Transcript Physical Science Chapter 2

Physical Science
Chapter 12
Forces and
Newton’s Laws of Motion
The Nature of Force
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By definition, a Force is a push or a pull.
Just like Velocity & Acceleration
Forces have both
magnitude and direction
components
Forces
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A force causes an object
to move, accelerate,
change speed, or
direction
Forces are represented
by:
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Arrows
Direction
Strength
represented by the length
of the arrow
Balanced &
Unbalanced Forces
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Balanced forces – opposite
and equal forces acting on the
same object
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result in NO motion of the object
Unbalanced forces – two or
more forces of unequal
strength or direction acting
upon on an object
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results in motion of the object
Balanced or Unbalanced Forces?
Balanced but Why?
Unbalanced but WHY?
Force/Free body diagrams
(to show forces)
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used to show the relative magnitude and
direction of all forces acting upon an object in a
given situation.
a special example of vector diagram
size of the arrow represents
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amount of force
Force / Free-body Diagrams
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direction of the arrow shows the direction which
the force is acting
each force arrow is labeled
The object is usually represented by a box. Force
arrows are drawn from the center of the box
outward in the direction which the force is
acting.
http://www.physicsclassroom.com/Class/newtl
aws/u2l2c.cfm#1 (look at types of forces also)
Combining Forces
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Net force: All forces are
added or subtracted to
create one total force
If net force is Zero then
there is no change in
motion.
If there is a net force
there is an acceleration
st
1
Newton’s Law of
Motion(cont.)
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If you roll a ball across the floor, why does it
eventually come to a stop.
Is there a force acting against it?
 If so, what is it?
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Friction—force that opposes motion.
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Always acts in the direction opposite of motion.
Anything slowing
this Coke as it
slides across the
bar?
nd
2
Newton’s
Law of
Motion
 called the “Law of Acceleration”.
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Relates force, mass, and acceleration.
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Force and acceleration directly related.
Mass and acceleration inversely related.
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Newton’s 2nd Law of
Motion(cont.)
Uses this Formula:
F=ma
 Where:
F
= force in Newton’s(N)
 m = mass in kilograms(kg)
 a = acceleration in m/s2
Ex. What force is required to accelerate a
1500 kg car by a rate of 1.5 m/s/s?
Given:
m = 1500 kg
a = 1.5 m/s/s
F=?
F=ma
F = 1500kg(1.5 m/s/s)
F = 2250 N
Ex. A force of 290 N is applied to a 20 kg
rock. At what rate will the rock
accelerate?
Given:
m = 20 kg
a=?
F = 290 N
F=ma
a = 14.5 m/s2
Ex. A force of 140 N is applied to a mover’s dolly
to accelerate a load of Cokes across the grocery
store at 3 m/s2. What is the mass of the load of
Cokes?
Given:
F = 140 N
a = 3 m/s2
m=?
A force is applied to accelerate a 50
kg mass at a rate of 2.5 m/s/s. What
force is required?
A.
B.
C.
D.
20 N
25 N
120 N
125 N
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D
F = 125 N
A 250 N force is applied to a 12 kg
mass. At what rate will the object
accelerate?
3000
m/s/s
B. 240 m/s/s
C. 20.8 m/s/s
D. .05 m/s/s
A.
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C
A = 20.8 m/s/s
Newton’s
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rd
3
Law
For every action, there is an equal but opposite
reaction.
Forces always occur in pairs!!
Which of the following scenarios does
not represent Newton’s 3rd Law?
A.
B.
C.
D.
A rocket being launched into space.
A book resting on a desk.
A person paddling a canoe.
A large force pushing on a small
mass, causing it to accelerate.
 Choice
D
 is Newton’s 2nd law.
Force Problem
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What is the net force on an object being pulled
toward the west with a force of 30N and
another force pulling the object toward the east
with a force of 75N?
