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Ch 4 lecture guide.notebook
1
April 24, 2015
kew. 71:30 PM
Chapter FOUR: Forces in One Dimension
kew. 71:30 PM
4.1 Force and Motion
force: a push or pull exerted on an object
forces cause objects to:
speed up
slow down
change direction
therefore, a force causes an acceleration
= change in velocity
force is a vector
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April 24, 2015
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when considering force problems,
the object of interest = the system
everything that acts on the system = the external world
ex: book on table
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Contact forces and Field forces
external world physically touches the system
exerted by external world without physical contact
Ex: Kyle shoves Mrs. Stowell out the window
Ex: gravity pulls the flower pot to Earth
agent: the cause of the force
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Freebody diagrams
sketch the situation
identify the system
identify the contact forces and the directions they push/pull
identify the field forces and the directions they push/pull
Flower pot is sitting on a shelf
system = flower pot
contact forces: shelf pushing up
field force: gravity pulling down
simplify to
agent: shelf
agent: earth's mass
Fshelf
FEarth's mass
length of vector arrows indicate magnitude
= length = constant velocity
Oct 281:10 PM
draw free body diagram of:
skydiver free falls out of a helicopter
(ignore air resistance)
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April 24, 2015
Oct 281:10 PM
draw free body diagram of:
skydiver free falls and accelerates toward the ground (DO factor in air resistance)
Oct 281:10 PM
draw free body diagram of:skydiver opens chute and falls at a constant velocity toward the ground
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April 24, 2015
Oct 281:10 PM
draw the free body diagram of:
crate is being pulled at a constant velocity to the right as friction opposes the motion
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Force and Acceleration
restated: a force causes a CHANGE in velocity ( an acceleration )
it is a linear relationship : the greater the force exerted, the larger the acceleration of the object
F = ma
larger mass = larger force necessary to cause the same acceleration
F = labeled in Newtons
m = kilograms
a = m/s2
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April 24, 2015
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Combining Forces
force = vector
ex: you and your friend push on a table
F1= 100N F2 = 75N
+ x x
100N + 75N = 25 N
positive (direction)
kew. 71:30 PM
Combining Forces
ex: you and your friend push on a table
F1 = 100N F2 = 75 N+ x
100N + 75N = 175 N
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April 24, 2015
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Combining Forces
ex: you and your friend push on a table
F1 = 100N F2 = 100 N+ x
100N + 100N = 0 N
no net force = no acceleration
clarification: doesn't mean no velocity
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F = ma
m = F/a
a = F/m
solving problems: draw diagram
identify all forces
find net force
use Newton's second Law
use previous formulas to find v, d, or t
Newton's Second Law label for force?
label for mass?
label for acceleration?
1 Newton = 1 kg m /s2
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April 24, 2015
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ex: What force is required to accelerate a 6.0 kg bowling ball at +2.0 m/s2?
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ex: What is the acceleration of a 35 kg mass that has a force of 270 N applied to it horizontally?
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Ex: What is the acceleration of a 125 kg mass when three different forces are exerted on it? The object was at rest and achieved a speed of 10.0 m/s. The first force is 10.0 N east, the second is 30.0 N east, and the third is 15.0 N west.
How far did the object travel by the time it reached 10.0 m/s?
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Newton's First Law
an object at motion will stay in motion and an object at rest will stay at rest unless an unbalanced force acts upon it
aka "Law of Inertia"
so....what is the motion of an object with no net force acting upon it?
nothing changes if there is NO NET force
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April 24, 2015
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Some types of forces: table 42 on page 94
you'll see these abbreviations in free body diagrams
ex: Fg instead of FEarth's mass
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inertia:
tendency of an object to resist a change in its motion
NOT a force
as mass of object increases = inertia increases
equilibrium:
when net force on object = 0
object is either: 1) at rest
2) moving at a constant velocity
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April 24, 2015
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4.2 Using Newton's Laws F = ma
Weight is a force Fg = mgIt is the mass of an object being pulled on by Earth's gravitational field
Fg and g are both straight downward toward center of the Earth
gravity is pulling down on objects even when they aren't falling
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scales
you have mass gravity pulls down on that mass you have weight
when you stand on the scale, your weight is a downward force
are you accelerating? (are you speeding up? slowing down? changing direction? )
so the scale must be pushing up on you with force equal in magnitude to your weight
Fsp
Fg
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April 24, 2015
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Felicia, a ballet dancer, has a mass of 45.0 kg.
a) what is Felicia's weight on Earth?
b) what is Felicia's mass on Jupiter where acceleration due to gravity is 25.0 m/s2?
c) what is Felicia's mass on Jupiter where acceleration due to gravity is 25.0 m/s2?
.
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A space ship has a mass of 9000 kg. The space ship is launched from Earth and lands on a distant planet where it has a weight of 390000 N. What is the acceleration of gravity on this planet?
