Physical Physical
Science Science
Chapter 12 Chapter 12
Forces
Forces
HW - Ch 12 Assessment HW - Ch 12 Assessment
p.385 in text p.385 in text
#1-23 #1-23
Also p. 387 #1-6 Also p. 387 #1-6
This is due Thursday!!! This is due Thursday!!!
Ch 12 Guided Reading Ch 12 Guided Reading
Do p.105-108 and 112 (due Friday) Do p.105-108 and 112 (due Friday)
Did you do Ch 11 also? (p.97-100, Did you do Ch 11 also? (p.97-100,
112) 112)
The Nature of Force The Nature of Force
By definition, a By definition, a ForceForce is a is a pushpush or a or a pullpull..
Just like Velocity &
Acceleration Forces have
both
magnitude and direction components
Just like Velocity &
Acceleration Forces have
both
magnitude and direction components
Forces Forces
A force causes an A force causes an object to move, object to move,
accelerate, change accelerate, change
speed, or direction speed, or direction
Forces are Forces are
represented by:
represented by:
ArrowsArrows
Direction Direction
StrengthStrength
represented by the represented by the length of the arrow length of the arrow
Balanced &
Balanced &
Unbalanced Forces Unbalanced Forces
Balanced forces – opposite and equal forces acting on the same object
result in NO motion of the object
Unbalanced forces Unbalanced forces – two – two or more forces of
or more forces of unequal strength or unequal strength or
direction acting upon direction acting upon
on an object on an object
results in motion of the objectresults in motion of the object
Balanced or Unbalanced Balanced or Unbalanced
Forces?
Forces?
balanced unbalanced
Force/Free body Force/Free body
diagrams diagrams
(to show forces) (to show forces)
used to show the relative magnitude used to show the relative magnitude and direction of all forces acting
and direction of all forces acting
upon an object in a given situation.
upon an object in a given situation.
a special example of vector diagram a special example of vector diagram
size of the arrow represents size of the arrow represents
amount of forceamount of force
Force / Free-body Force / Free-body
Diagrams Diagrams
direction of the arrow shows the direction of the arrow shows the direction which the force is acting direction which the force is acting
each force arrow is labeled each force arrow is labeled
The object is usually represented by a The object is usually represented by a box. Force arrows are drawn from the box. Force arrows are drawn from the
center of the box outward in the center of the box outward in the
direction which the force is acting.
direction which the force is acting.
http://www.physicsclassroom.com/Clashttp://www.physicsclassroom.com/Clas s/newtlaws/u2l2c.cfm#1
s/newtlaws/u2l2c.cfm#1
(look at types of forces also) (look at types of forces also)
Combining Forces Combining Forces
Net force: All Net force: All
forces are added or forces are added or
subtracted to create subtracted to create
one total force one total force
If net force is Zero If net force is Zero then there is no
then there is no change in motion.
change in motion.
If there is a net If there is a net force there is an force there is an
acceleration acceleration
Force Problem Force Problem
What is the net force on an object What is the net force on an object
being pulled toward the west with a being pulled toward the west with a
force of 30N and another force force of 30N and another force
pulling the object toward the east pulling the object toward the east
with a force of 75N?
with a force of 75N?
Friction Friction
a force that opposes the a force that opposes the motion of objects that
motion of objects that
touch as they move past touch as they move past
each other each other
acts at the surface where acts at the surface where objects are in contact
objects are in contact
All moving objects All moving objects encounter friction encounter friction
Without it most motion Without it most motion would be impossible
would be impossible
4 types, static, sliding, 4 types, static, sliding, rolling, fluid
rolling, fluid
Static Friction Static Friction
The force that keeps an object from The force that keeps an object from moving
moving
It is the largest frictional force It is the largest frictional force
Always opposite direction of the Always opposite direction of the applied force
applied force
Pushing a Car Pushing a Car
Walking Walking
Sliding Friction Sliding Friction
Once the object is in motion it Once the object is in motion it experiences sliding friction
experiences sliding friction
Opposite direction from applied Opposite direction from applied force
force
Less than static friction so less force Less than static friction so less force is needed to keep it in motion
is needed to keep it in motion
Rolling Friction Rolling Friction
As something rolls, the As something rolls, the object and floor bend object and floor bend
slightly slightly
This bend causes rolling This bend causes rolling friction
friction
It is a much smaller force It is a much smaller force than static friction
than static friction
As much as 1000 times As much as 1000 times smaller
smaller
Allows you to move heavy Allows you to move heavy objects
objects
Ball bearings reduce Ball bearings reduce friction
friction
Fluid Friction Fluid Friction
It opposes the It opposes the
motion in the liquid motion in the liquid
Like swimming, it is Like swimming, it is hard to move
hard to move
If you are in the air, If you are in the air, fluid friction is
fluid friction is
called air resistance called air resistance
At higher speeds it At higher speeds it is very noticable
is very noticable
Force of Gravity Force of Gravity
a force that acts between two objectsa force that acts between two objects
an attractive force, pulls objects togetheran attractive force, pulls objects together
causes objects to accelerate downwardcauses objects to accelerate downward
Falling objects, gravity pulls down, What Falling objects, gravity pulls down, What pushes up?