Types of Friction
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a force that opposes the
motion of objects that touch as
they move past each other
acts at the surface where
objects are in contact
All moving objects encounter
friction
Without it most motion would
be impossible
4 types, static, sliding,
rolling, fluid
Static Friction
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The force that keeps an object from moving
It is the largest frictional force
Always opposite direction of the applied force
Pushing a Car
Walking
Sliding Friction
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Once the object is in motion it experiences
sliding friction
Opposite direction from applied force
Less than static friction so less force is needed
to keep it in motion
Rolling Friction
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As something rolls, the
object and floor bend
slightly
This bend causes rolling
friction
It is a much smaller force
than static friction
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As much as 1000 times
smaller
Allows you to move
heavy objects
Ball bearings reduce
friction
Fluid Friction
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It opposes the motion in
the liquid
Like swimming, it is hard
to move
If you are in the air, fluid
friction is called air
resistance
At higher speeds it is
very noticeable
Force of Gravity
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a force that acts between two objects
an attractive force, pulls objects together
causes objects to accelerate downward
Falling objects, gravity pulls down, What
pushes up?
 air resistance acts in opposite
direction
Terminal Velocity- when Gravity and air
resistance cancel each other out (velocity
of falling body becomes constant)
Skydiving and falcons
Forces Montage
Terminal Velocity
See elephant problem on page 10 and 11 of Netwon’s laws Smart Notebook file!!
Projectile Motion
When an object not only goes up and down but out.
 It is a curved path
 Air resistance and gravity are the only forces acting
on a projectile
 What falls faster, an object that falls straight down,
or one that has a horizontal velocity? See Videos
They are very good….
 Myth busters bullet #2
 This one shows the end?
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May the “forces”be with you and
not against you!! haha
Forces Webquest: Do if I tell you.
Depends on Time..
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Log on to www.sascurriculumpathways.com
Click on Subscriber Logon at the top right.
For the student username, type “hickoryridge”
In the “Quick Launch” box, type “44” and hit
enter.
By the way, the first site you use
(www. physicsclassroom.com) is a GREAT site
to seek extra help on physics material!
Class Activity- You will do later.
Balanced and Unbalanced Forces
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1.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?
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2.Draw arrows showing the individual and combined
forces of the tugboats in #1.
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3.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?
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4.Draw arrows showing the individual and
combined forces of the tugboats in #3.
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5.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.
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Ck KEYS… for Feedback..
Newton’s Laws of Motion
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Begins Here!!
Great Scientists
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Aristotle- Incorrectly said
force was required to
keep an object in
constant motion
Galileo- with no
resistance objects would
move indefinitely
Newton- based on
Galileo’s findings he
came up with his law’s of
motion
SASinschools interactivity
Do if I Tell you.. Or skip
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Free Fall #1196
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Just show video clip at beginning
Newton’s

st
1
Law of Motion
AKA The Law of Inertia
Inertia- an objects tendency to resist
changes in motion
 an object at rest will remain at rest, and an
object in motion will remain in motion at a
constant velocity until acted on by another
force.
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Remember:
The greater the mass
of an object the
greater the inertia
Eureka Inertia Video
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Eureka Inertia Video
Check your Understanding on
Newton’s first law
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http://www.physicsclassroom.com/Class/newtl
aws/u2l1b.cfm
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http://www.physicsclassroom.com/Class/newtl
aws/u2l1d.cfm
You need to remember this about
Inertia!!!
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Mass is the measure of inertia of an object!!!
Remember:
So, which would have more inertia? Bowling ball
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The greater the mass of an object the greater the inertia
0.1 kg baseball traveling at 20 m/s
5 kg bowling ball traveling at 3 m/s
10 kg sled traveling at 0 m/s
0.001 kg bumblebee traveling at 2 m/s
Which would have the least?
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0.001 kg bumblebee traveling at 2 m/s
Newton’s
nd
2
Law
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The acceleration of an object is equal to the net
force acting on it divided by the objects mass
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A= F/m so F=ma
Acceleration is always in the same direction of
the net force
An object will have greater acceleration if a
greater force is applied
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Newton’s Second Law of Motion F=ma
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Force = mass x acceleration
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What is the basic unit for mass?
