motion and forces

1. Mr. Motuk (Room 124)
QQ
To me there has never been a higher source of earthly
honor or distinction than that connected with advances
in science.
Isaac Newton
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2. Frames of Reference
 You don't always need to see something move to
know that motion has taken place
 A reference point is needed to determine the
position of an object
 Ever felt like you were slowly moving backwards
when a semi truck passed you on the highway?
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3. Frames of Reference
 You have mistakenly made the truck your frame
of reference, measuring your motion relative to
the truck
 Both vehicles move forward relative to the
stationary tree (the ground is the proper frame
of reference)
Proper Frame of Reference 33

4. Describing One-Dimensional
 Motion- a change in position, measured by distance
and time
 The SI unit of length or distance is the meter (m)
 Shorter distances are measured in centimeters (cm)
 Longer distances are measured in kilometers (km)
 The following quantities are used to describe
motion:
 Speed
 Velocity
 Acceleration
Motion
The fastest “thing” travels
at ~670,000,000 mph…
What is it? Light
44

5. Change in Position
 Suppose a runner jogs to the 50-m mark and then
turns around and runs back to the 20-m mark
 Distance- quantity that tells you how far something has
moved
 The runner travels 50 m in the original direction (east)
plus 30 m in the opposite direction (west), so the total
distance she ran is 80 m
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6. Change in Position
 Sometimes you may want to know not only your
distance but also your direction from a reference
point, such as from the starting point
 Displacement- the distance AND direction of an object’s
position relative to a starting point
 Adding displacement: 50 m east, turn around and run 30
m west = 20 m east total displacement
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7. Speed
 Speed- the distance traveled by
a moving object over a period of
time
 Kilometers/sec, miles/hour,
meters/min
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8. Speed Formula
D = S X T
T = D/S
S = D/T
Example: A rifle bullet travels 1200 meters in 4
seconds.
What is the speed of the bullet?
S = 1200m/4 sec. S = 300 m/sec.
S = D / T
Step # 1 Step # 2 Step # 3
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9. Constant Speed
 A moving object that doesn’t change its speed
travels at constant speed
 Constant speed- equal distances are covered in
an equal amount of time (i.e. 25 miles/hour)
 This results in a linear position vs. time graph
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10. Changing Speed
 Usually speed is not
constant
 Usually the speed will
change for any
number of reasons
(wind, stop lights,
etc.)
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11. Instantaneous speed
Instantaneous speed-speed
at any instant
which the word “speed”
alone is representing
“My speed is 60 miles/h”
is referring to your speed
at that particular
moment, but likely to
change
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12. Average Speed
AA ccaarr ttrraavveellss aatt 5500 kkmm//hh,,
sslloowwss ddoowwnn ttoo 00 kkmm//hh,,
and speeds uupp aaggaaiinn ttoo 6600 kkmm//hh
Its average speed over the whole journey:
overall distance travelled
total time of travel
=
Instantaneous speeds
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13. Graphing Motion
 On a distance (or position)-
time graph, the distance,
or position, is plotted on
the vertical axis and the
time on the horizontal
axis
 Each axis must have a
scale that covers the
range of number to be
plotted
 The slope on a distance-time
graph is equal to
speed 1133