pushes up?
air resistance acts in opposite directionair resistance acts in opposite direction
Terminal Velocity- when Gravity and air Terminal Velocity- when Gravity and air resistance cancel each other out (velocity resistance cancel each other out (velocity of falling body becomes constant)
of falling body becomes constant)
Skydiving and falcons Skydiving and falcons
Forces MontageForces Montage
Terminal Velocity Terminal Velocity
See elephant problem on page 10 and 11 of Netwon’s laws Smart Notebook file!!
Projectile Motion Projectile Motion
When an object not only goes up and down When an object not only goes up and down but out
but out
It is a curved pathIt is a curved path
Air resistance and gravity are the only Air resistance and gravity are the only forces acting on a projectile
forces acting on a projectile
What falls faster, an object that falls straight What falls faster, an object that falls straight down, or one that has a horizontal velocity?
down, or one that has a horizontal velocity?
Myth busters bullet #2Myth busters bullet #2
This one shows the end?This one shows the end?
May the forces be with May the forces be with you and not against you!!
you and not against you!!
Forces Webquest Forces Webquest
Log on to www.sascurriculumpathways.com Log on to www.sascurriculumpathways.com
Click on Subscriber Logon at the top right.Click on Subscriber Logon at the top right.
For the student username, type For the student username, type
“hickoryridge”
“hickoryridge”
In the “Quick Launch” box, type “44” and In the “Quick Launch” box, type “44” and hit enter.
hit enter.
By the way, the first site you use By the way, the first site you use (www. physicsclassroom.com) is a GREAT (www. physicsclassroom.com) is a GREAT site to seek extra help on physics material!
site to seek extra help on physics material!
Warm-up – Balanced and Warm-up – Balanced and
Unbalanced Forces Unbalanced Forces
1.Two tugboats are moving a barge. 1.Two tugboats are moving a barge.
Tugboat A exerts a force of 3000 newtons Tugboat A exerts a force of 3000 newtons
on the barge. Tugboat B exerts a force of on the barge. Tugboat B exerts a force of
5000 newtons in the same direction. What 5000 newtons in the same direction. What
is the combined force on the barge?
is the combined force on the barge?
2.Draw arrows showing the individual and 2.Draw arrows showing the individual and combined forces of the tugboats in #1.
combined forces of the tugboats in #1.
3.Now suppose that Tugboat A exerts a 3.Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and force of 2000 newtons on the barge and
Tugboat B exerts a force of 4000 newtons Tugboat B exerts a force of 4000 newtons
in the opposite direction. What is the in the opposite direction. What is the
combined force on the barge?
combined force on the barge?
4.Draw arrows showing the individual and 4.Draw arrows showing the individual and combined forces of the tugboats in #3.
combined forces of the tugboats in #3.
5.Could there ever be a case when 5.Could there ever be a case when Tugboat A and Tugboat B are both Tugboat A and Tugboat B are both
exerting a force on the barge but the exerting a force on the barge but the
barge doesn't move? Draw arrows barge doesn't move? Draw arrows
showing the individual and showing the individual and
combined forces in such a situation.
combined forces in such a situation.
Newton’s Laws of Motion Newton’s Laws of Motion
Begins Here!! Begins Here!!
Great Scientists Great Scientists
Aristotle- Incorrectly Aristotle- Incorrectly said force was required said force was required
to keep an object in to keep an object in
constant motion constant motion
Galileo- with no Galileo- with no resistance objects resistance objects
would move indefinitely would move indefinitely
Newton- based on Newton- based on Galileo’s findings he Galileo’s findings he
came up with his law’s came up with his law’s
of motion of motion
SASinschools SASinschools
interactivity interactivity
Free Fall #1196 Free Fall #1196
Just show video clip at beginning Just show video clip at beginning
Newton’s 1
Newton’s 1
ststLaw of Law of Motion
Motion
AKA AKA The Law of Inertia The Law of Inertia
Inertia- an objects tendency to resist Inertia- an objects tendency to resist changes in motion
changes in motion
an object at rest will remain at rest, and an object in motion will remain in 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.
motion at a constant velocity until acted on by another force.
