion
Forces and Motion
Newton’s second law also applies when a net force acts in the direction opposite to the object’s motion.

41. 12.2 Newton's First and Second Laws of Motion
Forces and Motion

42. 12.2 Newton's First and Second Laws of Motion
Forces and Motion

43. 12.2 Newton's First and Second Laws of Motion
Forces and Motion

44. 12.2 Newton's First and Second Laws of Motion
Forces and Motion

45. Effects of a Force on Acceleration
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Effects of a Force on Acceleration

46. Effects of a Force on Acceleration
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Effects of a Force on Acceleration

47. Effects of a Force on Acceleration
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Effects of a Force on Acceleration

48. Effects of a Force on Acceleration
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Effects of a Force on Acceleration

49. Effects of a Force on Acceleration
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Effects of a Force on Acceleration

50. Effects of a Force on Acceleration
12.2 Newton's First and Second Laws of Motion
Forces and Motion
Effects of a Force on Acceleration

51. 12.2 Newton's First and Second Laws of Motion
Forces and Motion
Weight and Mass
Although related to each other, mass and weight are not the same.
Weight is the force of gravity acting on an object.
An object’s weight is the product of the object’s mass and acceleration due to gravity acting on it.
The equation is W=mg.

52. The weight formula is basically Newton’s second law.
12.2 Newton's First and Second Laws of Motion
Forces and Motion
The weight formula is basically Newton’s second law.
Weight is substituted for force
Acceleration due to gravity is substituted for acceleration.

53. The value of g (at Earth’s surface) in the formula is 9.8 m/s2.
12.2 Newton's First and Second Laws of Motion
Forces and Motion
The value of g (at Earth’s surface) in the formula is 9.8 m/s2.
Mass is a measure of the inertia of an object
Weight is a measure of the force of gravity acting on an object.

54. 12.3 Newton’s Third Law of Motion and Momentum
Forces and Motion
12.3 Newton’s Third Law of Motion and Momentum
Newton’s Third Law
A force cannot exist alone.
Forces always exist in pairs.
According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

55. These two forces are called action and reaction forces.
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
These two forces are called action and reaction forces.
The forces are equal in size and opposite in direction.
The forces do not cancel each other because they act on different objects.

56. 12.3 Newton's Third Law of Motion and Momentum
Forces and Motion

57. Momentum Momentum is the product of an object’s mass and its velocity.
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Momentum
Momentum is the product of an object’s mass and its velocity.
The momentum for any object at rest is zero.
Momentum is calculated by multiplying an object’s mass (in kg) and its velocity (in m/s).

58. Momentum is measured in kg∙m/s.
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
The formula is p=mv.
Momentum is measured in kg∙m/s.

59. Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum
Under certain conditions, collisions obey the law of conservation of momentum.
In physics, conservation means that something has a constant value.
Conservation of momentum means that momentum does not increase or decrease.

60. Objects within the system can exert forces on one another.
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
A closed system means other objects and forces cannot enter or leave the system.
Objects within the system can exert forces on one another.

61. 12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change.
In a closed system the loss of momentum of one object equals the gain in momentum of another object
momentum is conserved

62. Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Figure 17A and 17B
Forces and Motion
Conservation of Momentum

63. Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum

64. Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum

65. Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum

66. Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum

67. Conservation of Momentum
12.3 Newton's Third Law of Motion and Momentum
Forces and Motion
Conservation of Momentum

68. 12.4 Universal Forces
Forces and Motion
12.4 Universal Forces
Observations of planets, stars, and galaxies strongly suggest four different forces exist throughout the universe.
The four universal forces are
electromagnetic
strong nuclear
weak nuclear
gravitational

69. Electromagnetic Forces
12.4 Universal Forces
Forces and Motion
Electromagnetic Forces
Electric and magnetic forces are two different aspects of the electromagnetic force.
Electromagnetic force is associated with charged particles.
Electric force and magnetic force are the only forces that both attract and repel.

70. Electric Forces Objects with opposite charges attract each other.
12.4 Universal Forces
Forces and Motion
Electric Forces
Objects with opposite charges attract each other.
Objects with like charges repel each other.

71. 12.4 Universal Forces
Forces and Motion
Magnetic Forces
Magnetic forces act on certain metals, on the poles of magnets, and on moving charges.
Magnets have two poles that attract each other.
Like poles repel each other.

72. 12.4 Universal Forces
Forces and Motion
Nuclear Forces
Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together.

73. 12.4 Universal Forces
Forces and Motion
Strong Nuclear Force
The strong nuclear force is a powerful force of attraction that acts only on the neutrons and protons in the nucleus.
Although this force acts over only extremely short distances, it is 100 times stronger than the electric force of repulsion at these distances.

74. 12.4 Universal Forces
Forces and Motion
Weak Nuclear Force
The weak nuclear force is an attractive force that acts only over a range that is shorter than the strong nuclear force.
The existence of beta decay indicates that there must a force much weaker than the strong nuclear force.

75. Gravitational Force Gravity is the weakest universal force.
12.4 Universal Forces
Forces and Motion
Gravitational Force
Gravity is the weakest universal force.
Gravitational force is an attractive force that acts between any two masses.
Newton’s law of universal gravitation states that every object in the universe attracts every other object.
F=G(m1m2/d2)
Value of G is 6.67×10-11N∙m2/kg2

76. Gravity Acts Over Large Distances
12.4 Universal Forces
Forces and Motion
Gravity Acts Over Large Distances
The gravitational force between two objects is proportional to their masses.
Gravitational force decreases with the square of the distance between the objects.
Gravity is the weakest universal force but it is the most effective force over long distances.

77. Gravitational Forces Acting on Masses at Different Distances
12.4 Universal Forces
Forces and Motion
Gravitational Forces Acting on Masses at Different Distances

78. Gravitational Forces Acting on Masses at Different Distances
12.4 Universal Forces
Forces and Motion
Gravitational Forces Acting on Masses at Different Distances

79. Gravitational Forces Acting on Masses at Different Distances
12.4 Universal Forces
Forces and Motion
Gravitational Forces Acting on Masses at Different Distances