Title: AND chapter 12
1 AND chapter 12
FORCES
MOTION
2What 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
- Accelerate a moving object by
- changing the objects speed
- direction
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4How do we MEASURE force?
- Forces can be measured on a spring scale
- WEIGHT is a FORCE
- UNITS OF FORCE
- Newton (N)
- A Newton is a force that causes a 1 kg mass to
accelerate at a rate of 1 meter per second each
second - Which is written as 1 N 1 kg ? m/s2
5Representing Forces
- Forces can be represented with an arrow
- The length of the arrow shows strength or
magnitude - Direction of the arrow shows the direction of the
force - Figure 2 on page 357 shows an example of weight
Normal Force
Applied Force
Friction Force
6Combining Forces
- Net Force the overall force acting on an object
after all forces are combined
7Combining Forces
8 9Net Force
10Balanced Forces
- Balanced Forces the net force is zero, there is
no change in the objects motion - Question what are some examples where the net
force would equal zero?
11ANSWER
- Net force would be zero when
- You play tug-of-war and neither team moves
- You arm wrestle and neither person wins
- A car using cruise control constant speed!
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13Unbalanced Forces
- Unbalanced Forces the net force acting on an
object does not equal zero
14- Question What happens to the object when the net
force acting on an object is NOT ZERO, or
unbalanced? (like the example below)
15Answer
- When an unbalanced force acts on an object, the
object accelerates
16- 3 examples of a net force causing an object to
accelerate - Pushing against the side of a book getting it
to move - A team winning a game of tug-of-war pulling the
other team towards them - A person in freefall
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18Watch video clip The Effects of Forces on Speed
- On video notes page, draw an example of when the
forces are balanced when they are unbalanced.
Then describe the result of these forces.
19Try these problems
- Two tugboats are moving a barge. Tugboat A exerts
a force of 3000 newtons on the barge. Tugboat B
exerts a force of 5000 newtons in the same
direction. What is the combined force on the
barge? Draw arrows showing the individual and
combined forces of the tugboats in this problem
to help you answer the question
20- Same direction so add
- 3000 N 5000 N 8,000 N
- ? ---? ------?
21- 2. Now suppose that Tugboat A exerts a force of
2000 newtons on the barge and Tugboat B exerts a
force of 4000 newtons in the opposite direction.
What is the combined force on the barge? Draw
arrows showing the individual and combined forces
of the tugboats in this problem.
22- Opposite directions, so subtract
- 4000 N 2000 N 2000 N
- ?-- ? ?
23- 3. Could there ever be a case when Tugboat A and
Tugboat B are both exerting a force on the barge
but the barge doesn't move? Draw arrows showing
the individual and combined forces in such a
situation.
24- ? ? 0
- Equal in size opposite in direction
25FRICTION
26FRICTION
- Friction a force that opposes the motion of
objects that touch as they move past each other - Without friction, it would be a very different
world!!! - Food would not stay on your fork!
- Cars would slide all over the road!
- Walking would be almost impossible!
- Friction acts at the surface where objects are
in contact
27The 4 Types of Friction
- There are 4 main types of friction
- Static Friction
- Sliding Friction
- Rolling Friction
- Fluid Friction
28STATIC FRICTION
- friction force that acts on objects that are not
moving - Static friction always acts in the direction
opposite to the applied force - Examples
- every time you take a step and push off
- glass of water sitting stationary on the table
29SLIDING FRICTION
- a force that opposes the direction of motion of
an object as it slides over a surface - Sliding friction is a weaker force than static
friction - This is why less force is needed to keep an
object moving than it is to start it moving
30ROLLING FRICTION
- The friction force that acts on rolling objects
- Rolling friction is about 100-1000 times less
than static or sliding friction - This is why we use wheeled dollies to move heavy
objects! - In machines, ball bearings, are often used to
reduce friction between two surfaces
31FLUID FRICTION
- Force that opposes the motion of an object
through a fluid - Water and a mixture of gases such as air are
known as fluids - Example a submarine moving through water
- The motion of the submarine is slowed by fluid
friction - Fluid friction increases as the speed of the
object moving through the fluid increase - So the faster the sub goes, the greater the
friction!!!