Remember:
The greater the mass of an object the greater the inertia
Eureka Inertia Video Eureka Inertia Video
Eureka Inertia Video Eureka Inertia Video
Check your Check your
Understanding on Understanding on
Newton’s first law Newton’s first law
http://www.physicsclassroom.com/Cl http://www.physicsclassroom.com/Cl ass/newtlaws/u2l1b.cfm
ass/newtlaws/u2l1b.cfm
http://www.physicsclassroom.com/Cl http://www.physicsclassroom.com/Cl ass/newtlaws/u2l1d.cfm
ass/newtlaws/u2l1d.cfm
You need to remember this about Inertia!!!
Mass is the measure of inertia of an object!!!
So, which would have more 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?
0.001 kg bumblebee traveling at 2 m/s
Newton’s 2
Newton’s 2
ndndLaw Law
The acceleration of an object is equal to the net The acceleration of an object is equal to the net force acting on it divided by the objects mass
force acting on it divided by the objects mass
A= F/m so A= F/m so F=maF=ma
Acceleration is always in the same direction of Acceleration is always in the same direction of the net force
the net force
An object will have greater acceleration if a An object will have greater acceleration if a greater force is applied
greater force is applied
Force = mass x accelerationForce = mass x acceleration
F=ma ; a= F/m ; m= F/aF=ma ; a= F/m ; m= F/a
What is the basic unit for mass? What is the basic unit for mass?
KilogramKilogram
What is the basic unit for acceleration? What is the basic unit for acceleration?
Meter/sec/secMeter/sec/sec
Therefore the basic unit for Force is Therefore the basic unit for Force is
(kilogram)( meter/sec/sec) (kilogram)( meter/sec/sec)
An object with a mass of 1 kg accelerating at 1 m/s/s has a An object with a mass of 1 kg accelerating at 1 m/s/s has a force of 1 Newton
force of 1 Newton
Newton’s Second Law of Motion Newton’s Second Law of Motion
F=ma F=ma
Eureka – Weight vs. Mass
Newton’s 2
Newton’s 2
ndndLaw & Force of Law & Force of Gravity
Gravity
Have you heard of the FORCE of gravity?Have you heard of the FORCE of gravity?
Gravity: the force that pulls objects towards each otherGravity: the force that pulls objects towards each other
Since gravity is a force it also obeys Newton’s second lawSince gravity is a force it also obeys Newton’s second law
F=maF=ma
With this experiment, Galileo proved
Aristotle wrong
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, Apollo 15 astronauts proved Galileo right.
(link to You Tube) Air
resistance keeps things from falling equally
Elephant and Terminal
Velocity
Weight and Mass Weight and Mass
Weight is the force of gravity acting on an Weight is the force of gravity acting on an object
object
Weight (N) = mass (g) x acceleration of gravity Weight (N) = mass (g) x acceleration of gravity (m/s(m/s22))
Note: just a version of F=ma, F = mass x gravity Note: just a version of F=ma, F = mass x gravity
FYI:FYI:
1 pound = 4.448 Newtons1 pound = 4.448 Newtons
so 1/4 lb is slightly more that 1 Nso 1/4 lb is slightly more that 1 N
so a 1/4 lb burger is a "Newton" burgerso a 1/4 lb burger is a "Newton" burger
F=ma F=ma
So, weight is a type of ForceSo, weight is a type of Force
The formula for weight: Weight = mass x AThe formula for weight: Weight = mass x Agg
SinceSince A Agg= 9.8 m/s= 9.8 m/s2 2 then then
Weight = mass x 9.8 m/sWeight = mass x 9.8 m/s2 2
Newton’s 2
Newton’s 2
ndndLaw & Weight Law & Weight
Remember:
1 newton = 0.22 pounds
Your weight on other planets Your weight on other planets
& 3 different types of stars
& 3 different types of stars
Newton’s 2
Newton’s 2
ndndLaw Law
Math Practice p.367 and Math Practice p.367 and
369 369
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?
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?
During a test crash, an air bag During a test crash, an air bag
inflates to stop a dummy’s forward inflates to stop a dummy’s forward
motion. The dummy’s mass is 75 kg.
motion. The dummy’s mass is 75 kg.
If the net force on the dummy is 825 If the net force on the dummy is 825
N toward the rear of the car, what is N toward the rear of the car, what is
the dummy’s deceleration?