14. Check for Understanding
 What is the difference between
distance and displacement?
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15. Check for Understanding
 __________ is the distance an
object travels per unit of time.
A. acceleration
B. displacement
C. speed
D. velocity
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16. Check for Understanding
Name two observations you can make about the
cars speed from looking at the graph.
Calculate the speeds of both cars from the graph
by choosing two points on each line.
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17. Check for Understanding
Calculate the average speed of the car below:
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18. Velocity
 Velocity- a speed in a given direction
 It’s possible for two objects to have the same
speed, but different velocities
velocity
Has
directio
direction n!
magnitude
(speed)
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19. Earth’s speed at the equator: 1670 km/h
Earth’s velocity at the equator: 1670 km/h to the East
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20. Velocity
 Velocity depends on direction as well as speed,
so the velocity of an object can change even if
the speed of the object remains constant
 The speed of this car might be constant, but its
velocity is not because the direction of motion
is always changing
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21. Velocity and Momentum
 A moving object has a property called momentum
that is related to how much force is needed to
change its motion
 Momentum (p) takes into consideration not only
an object’s velocity AND mass
 Mass- the amount of matter (atoms) in an object
(kg)
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22. Velocity and Momentum
 Momentum is given the symbol p and can
be calculated with the following equation
p = mass (kg) X velocity (m/s)
 The unit for momentum is kg · m/s. Notice
that momentum has a direction because
velocity has a direction.
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23. Velocity and Momentum
 When two objects have the same velocity, the one with the
larger mass has the larger momentum
 The 1,000-kg car traveling at 20 m/s east has a momentum
of 20,000 kg•m/s east.
 p = m X v = 1000kg X 20 m/s
 What about the truck?
 Law of conservation of momentum- the total momentum of
a system stays the same before and after an interaction
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24. Check for Understanding
 Speed or Velocity?
 A race car traveling 155 miles per hour
turning left on a circular racetrack
 A sprinter running 3 meters/sec
 A tornado heading west at 15 km/hour
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25. Check for Understanding
 Speed or Velocity?
 A race car traveling 155 miles per hour V
turning left on a circular racetrack
 A sprinter running 3 meters/sec S
 A tornado heading west at 15 km/hour V
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26. Check for Understanding
 A 1,500-kg car is traveling west at
100 m/s. What is the car’s
momentum?
A. 1,500 kg•m/s
B. 150,000 kg•m/s
C. 1,400 kg•m/s
D. 1,600 kg•m/s
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27. Check for Understanding
 A 1,500-kg car is traveling west at
100 m/s. What is the car’s
momentum?
B. 150,000 kg•m/s
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28. Change in Velocity
 Velocity rarely stays constant
 Acceleration is the rate of change of
velocity
 When the velocity of an object
changes, the object is accelerating
 A change in velocity can be either
a change in how fast something is
moving, or a change in the
direction it is moving
 Acceleration occurs when an object
changes its speed, its direction, or both
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29. Change in Velocity
 In a car we can change our velocity 3
ways:
 Speed up
 Slow down
 Change direction
 All of these would be considered
acceleration
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30. Change in Velocity
We say that this car is accelerating
because its velocity is increasing
We say that this car is accelerating
because its direction is changing as it
turns, which means its velocity is
changingeven though its speed stays
constant
We say that this car is accelerating
because its velocity is decreasing.
Decreasing velocity is still acceleration,
although it is a negative
acceleration
30 km/h 60 km/h
60 km/h
60 km/h
60 km/h 30 km/h 0 km/h
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31. Change in Velocity
 Changing speed changes velocity and is
therefore considered acceleration
 Positive acceleration speeding up
 Negative acceleration slowing down
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32. Velocity vs. Time Graphs
The slope of the line on a speed-time graph
equals the object’s acceleration
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PPoossiittiivvee
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33. Change in Velocity
 Is the velocity for each car constant or changing?
 Which car has the highest velocity?
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34. Acceleration Formula
A = Vfinal–Vinitial
T
OR
Example: A cars velocity chang e s from 0.0m/s south to 50.0m/s
south in 10.0 seconds. Calculate the cars acceleration.
A = 5.0 m/s/s
or m/s2
A = Vfinal – Vinitial
T
A = 50.0m/s – 0.0m/s
10.0s
Step # 1 Step # 2 Step # 3
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35. Check for Understanding
A car traveling at 60 mph accelerates to
90 mph in 3 seconds. What is the
car’s acceleration?
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36. Check for Understanding
A car traveling at 60 mph accelerates to
90 mph in 3 seconds. What is the
car’s acceleration?
Acceleration = Velocity(final) - Velocity(initial)
time
= 90 mph - 60 mph
3 seconds
=
30 mph
3 seconds
= 10 mph/second
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37. PPoossiittiivvee
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Acceleration
Velocity vs. Time Graph
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38. Check for Understanding
A car traveling at 60 mph slams on the breaks to
avoid hitting a deer. The car comes to a safe stop
6 seconds after applying the breaks. What is the
car’s acceleration?
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39. Check for Understanding
A car traveling at 60 mph slams on the breaks to
avoid hitting a deer. The car comes to a safe stop
6 seconds after applying the breaks. What is the
car’s acceleration?
Acceleration = Velocity(final) - Velocity(initial)
time
= 0 mph - 60 mph
6 seconds
= -60 mph
6 seconds
= -10 mph/second
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40. NNeeggaattiivvee
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Acceleration
Velocity vs.Time Graph
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41. Acceleration in 2D
 The speed of the horses
in this carousel is
constant, but they are
accelerating because their
direction is changing
 This would be considered
centripetal acceleration-acceleration
of an object
toward the center of a
curved or circular path
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42. Horizontal & Vertical Motion
Are Independent
Gravity
makes
both
bullets
fall at
the
same
rate
The bullet from the gun keeps going
forward while it falls.
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43. What if the Projectile is Thrown
Upward?
Projectiles keeps moving forward with
.
the same speed.
Gravity
slows projectiles down
while going up
and speeds them up
while going down.
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44. CChheecckk ffoorr UUnnddeerrssttaannddiinngg
 Which is NOT a form of acceleration?
A. maintaining a constant speed and
direction
B. speeding up
C. slowing down
D. turning
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45. Check for Understanding
 Which is NOT a form of acceleration?
A. maintaining a constant speed and
direction
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46. The question is… why?
Why does
everything in
the universe
move?
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47. The answer…
Big, huge, massive forces!
And little ones too.
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48. Forces
A force is a pull (an attraction)
Or, a push (a repulsion)
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49. Forces
 All forces have two properties:
 Direction
 Size
 A newton (N) is the unit that describes the size
of a force and is equal to 1kg X m/s2
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50. Changing Motion
 A force can cause the motion of an object to
change
 If you have played pool, you know that you can force a
ball at rest to roll into a pocket by striking it with
another ball
 The force of the moving ball causes the ball at rest to
move in the direction of the force
 Force does not always change motion, though