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Kilogram
What is the basic unit for acceleration?
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F=ma ; a= F/m ; m= F/a
Meter/sec/sec
Therefore the basic unit for Force is
(kilogram)( meter/sec/sec)
An object with a mass of 1 kg accelerating at 1 m/s/s
has a force of 1 Newton
Eureka – Weight vs. Mass
Newton’s 2nd Law & Force of Gravity
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Have you heard of the FORCE of gravity?
Gravity: the force that pulls objects towards each other
Since gravity is a force it also obeys Newton’s second law
F=ma
Since objects fall at the same speed,
their acceleration is the same.
All objects accelerate at the rate.
Here on Earth the rate is:
Ag=9.8 m/s2
Or
Ag=32 ft/s2
With this experiment, Galileo
proved Aristotle wrong
Air resistance
keeps things
from falling
equally
With this experiment, Apollo 15
astronauts proved Galileo right.
(link to You Tube)
Weight and Mass
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Weight is the force of gravity acting on an object
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Weight (N) = mass (g) x acceleration of gravity (m/s2)
Note: just a version of F=ma, F = mass x gravity
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FYI:
1 pound = 4.448 Newtons
 so 1/4 lb is slightly more that 1 N
 so a 1/4 lb burger is a "Newton" burger
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Newton’s 2nd Law & Weight
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F=ma
So, weight is a type of Force
The formula for weight: Weight = mass x Ag
Since Ag= 9.8 m/s2 then
Weight = mass x 9.8 m/s2
Remember:
1 newton = 0.22 pounds
Your weight on other planets
& 3 different types of stars
Newton’s
nd
2
Law
PRACTICE:
Look thru Text Ch 12
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Assessment p.385 in text Questions #1-20
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Also p. 387 #1-6
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1. C
2. A
3. A
4. D
5. A
6. C
7. B
8. A
9. D
10. B
Answers p385 1-23
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11. The Net Force is zero.. Crate doesn’t move
12 . 8 N
13. Yes, all of the forces can cancel one another
14. It Accelerates
15. Static Friction
17. Air Resistance also increases pushing up.
Obj will reach terminal velocity.
18. Weight is the FORCE of Gravity. Mass is
amount of MATTER AN OBJ CONTAINS.
20. The MASS of each vehicle. p = mv
F= ma Math Practice p.367 & 369
A boy pushes forward a cart of groceries with a
total mass of 40.0 kg. What is the acceleration of
the cart if the net force on the cart is 60.0 N?
What is the upward acceleration of a helicopter
with a mass of 5000 kg if a force of 10,000 N acts
on it in an upward direction?
F= m a formula problems…
An automobile with a mass of 1200 kg accelerates
at a rate of 3.0 m/s2 in the forward direction. What
is the net force acting on the automobile?
A 25-N force accelerates a boy in a wheelchair at
0.5 m/s2 What is the mass of the boy and the
wheelchair?
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During a test crash, an air bag inflates to stop a
dummy’s forward motion. The dummy’s mass
is 75 kg. If the net force on the dummy is 825
N toward the rear of the car, what is the
dummy’s deceleration?
A bicycle takes 8.0 seconds to accelerate at a
constant rate from rest to a speed of 4.0 m/s. If
the mass of the bicycle and rider together is
85 kg, what is the net force acting on the
bicycle? (Hint: First calculate the acceleration.)
a=(vf-vi)/t
= (4.0 m/s) / 8.0 s
= 0.50 m/s2
F=ma
= 85 kg x 0.50 m/s2
= 43 N
Good Videos
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Eureka – Acceleration I
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Eureka – Acceleration II
Newton’s 3rd Law of Motion:
 For
every action there is an equal & opposite
reaction.
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This means every time a force is applied in one
direction an equal force is applied in the opposite
direction.
If an object is not in motion, then all forces acting on it
are balanced and the net force is zero!