32AIR RESISTANCE
- Air resistance is a type of fluid friction
- Remembergases are considered fluids
- Fluid Friction acting on an object moving through
the air is called AIR RESISTANCE
33GRAVITY
34GRAVITY
- Gravity a force that acts between any two masses
- Gravity is an attractive force so it pulls
objects together - Gravity does not require objects to be in contact
for it to act on them - Gravity can act over large distances!!!
35More GRAVITY!
- Earth's gravity acts downward toward the center
of the Earth. - There is usually an upward force that acts
against gravity to balance out the forces and
allow objects to remain still.
36FALLING OBJECTS
- QUESTION What forces are acting on an object as
it falls?
37- ANSWER Only two forces acting on a falling
object are gravity and air resistance
38Forces Acting on Falling Objects
- Gravity causes object to accelerate downward
- Air resistance acts in the opposite direction of
the motion - Which means it reduces acceleration
39- Recallwhat happens to the amount of fluid
friction as an object speeds up?
40Watch video clip The Physics of Skydiving
- On the video notes page, describe what is meant
by terminal velocity describe the forces on the
skydiver at this point.
41Falling Objects
- If an object in freefall falls for long enough,
the upward force of air resistance will become
equal to the downward force of gravity. - At this point, the two forces are BALANCED
- Acceleration is zero when forces are balanced
- The object will continue to fall at a constant
velocity
42Terminal Velocity
- Terminal Velocity the constant velocity of a
falling object when the force of air resistance
equals the force of gravity
43Projectile Motion
- Projectile Motion The motion of a falling object
(projectile) after it is given an initial forward
velocity
44Projectile Motion
- Question What are the ONLY 2 FORCES that act on
a projectile???
All 3 balls are experiencing projectile motion!
45Projectile Motion
- Answer Air resistance and gravity!!!
- Refer to figure 9 on page 362 in textbook
- The combination of an initial forward velocity
and the downward vertical force of gravity causes
the ball to follow a curved path
46- If I shoot a bullet horizontally and at the same
time drop a bullet from the same height as the
gunwhich will hit the ground first?
47FALLING OBJECTS
- The two bullets WILL hit the ground at the same
time!
48The two balls fall with the same acceleration and
strike the ground at the same time!!!
Remember the Investigating Freefall Lab with
the marbles? How did that lab compare with the
previous statement???
4912.2 Newtons First Second Laws of Motion
50Its not ALL about Newton
- Aristotle incorrectly proposed that force is
required to keep an object moving at a constant
speed - Galileo studied how gravity produces constant
acceleration. He concluded that objects not
subjected friction or any other force would
continue to move indefinitely - Newton built off the work of Galileo and later
published his work in a book entitled Principia - Newton summarized his study of force and motion
in several laws of motion
511st Law of Motion Law of Inertia
521st Law of Motion Law of Inertia
- 1st Law The state of motion of an object does
not change as long as the net force acting on the
object is zero - In other words Unless an unbalanced force acts,
an object at rest remains at rest, and an object
in motion remains in motion with the same speed
and direction
531st Law Law of Inertia
- Example A soccer ball resting on the grass
remains motionless until a force is applied to it.
54And
- Example A soccer ball in motion remains in
motion unless a force acts on it.
55Why is it called the Law of Inertia
- INERTIA the tendency of an object to resist a
change in its motion so - an object at rest tends to remain at rest
(resist moving), and an object in motion tends to
remain in motion (resist stopping)
56- Remembersoccer ball sat motionless (forces were
balanced) until an unbalanced force acted on it - The ball has inertia
- Everything with mass has inertia
- The more mass, the more inertia
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58InertiaFront-end collision
- Example Front-end collision
- Collision makes car stop suddenly
- Since you have inertia you continue moving
forward - Page 364/365 Figure 12
59Newtons Second Law of Motion
602nd Law of MotionRelates the acceleration of an
object to the force acting on it the objects
mass.
- 2nd Law The acceleration of an object is equal
to the net force acting on it divided by the
objects mass. A F/M - In other words, the amount of acceleration an
object has depends on - How hard it is pushed (force)
- How heavy it is (mass)
61- Net Force
- Acceleration --------------
Mass -
62AccelerationForce Mass Relationship What is
the acceleration?