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51. Net Force
 When all the forces acting on an object are
considered together, you determine the net force
on the object
 An object with a net force of anything other than 0
N on it will change its state of motion
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52. Forces in the Same Direction
 When forces are applied in the same
direction, they are added to determine the
size of the net force
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53. 5533

54. Forces in Different Directions
 When two forces act in opposite directions, you
subtract the smaller force from the larger force
to determine the net force
 The net force will be in the same direction as the
larger force
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55. Balanced Forces
 Balanced forces cancel each other out!
They are forces that are equal in size and
opposite in direction
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56. Types of Forces
1. Friction
2. Gravity
3. Electromagnetic
4. Nuclear
5. Etc.
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57. 1. Friction
 Friction- the force that
opposes the sliding motion of
two surfaces that are
touching each other
 i.e. skateboard stops rolling
 It always slows a moving object
down
 The amount of friction between
two surfaces depends on two
factors¾the kinds of surfaces
and the force pressing the
surfaces together.
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58. 1. Friction
Force on person
by box
Force on floor by box Force on box
by floor
Force on box
by person
Corrugations and imperfections in the surfaces
grind when things slide.
How can we reduce friction?
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59. Cause of Friction
•The larger the force pushing the two surfaces
together is, the stronger these microwelds will
be, because more of the surface bumps will
come into contact
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60. Types of Friction
 Static-prevents two surfaces from sliding
past each other at all (move a box of books)
 Sliding- opposes sliding motion (box of books
that is sliding stops moving)
 Rolling- acts over the area where the wheel
and surface meet like traction (skateboard
with box of books on it stops moving)
 Fluid (Viscous)- opposes the motion of
objects traveling through a fluid (air or
water)
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61. 2. Gravity
 Galileo-1600’s studied how things fell
 Gravity is an attractive force
between any two objects that
depends on the masses of the
objects and the distance between
them
 Isaac Newton formulated the law of universal
gravitation, which he published in 1687
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62. Law of Universal Gravitation
 This law can be written as the following equation
2
 F 1
is the mmforce of gravity, G is a constant called
the universal gravitational constant, and d is the
distance between the two masses, and  The greater the mass of two objects, the greater the
gravitational force (F) between them
 The greater the distance between two objects, the
less the gravitation force between them 6622