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
Eureka – Acceleration I
Eureka – Acceleration II
Forces Problem Worksheet
Have any questions before we take
your quiz?
For every action there is an equal & opposite reaction.For every action there is an equal & opposite reaction.
This means every time a force is applied in one direction an equal force is applied in the This means every time a force is applied in one direction an equal force is applied in the opposite direction.
opposite direction.
If an object is not in motion, then all forces acting on it are balanced and the net force is zero!If an object is not in motion, then all forces acting on it are balanced and the net force is zero!
Newton’s 3
Newton’s 3
rdrdLaw of Law of Motion
Motion : :
Action-Reaction Forces Action-Reaction Forces
If you push against a If you push against a wall it pushes back wall it pushes back
These forces can These forces can
cause motion, like a cause motion, like a
swimmer swimmer
They do not cancel They do not cancel each other out,
each other out,
because they act on because they act on
different objects.
different objects.
2 ice skaters2 ice skaters
Rocket man video clip (stop after Rocket man video clip (stop after balloon)
balloon)
Space Walk Space Walk
Momentum Momentum
The product of an object’s mass x velocityThe product of an object’s mass x velocity
Influences how easily an object can be stoppedInfluences how easily an object can be stopped
High velocity or high mass cause a high momentumHigh velocity or high mass cause a high momentum
For some reason, maybe because mass is designated as “m” For some reason, maybe because mass is designated as “m”
in formulas, momentum is designated as “
in formulas, momentum is designated as “p”. p”.
Therefore: Therefore: pp = mv = mv
The unit for The unit for mass is kgmass is kg, the unit for , the unit for velocity is meter/secondvelocity is meter/second, , therefore the unit for
therefore the unit for momentum is kg m/secmomentum is kg m/sec
Which has more Which has more
momentum?
momentum?
a 0.046 kg golf ball with a
speed of 60.0 m/s a 7.0 kg bowling ball with a speed of 6.0 m/s
.046 kg x 60.0 m/s = 2.8 kg m/s 7.0 kg x 6.0 = 42 kg m/s
Conservation of Conservation of
Momentum Momentum : :
When two or more objects interact (collide) the When two or more objects interact (collide) the total momentum before the collision is equal to total momentum before the collision is equal to
the total momentum after the collision the total momentum after the collision
If no net force acts on a system, then the total If no net force acts on a system, then the total momentum of the system does not change
momentum of the system does not change
In a closed system, loss of momentum of one In a closed system, loss of momentum of one
object equals the gain in momentum of another object equals the gain in momentum of another object
object
Momentum – 2 moving Momentum – 2 moving
objects objects
During this collision the speed of both box cars During this collision the speed of both box cars changes. The total momentum remains constant changes. The total momentum remains constant
before & after the collision. The masses of both cars is before & after the collision. The masses of both cars is
the same so the velocity of the red car is transferred the same so the velocity of the red car is transferred
to the blue car.
to the blue car.
Momentum – 1 moving Momentum – 1 moving
object object
During this collision the speed red car is transferred During this collision the speed red car is transferred to the blue car. The total momentum remains constant to the blue car. The total momentum remains constant before & after the collision. The masses of both cars is before & after the collision. The masses of both cars is
the same so the velocity of the red car is transferred the same so the velocity of the red car is transferred
to the blue car.
to the blue car.
Momentum – 2 Momentum – 2 connected objects connected objects
After this collision, the coupled cars make one object w/ After this collision, the coupled cars make one object w/
a total mass of 60,000 kg. Since the momentum after a total mass of 60,000 kg. Since the momentum after
the collision must equal the momentum before, the the collision must equal the momentum before, the
velocity must change. In this case the velocity is velocity must change. In this case the velocity is
reduced from 10 m/sec. to 5 m/sec.
reduced from 10 m/sec. to 5 m/sec.
Momentum Videos Momentum Videos
Bill Nye – Momentum Bill Nye – Momentum
Momentum and collisions video clip Momentum and collisions video clip
Data Analysis –
Data Analysis –
Momentum p.377
Momentum p.377
Questions that make you Questions that make you
go Hmmmmmmm….
go Hmmmmmmm….
1. Why can you exert greater force on the pedals of a 1. Why can you exert greater force on the pedals of a bicycle if you pull up on the handlebars?
bicycle if you pull up on the handlebars?
When you pull up on the handlebars, the handlebars push down When you pull up on the handlebars, the handlebars push down on you, and this force is transmitted to the pedals.
on you, and this force is transmitted to the pedals.