Action-Reaction Forces
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If you push against a wall
it pushes back
These forces can cause
motion, like a swimmer
They do not cancel each
other out, because they
act on different objects.
2 ice skaters
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Rocket man video clip (stop after balloon)
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Space Walk
Part 4 Make sure you finish this.
Momentum is sm p.
p = mv
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The product of an object’s mass x velocity
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Influences how easily an object can be stopped
High velocity or high mass cause a high momentum
For some reason, maybe because mass is designated as “m” in
formulas, momentum is designated as “p”.
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p = mv
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Therefore:
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The unit for mass is kg, the unit for velocity is meter/second,
therefore the unit for momentum is kg m/sec
Which has more momentum?
p = mv
a 0.046 kg golf ball with a
speed of 60.0 m/s
.046 kg x 60.0 m/s = 2.8 kg m/s
a 7.0 kg bowling ball with
a speed of 6.0 m/s
7.0 kg x 6.0 = 42 kg m/s
Conservation of Momentum:
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When two or more objects interact (collide) the
total momentum before the collision is equal to
the total momentum after the collision
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If no net force acts on a system, then the total
momentum of the system does not change
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In a closed system, loss of momentum of one
object equals the gain in momentum of another
object
Momentum – 2 moving objects
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During this collision the speed of both box cars
changes. The total momentum remains constant before
& after the collision. The masses of both cars is the
same so the velocity of the red car is transferred to the
blue car.
Momentum – 1 moving object
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During this collision the speed red car is transferred to
the blue car. The total momentum remains constant
before & after the collision. The masses of both cars is
the same so the velocity of the red car is transferred to
the blue car.
Momentum – 2 connected objects
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After this collision, the coupled cars make one object
w/ a total mass of 60,000 kg. Since the momentum
after the collision must equal the momentum before,
the velocity must change. In this case the velocity is
reduced from 10 m/sec. to 5 m/sec.
Momentum Videos
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Bill Nye – Momentum
Momentum and collisions video clip
Eureka – Momentum (Speed)
Universal Forces
Forces that can be found anywhere in
the universe
Electromagnetic Force
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Electric and Magnetic
Forces are the only
forces that can both
attract and repel
Electric forces- between
charged particle, positive
attracted to negative
Clothes in a dryer
Magnetic forces- N and
S poles, opposites attract,
likes repel
Nuclear Forces
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Two forces- a strong and
a weak force hold an
atom together
The strong nuclear force
overcomes the protons
repulsion force
This force is over 100x
stronger than electric
forces
The weak force is
involved in radioactive
processes
Gravitational Force
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Attractive force between
any two masses
This force is very small
compared to all the rest
Newton came up with
the Law of Universal
Gravitation, which
explains how we can find
the gravitational force of
any object
A large mass is required
for gravity to be felt
Gravity is relational to distance
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The further apart two
objects are the less the
gravitational force
Gravity still has pull over
millions and millions of
miles apart
As distance doubles the
force gets quartered
Eureka - Gravity
The Earth Moon System
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The moon stays in orbit
because of Earth’s
gravity
It works very similar to a
centripetal force
It pulls in on an object
So as the moon follows
this circular path, it’s
gravitational force has an
affect on Earth.
Satellites
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Satellites stay in orbit with centripetal force of
gravity
If it were to slow down it would lose altitude
Physical Science Formula
Reference Sheet
Eureka - Mass
Tacoma Bridge Collapse
Newton’s Laws Tutorial
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Having problems? Try doing this tutorial:
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http://www.usoe.k12.ut.us/CURR/science/scib
er00/8th/forces/sciber/newtons.htm
Video Clips on Newton’s Laws in
case you need more!!!
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Newton’s Laws of Motion
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Another Newton’s Laws of Motion Video
There is a 2nd ppt – for this
Chapter 12.
In Class or at HOME - be sure to run thru
Chapt 12 : Force Diagrams & Newton’s Laws ppt
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