63. Gravitational Force
 No matter how far apart two objects are,
the gravitational force between them never
completely goes to zero
 Because of this gravity is called a long-range
force
 The strength of the gravitational field is 9.8
N/kg near Earth’s surface and gets smaller as
you move away from Earth
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64. Weight
 Because the weight of an object on Earth is
equal to the force of Earth’s gravity on the
object, weight can be calculated from this
equation:
or (m/s2)
 Where Fg is the force of gravity on an
object…..in other words, its weight…and g is 9.8
N/kg near Earth’s surface (9.8N/kg = 9.8 m/s2)
6644

65. Mass
 Weight and mass are not the same
 Weight is a force and mass is a measure of
the amount of matter an object contains
 Weight and mass are related. Weight
increases as mass increases
or (m/s2)
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66. Mass vs. Weight
The amount of matter
(atoms) in an object
A measure of gravity’s
pull on an object
Measure with a balance Measure with a Newton
scale
Never changes
Changes due to gravity
Both are
measure-ments
of
matter
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67. Check for Understanding
• What is the weight of a 10-kg block?
10 kg m
9.8
N/kg Fg
Fg = mg = (10 kg)(9.8 N/kg)
Fg F = 98 N g = 98 N
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68. Newton’s Laws of Motion
 Newton lived from 1642–1727
 #1 An object in motion stays in motion and
an object at rest stays at rest unless
acted upon by an unbalanced force
 #2 Force equals mass times acceleration
(F = ma)
 #3 For every action there is an equal and
opposite reaction
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69. Newton’s First Law
An object in motion stays
in motion and an object at
rest stays at rest unless
acted upon by an
unbalanced force
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70. Newton’s First Law
 What does this mean?
 AAn object will keep doing what it’s doing UNLESS
acted on by an unbalanced force like friction
 If it is moving at a constant velocity it will
continue
 If it is at rest, it stays at rest
 In outer space, away from gravity and any
sources of friction, a rocket ship launched with a
certain speed and direction would keep going in
that same direction and same speed forever
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71. Newton’s First Law
 Called the Law of Inertia- the tendency of an
object to resist changes in its state of motion
 Recall that mass is the amount of matter (atoms)
in an object
 Newton’s First Law states that all objects have
inertia
 The more mass an object has, the more inertia it
has (and the harder it is to change its motion)
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72. Then why don’t moving objects keep moving
forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon it
A book sliding across a table
slows down and stops because of
the force of friction
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity
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73. Newton’s Second Law
Force equals mass times acceleration
F = ma
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74. Newton’s Second Law
 What Does F = ma Mean?
 The force of an object comes from its mass and its
acceleration so that the acceleration of an object is in
the same direction as the net force on the object
 A massive glacier that’s
changing speed very slowly
(low acceleration) can still have
great force due to its mass
 Something very small (low mass)
like a bullet that’s changing
speed very quickly
(high acceleration) can still have
a great force
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75. Force = Mass X Acceleration
 Force is directly proportional to mass
and acceleration
 First ball: has a certain mass, m, moving
at a certain acceleration, a, and therefore
a certain force, f.
 Second ball: has double the mass of the
first ball, 2m, and the same acceleration,
a, therefore has twice the force of the
first ball, 2f
 Third ball: has mass m moving at twice
the first ball’s acceleration, 2a, would
have a force of 2f.
a
a
a
m
m
m
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76. Newton’s Third Law
For every action there is an equal and
opposite reaction
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77. Newton’s Third Law
 What Does this Mean?
 When one object exerts a force on a second object, the
second one exerts a force on the first that is equal in
strength and opposite in direction
 Gravity is pulling you down in your seat, but Newton’s
Third Law says your seat is pushing up against you with
equal force
 There are balanced forces acting on you– gravity pulling
down and your seat pushing up- so you are not moving
gravity
your seat 7777

78. Newton’s Third Law
 For every action force, there must be an equal
and opposite reaction force
 Forces occur in pairs
The action force is exerted
by the _____ hhaannddss on the _____.
bbaarr
The reaction force is
exerted by the _____ bbaarr
on
the _____.
hhaannddss
Action
Reaction
Newton’s Laws on teachersdomain 7788

