Holt Science & Technology - Physical Science Series

1. Forces & Motion
Book M - Chapter 2

2. Warmup
If Wile E. Coyote and a boulder fall off a cliff at
the same time, which do you think will hit the
ground first?

3. Section 1: Gravity and Motion
By the end of this section, you should be able to:
 Explain the effect of gravity and air resistance on
falling objects.
 Explain why objects in orbit are in free fall and
appear to be weightless.
 Describe how projectile motion is affected by
gravity.

4. Gravity and Falling Objects
 Objects fall to the ground at the same rate
because the acceleration due to gravity is the
same for all objects.
 For every second that an object falls, the
object’s downward velocity increases by 9.8
m/s.

6. Calculating the Velocity of Falling Objects
Δv  g · t
change in velocity = accelerationgravity · time
 If an object starts at rest, this equation yields
the velocity of the object after a certain time
period.

7. Warmup
Complete #7 on p.43

9. Calculating Time
Katey drops a ball off the top of a parking
garage. Dakota uses a radar gun to determine
the speed of the ball when it hits the ground is
44.1 m/s.
How long was the ball in the air?

10. Air Resistance and Falling Objects
 Air resistance is the force that opposes the
motion of objects through air.
 The amount of air resistance acting on an
object depends on the size, shape, and speed
of the object.
 As the speed of a falling object increases, air
resistance increases.

12. Air Resistance and Falling Objects
 The upward force of air resistance continues
to increase until it is equal to the downward
force of gravity.
 The object then falls at a constant velocity
called the terminal velocity.

13. Air Resistance and Falling Objects
 An object is in free fall only if gravity is pulling
it down and no other forces are acting on it.
 A vacuum is a place in which there is no
matter. Objects falling in a vacuum are in free
fall because there is no air resistance.

14. Orbiting Objects are in Free Fall
 Astronauts float in orbiting spacecrafts
because of free fall.

15. Orbiting Objects are in Free Fall
 An object is orbiting when it is traveling
around another object in space.

16. Orbiting Objects are in Free Fall
 The unbalanced force that causes objects to
move in a circular path is called a centripetal
force.
 Gravity provides the centripetal force that
keeps objects in orbit.

17. Projectile Motion and Gravity
 Projectile motion is the curved path an object
follows when it is thrown or propelled near the
surface of the Earth.
 Projectile motion has two components—
horizontal motion and vertical motion. These
components are independent, so they have
no effect on each other.

18. Projectile Motion and Gravity
 Horizontal motion is a motion that is parallel to
the ground.
 When you throw a ball, your hand exerts a
force on the ball that makes the ball move
forward. This force gives the ball its horizontal
motion.

19. Projectile Motion and Gravity
 Vertical motion is motion that is perpendicular
to the ground.
 A ball in your hand is prevented from falling by
your hand. After you throw the ball, gravity
pulls it downward and gives the ball vertical
motion.

22. Section Review
p.43 #2-7, 9

23. Warmup
If you are sitting still in your seat on a bus that is
traveling 100 km/h on a highway, is your body
at rest or in motion?
Explain your answer. Use a diagram if it will
help make your answer clear.

24. Section 2: Newton’s Laws of Motion
By the end of this section, you should be able to:
 Describe Newton’s first law of motion, and explain
how it relates to objects at rest and objects in
motion.
 State Newton’s second law of motion, and explain
the relationship between force, mass, and
acceleration.
 State Newton’s third law of motion, and give
examples of force pairs.

25. Newton’s First Law of Motion
 An object at rest remains at rest, and an
object in motion remains in motion at a
constant speed and in a straight line unless
acted on by an unbalanced force.
 Newton’s first law of motion describes the
motion of an object that has a net force of 0 N
acting on it.

26. Newton’s First Law of Motion
 Part 1: Objects at rest will stay at rest unless
they are acted on by an unbalanced force.

27. Newton’s First Law of Motion
 Part 2: Objects will continue to move with the
same velocity unless an unbalanced force
acts on them.
 Friction between an object and the surface it
is moving over is an example of an
unbalanced force that stops motion.

29. Newton’s First Law of Motion
 Newton’s first law is sometimes called the law
of inertia. Inertia is the tendency of all objects
to resist any change in motion.
 Mass is a measure of inertia. An object that
has a small mass has less inertia than an
object that has a large mass.

30. Newton’s First Law of Motion
 So, changing the motion of an object that has
a small mass is easier than changing the
motion of an object that has a large mass.

31. Newton’s Second Law of Motion
 The acceleration of an object depends on the
mass of the object and the amount of force
applied.
 Newton’s second law describes the motion of
an object when an unbalanced force acts on
the object.

32. Newton’s Second Law of Motion
 Part 1: The acceleration of an object
decreases as its mass increases. Its
acceleration increases as its mass decreases.
 Part 2: An object’s acceleration increases as
the force on the object increases. The
acceleration of an object is always in the
same direction as the force applied.

33. Newton’s Second Law of Motion

35. Newton’s Second Law of Motion


38. 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.
 Newton’s third law of motion can be simply
stated as follows: All forces act in pairs.

39. Newton’s Third Law of Motion
 A force is always exerted by one object on
another object. This rule is true for all
forces, including action and reaction forces.

40. Newton’s Third Law of Motion
 Action and reaction forces in a pair do not act
on the same object. If they did, the net force
would always be 0 N and nothing would ever
move!

41. Newton’s Third Law of Motion
 Newton’s third law says that all forces act in
pairs. When a force is exerted, there is always
a reaction force.

42. Newton’s Third Law of Motion
 When an object falls, gravity pulls the object
toward Earth and pulls Earth toward the
object.
 You don’t notice Earth being pulled upward
because the mass of Earth is much larger
than the mass of the object. Thus, the
acceleration of Earth is much smaller than the
acceleration of the object.

43. Section Review
p.51 #2-6, 8

44. Warmup
Make a list of 5 things you think have
momentum and 5 things you think don’t have
momentum.

45. Section 3: Momentum
By the end of this section, you should be able to:
 Calculate the momentum of moving objects.
 Explain the law of conservation of momentum.

46. Momentum, Mass, and Velocity
 The momentum of an object depends on the
object’s mass and velocity.

47. Calculating Momentum


48. Calculating Momentum

49. The Law of Conservation of Momentum
 The law of conservation of momentum states
that any time objects collide, the total amount
of momentum stays the same.
 After two objects stick together, they move as
one object. The mass of the combined objects
is equal to the masses of the two objects
added together.

50. The Law of Conservation of Momentum
 The combined objects have a different
velocity because momentum is conserved and
depends on mass and velocity.
 So, when the mass changes, the velocity
must change, too.

51. The Law of Conservation of Momentum
 When two objects bounce off each
other, momentum is usually transferred from
one object to the other.
 The transfer of momentum causes the objects
to move in different directions at different
speeds.

52. The Law of Conservation of Momentum
 Conservation of momentum can be explained by
Newton’s third law.
 Because action and reaction forces are equal and
opposite, momentum is neither gained or lost in a
collision.

53. Section Review
p.55 #2-7