63ANSWER
- Acceleration is calculated by dividing the FORCE
by the MASS - Acceleration 100 N / 50 kg
- A 2 m/s2
642nd Law
- Example You apply force to a ball when you throw
it - The harder you throw, the more the ball
accelerates - If you double the force, the acceleration of the
ball doubles as well - If you double the mass of the ball the
acceleration is cut in half - More Examples
65Important NotesRegarding Newtons 2nd Law of
Motion
- The acceleration of an object is always in the
same direction as the net force. - In using Newtons second law, it is helpful to
realize that the units N/kg and m/s2 are
equivalent - Newtons second law also applies when a net force
acts in the direction opposite to the objects
motion - producing deceleration (See figure 13, page 368)
- This is the principle used by automobile seat
belts - See Math Skills page 367 (in textbook) for extra
help
66Weight MassWhat really is the difference?
67- We often talk about weight and mass as if they
were the same thing - Weight is the force of gravity acting on an
object. - An objects weight is the product of the objects
mass and acceleration due to gravity acting on
it. - Written mathematically as w m x g
68Weight lessens as
Weight will DECREASE if gravity DECREASES!!!
69Mass Versus Weight
- Mass is a measure of the inertia of an object,
- Measure with a balance
- Weight is a measure of the force of gravity
acting on an object - Measure with a spring scale
- Weight Mass x Acceleration due to gravity
- W mg
- Value of g 9.8 m/s2 (Earth)
70Important Note Regarding Units
- Be sure when you use the weight formula or
Newtons second-law formula that you use the
CORRECT units!!! - Force (F or W) in N (newtons)
- Acceleration (a or g) in m/s2 (meters per second
per second) - Mass (m) in kg (kilograms)
71Practice Problem
- 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? - What would his weight be on the moon (g1.62
m/s2)?
72Answer to Practice Problem
- ON EARTH
- Weight Mass ? Acceleration due to gravity
- W 112 kg x 9.8 m/s2
- W 1097 kg?m/s2 1097 N
- ON MOON
- W 112 kg x 1.62 m/s2
- W 181 kg?m/s2 181 N
73Mass Weight are Related
- Doubling the mass of an object also doubles the
objects weight (if gravity remains the same) - If Gravity changes then
- Example Think about the astronaut above
- On the moon, the acceleration due to gravity is
only about 1/6 that on Earth - Thus, the astronaut weighs only about 1/6 as much
on the moon as on Earth - In both locations, the mass of the astronaut is
the same!
7412.3 Newtons 3rd Law of Motion
75Forces CANT Exist Alone
- Forces always exist in pairs.
- According to Newtons 3rd 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. - These two forces are called action and reaction
forces
76Action Reaction Forces
- Example The book lying on the table is exerting
a downward force on the table, while the table is
exerting an upward reaction force on the book.
77Newtons Third Law Ex A rocket
78- More Examples
- Jumping off of a raft
- Action force you push on the raft
- Reaction force raft pushes back
- A swimmer in a pool
- Action force swimmers arm pushes on water
- Reaction force water pushes back on swimmer
79Action Reaction Forces
- Question Since these action/reaction forces are
equal in size and opposite in direction then why
dont they cancel each other out and produce a
net force of zero?
80Action Reaction Forces
- Because the action/reaction forces do not act on
the same object! - Swimmer Example
- The action force acts on the water
- The reaction force acts on the swimmer
81Momentum
- Momentum the product of an objects mass and
its velocity - An object with a large momentum is hard to stop
82- Momentum mass X velocity
- Practice Problem What is the momentum of a rock
with a mass of 0.5kg that is moving at a velocity
of 5m/s?
83- Momentum 0.5kg X 5m/s 2.5kg x m/s
- The units do NOT cancel!
84- Momentum can be transferred from one object to
another during a collision
8512.4 Universal Forces
86Four Universal Forces
- Electromagnetic forces- only forces that both
attract and repel. - Strong Nuclear forces- holds neutrons protons
together - Weak Nuclear forces- attractive force between
particles in nucleus - Gravitational forces- attractive force between
any two objects
87GRAVITY
- Gravity is the weakest universal force
- On a daily basis, you dont notice the force of
gravity that you exert on objectsthis is because
your mass is sooooooo small. - It takes a huge mass like the Earths to exert a
large gravitational force.
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89GRAVITY
- Gravitational force depends on two things the
mass of an object and the distance between two
objects. - A greater mass will exert a greater gravitational
force on an object. - The greater the distance between two masses
significantly decreases the gravitational force.
90Attractive Force of Gravity
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92GRAVITYhow can it be so weak?