79. Check for Understanding
One newton is a force which imparts an
acceleration of 1 m/s2 to a mass of 1 kg.
FF ((NN)) == mm ((kkgg)) aa ((mm//ss22))
What resultant force will give a 3 kg mass an
acceleration of 4 m/s2?
F = m a
3 kg F = 3 kg X 4 m/s2
FF == 1122 NN
F = ?
a = 4 m/s2
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80. Check for Understanding
 Inertia is__________.
 A. the tendency of an object to
resist any change in its motion
 B. the tendency of an object to
have a positive acceleration
 C. The tendency of an object to
have a net force of zero.
 D. The tendency of an object to
change in speed or direction.
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81. Check for Understanding
 Inertia is__________.
 A. the tendency of an object to
resist any change in its motion
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82. Check for Understanding
 Newton’s second law of motion
states that _________ of an
object is in the same direction as
the net force on the object.
 A. acceleration
 B. momentum
 C. speed
 D. velocity
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83. Check for Understanding
 Newton’s second law of motion
states that _________ of an
object is in the same direction as
the net force on the object.
 A. acceleration
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84. Newton’s Law Applied to Life
 Newton’s 3 laws can be used to explain
everyday events, such as falling, and collision
 These laws have been applied to aid in
technology, safety, and countless other ways
 Newton’s Laws on Science360
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85. Newton’s First Law with Seat
Belts
 Don’t let this be you
 Due to inertia, objects (including you) resist
changes in their motion. When you and the car going
80 km/hour is stopped by the brick wall, your body
keeps moving at 80 km/hour
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86. Newton’s First Law with Air
Bags
 Air bags also reduce injuries in car crashes by
providing a cushion that reduces the force on the
car's occupants
 When impact occurs, a chemical reaction occurs
in the air bag that produces nitrogen gas
 The air bag expands rapidly and then deflates
just as quickly as the nitrogen gas escapes out of
tiny holes in the bag
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87. Newton’s First Law and
Centripetal Force
 According to Newton, as a car tries to make a
turn, the car would continue in a straight line
unless there was a force acting on the car to turn
it
 This force of friction acting upon the turned
wheels provides centripetal force required for
circular motion
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88. Newton’s First Law and
Centripetal Force
Inertia
Without a centripetal force,
an object in
motion continues along a
straight-line path
With a centripetal force,
an object in motion will
be accelerated and change
its direction
Centripetal Force
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89. Newton’s First Law and
Centripetal Force
 As a bucket of water is spun in a circle, the
tension force acting upon the bucket provides the
centripetal force required for circular motion
 The force of gravity acting upon the moon
provides the centripetal force required for orbit
 Nascar and Centripetal Force
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90. Newton’s Second Law and
Gravitational Acceleration
 If gravity is the only force being exerted on an
object’s mass then the net force is Fg

 ****Combining the above gravitational law with
Newton’s second law, F=ma, the force due to
gravity only would cause an object to accelerate at
9.8 m/s/s (m/s2)
 Papers falling demo 9900

91. Acceleration Due to Gravity
 Gravity causes objects to
accelerate at the SAME rate,
9.8 m/s/s (~10 m/s/s)
 WITHOUT air resistance, a friction-like
force, all objects would fall at
the same speed
 Galileo on the moon
 Doesn’t depend on mass
 After 1 second falling at ~10 m/s
 After 2 seconds ~20 m/s
 3 seconds ~30 m/s
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92. Terminal Velocity
 Air resistance (fluid
friction) will increase as
object falls faster
causing an upward force
on the object
 Eventually gravity will
balance with air resistance
 Reaches terminal velocity
- highest speed reached
by a falling object
 Terminal velocity
No air resistance Air resistance
which is greater
on the feather
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93. 9933

94. Summary of Formulas
 Speed = distance traveled (m)
time (s)
 Velocity = displacement (distance with direction) (m)
time (s)
 Momentum (p) = velocity (m/s) X mass (kg)
 Acceleration = change in velocity (m/s) or m/s2
time (s)
 Force of gravity (weight in N) = mass (kg) X gravitational
strength 9.8 (N/kg)
 Force = mass X acceleration (9.8 m/s2 if due to gravity)
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95. 9955