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April 24, 2015
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Arnold needs to lift a 35.0 kg rock. If he exerts an upward force of 502 N on the rock, what is the rock's acceleration?
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The elevator in the new office building warns that it can safely lift 300 kg at one time. If John (850 N), Betty (530 N), Robert (740 N) and Alice (610 N) all get in the elevator at once, can the elevator safely carry them up to the third floor?
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April 24, 2015
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A giant crane in Washington DC was tested by lifting a load with a mass of 2.232 x 103 kg.
Find the force needed to lift the load with an acceleration of 2.20 m/s2.
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If friction exerts a force of 15.0 N and a crate of 105 kg is accelerated to the right at a rate of 0.652 m/s2, what is the force pushing the crate to the right?
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Apparent Weightobserved when an object experiences a vertical acceleration
elevator:
at constant velocity ( no acceleration)
scale will read your actual weight
accelerating upward
stronger upward force than the downward gravity force
scale will read that you are heavier than your actual wt.
accelerating downward
scale will read less than your actual weight
cable breaks: you and elevator room are in free fall
only acceleration is g
you will appear weightless on the scale
.
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You are in an elevator standing on a balance scale such as the kind you find in a doctor's office. You notice that the scale reads many kilograms less than you really weigh.
a. Is the elevator moving at constant velocity or is it accelerating?
b. What is the direction of the acceleration?
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Drag force and terminal velocity
So far, we've ignored air resistance
In reality, when an object moves through any fluid (liquid/gas) the fluid exerts a force opposing the motion
the magnitude of the drag force depends on:
the speed the object is moving
faster object speed = greater drag forceslower object speed = smaller drag force
the size and shape of the object that is moving
sheet of paper has a greater drag force than a spherical ball
properties of the fluidtemperature and viscosity
.
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When drag force EQUALS the force of gravity ( Fg = mg), the object is in equilibrium
Fg
Fair
object falls, and accelerates due to "g"
as velocity increases, so does the drag force
if the object falls long enough, it speeds up until drag force has increased to = "g"FgFg
FairFair
then it no longer accelerates = constant velocity = TERMINAL VELOCITY
(initial velocity is small, so drag force is small)
.
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April 24, 2015
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4.3 Interaction Forces
Forces exist in pairs
example: two people wearing ice skates ( to reduce friction )one person pushes the other, but both will move
Grandma Jedidiah James
Fgrandma on JJFJJ on grandma
opposite directions, but equal magnitude
they act on EACH OTHER: so, JJ is the agent when Grandma is the systemGrandma is the agent when JJ is the system
.
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Newton's Third Law:
For every force, there is an equal and opposite force
F A on B = F B on AForce of Grandma on JJ = Force of JJ on Grandma in opposite
direction
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April 24, 2015
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F lady on floor = F floor on ladyF floor on lady
F lady on floor
equal and opposite = interaction pair
Fg
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Fg is always present
if Earth exerts a downward force on the person
the person also exerts an upward force on earth
interaction pair
the FORCES are equal and opposite
but Earth's mass is so large compared to the person .....
it makes the acceleration of the Earth very small
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April 24, 2015
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Forces of Ropes and Strings
Tension: force exerted by rope/string.
Fg
FT
Tension is distributed equally throughout the length of the rope/string
Tension in the string would equal the weight of the bucket
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#32, page 106
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April 24, 2015
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You are fishing and catch a fish with a mass of 2.0 kg. If the fishing line can withstand a maximum tension of 30.0 N, what is the maximum acceleration you can give the fish as you reel it in?
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The Normal Force : the perpendicular contact force exerted by a surface on another object
any two objects in contact with each other exert a force on each other
F box on table
F table on box
the surface that the object makes contact with is the table (horizontal)
so the normal force is perpendicular to and away from the surface Fn
Fn = the force of the table on the box
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April 24, 2015
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Fn often equals Fg in magnitude
but not always.....
10kg
this crate is in equilibrium
Fn = Fg
what is the normal force?
ex:
10kg
Fg = 10kg (9.8m/s2) = 98 N
What if a little kid tried to lift it with a force of 10 N upward? It isn't enough force to move the box, but it is still exerted on the box.
Fg = 10kg (9.8m/s2) = 98 N
(0 m/s, no acceleration)
still in equilibrium, so up forces have to balance with down forces
Fkid = 10 N
What is the normal force?
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10kg
Fg = 10kg (9.8m/s2) = 98 N
still in equilibrium, so up forces have to balance with down forces
Fkid = 10 N
What is the normal force?
10kg
Fg = 10kg (9.8m/s2) = 98 N
What if something pushed down on the box with 5 N of force.
Fg
Fpush
What is the normal force?
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April 24, 2015
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A 12.0 kg block sits on a table. A 10.0 kg block sits on top of the 12.0 kg block. If there is nothing on top of the 10.0 kg block, what is the force that the table exerts on the 12.0 kg block?