- Even though gravity is the weakest universal
force, it is the most effective over large
distances. - Gravity holds you on the ground
- It holds the moon in orbit around the Earth
- It holds the planets in orbit around the Sun
- It holds the stars in orbit around their galaxies
93Centripetal Force
- The force of gravity from the Earth continuously
pulls the moon in a nearly circular orbit around
the Earth. - Centripetal force center-directed force that
continuously changes the direction of an object
to make it move in a circle. - As an object moves in a continuous (constant)
circular motion, it is accelerating. - How is it accelerating if it does not change
speed? - Change in direction!!!
94Centripetal Force
- Its a lot like a string tied to an eraser
- The force from the center of the string allows
the eraser to twirl in a circle over your head - As you twirl the eraser, the string exerts a
centripetal force on the eraser.
95Centripetal force Orbital Motion
- Objects need only a centripetal force and their
own inertia to maintain an orbit. - Orbital motion is a balance between the
centripetal force and inertia. - If Centripetal force is stronger it will crash,
if inertia is stronger it will fly off into space
96- Examples of objects in orbit
- Satellites
- Our moon is a natural satellite of our planet
97Orbital Motion
http//upload.wikimedia.org/wikipedia/commons/4/4e
/Orbital_motion.gif
98How is this possible?!?!
Same PlaceSame Day??? HOW!!!???
99How is that possible?
- Ever been to the seashore? What do you notice
about the level of the water throughout the day?
Why does it change? - The gravitational pull from the Moon produces two
tides in the Earths oceans as it moves around
the Earth. - One bulge where the moon is closest to the Earth
- One bulge where the moon is farthest from the
Earth
100Tides
- Since the Earth rotates once per day, it results
in two high tides and two low tides per day on
Earth!
http//home.hiwaay.net/krcool/Astro/moon/moontide
s/
101Rotation versus Revolution
- The Earth moves in two major ways
- Rotation- the spinning of the Earth on its axis
- This rotation causes day and night
- Revolution- the movement of one body in space
around another - When does the Earth show revolution? Revolves
around the Sun - How long does it take for the Earth to revolve?
- 365 Days
102Ch 13 Forces in Fluids
10313.1 Fluid Pressure
- Why is an Aeron chair so much more comfortable
than a bicycle seat?
104- Pressure the result of a force distributed over
an area
105- Ex the tip of a pencil easily pokes through a
sheet of paper while the eraser end does not - Ex finger poke vs needle poke
106- Pressure Force/Area
- Units of Pressure N/m2 Pascal (Pa)
107- Practice problem A box weighing 2700 N is
resting on the ground. If the box is covering
1.5 square meters of area, what is the pressure?
108- Pressure 2700 N/1.5 m2 1800 Pa 1.8 kPa
109Pressure in Fluids
- Recall a fluid is a substance that takes the
shape of its container, both liquids and gases
are fluids
110- What happens to the pressure exerted on your body
as you dive deeper in a pool?
111- Water pressure increases as depth increases
- The relationship between pressure and the depth
of a fluid can be shown with this formula P d
x h x g - d density
- h depth
- g accel due to gravity
112- Fluid pressure is determined by the type of fluid
and its depth - Which would exert more fluid pressure at a depth
of 20 m, Lake MI or the Gulf of Mexico?
113Air Pressure the Atmosphere
- We live at the bottom of a vast ocean of air.
- The weight of the Earths atmosphere exerts
pressure. At sea level 101 kPa - We refer to a certain altitude instead of a depth
- Air pressure decreases as the altitude increases
114- Why do your ears pop while traveling up a
mountain?
115- The difference in pressure between the inside of
your ear and the air outside causes air to pass
through a small tube within your ear to equalize
the pressure. - This is the popping sensation you feel
116- The atmosphere is exerting more than 1000 N on
the top of your head. Why arent we crushed by
this?
117- The inside of your body exerts pressure against
the air pressure outside which balances and
results in a net force of zero.
118Bernoullis Principle
- Bernoullis Principle as the speed of a fluid
increases, the pressure within the fluid
decreases - Demo take a piece of paper and blow across the
top of the paper
119Wings Lift
- The ability of birds and airplanes to fly is
explained by Bernoullis principle
120- The air traveling over the top of the curved wing
travels faster so the pressure is less which
create the lift
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