2. Lets say you are weighing yourself on a set of 2. Lets say you are weighing yourself on a set of
bathroom scales. You are standing next to the sink in bathroom scales. You are standing next to the sink in
the bathroom. If at the same time while you are the bathroom. If at the same time while you are
standing on the scales you reach under the sink and pull standing on the scales you reach under the sink and pull up on the sink, will the scales register your weight to be up on the sink, will the scales register your weight to be more or less than what they would register if you didn't more or less than what they would register if you didn't
pull up on the sink?
pull up on the sink?
the scales would register heavy. Since you are lifting up on the the scales would register heavy. Since you are lifting up on the sink, the sink is pushing down on you with an equal but opposite sink, the sink is pushing down on you with an equal but opposite force and this would be transmitted to the scales.
force and this would be transmitted to the scales.
3. Lets repeat the question in the situation 3. Lets repeat the question in the situation above except that you push down on the top above except that you push down on the top
of the sink instead of pulling up on the of the sink instead of pulling up on the
bottom of the sink. What will your weight be bottom of the sink. What will your weight be
this time compared to what it would be if this time compared to what it would be if
you did not push down on the sink?
you did not push down on the sink?
The scales would register lighter. Since you are The scales would register lighter. Since you are pushing down on the sink it is pushing up on you pushing down on the sink it is pushing up on you with an equal force which tends to lift you up off with an equal force which tends to lift you up off the scales some and therefore they register light.
the scales some and therefore they register light.
Eureka – Momentum
(Speed)
Universal Forces Universal Forces
Forces that can be found
Forces that can be found
anywhere in the universe
anywhere in the universe
Electromagnetic Force Electromagnetic Force
Electric and Magnetic Electric and Magnetic Forces are the only
Forces are the only forces that can both forces that can both attract and repel
attract and repel
Electric forces- between Electric forces- between charged particle,
charged particle,
positive attracted to positive attracted to negative
negative
Clothes in a dryerClothes in a dryer
Magnetic forces- N and Magnetic forces- N and S poles, opposites
S poles, opposites attract, likes repel attract, likes repel
Nuclear Forces Nuclear Forces
Two forces- a strong Two forces- a strong and a weak force hold and a weak force hold an atom together
an atom together
The strong nuclear The strong nuclear force overcomes the force overcomes the protons repulsion force protons repulsion force
This force is over 100x This force is over 100x stronger than electric stronger than electric forces
forces
The weak force is The weak force is
involved in radioactive involved in radioactive processes
processes
Gravitational Force Gravitational Force
Attractive force between Attractive force between any two masses
any two masses
This force is very small This force is very small compared to all the rest compared to all the rest
Newton came up with Newton came up with the Law of Universal the Law of Universal
Gravitation, which Gravitation, which
explains how we can explains how we can find the gravitational find the gravitational
force of any object force of any object
A large mass is required A large mass is required for gravity to be felt
for gravity to be felt
Gravity is relational to Gravity is relational to
distance distance
The further apart The further apart two objects are the two objects are the less the gravitational less the gravitational force
force
Gravity still has pull Gravity still has pull over millions and
over millions and millions of miles millions of miles apart
apart
As distance doubles As distance doubles the force gets
the force gets quartered
quartered
Eureka - Gravity
The Earth Moon System The Earth Moon System
The moon stays in The moon stays in orbit because of orbit because of Earth’s gravity Earth’s gravity
It works very similar to It works very similar to a centripetal force
a centripetal force
It pulls in on an objectIt pulls in on an object
So as the moon follows So as the moon follows this circular path, it’s this circular path, it’s gravitational force has gravitational force has an affect on Earth.
an affect on Earth.
Satellites Satellites
Satellites stay in orbit with Satellites stay in orbit with centripetal force of gravity centripetal force of gravity
If it were to slow down it would lose If it were to slow down it would lose altitude
altitude
Physical Science Formula
Reference Sheet
Eureka - Mass
Tacoma Bridge Collapse
Newton’s Laws Tutorial Newton’s Laws Tutorial
Having problems? Try doing this Having problems? Try doing this tutorial:
tutorial:
http://www.usoe.k12.ut.us/CURR/sci http://www.usoe.k12.ut.us/CURR/sci ence/sciber00/8th/forces/sciber/newt ence/sciber00/8th/forces/sciber/newt
ons.htm
ons.htm
Video Clips on Newton’s Video Clips on Newton’s
Laws in case you need Laws in case you need
more!!!
more!!!
Newton’s Laws of Motion Newton’s Laws of Motion
