Transcript Newton`s Laws of Motion - San Juan Unified School District

Newton’s
Laws of
Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction
p29
Demo: table cloth trick
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Draw the apparatus
What do you predict will
happen?
Record what does happen
Why do you think this
happened?
Demo: eggs in beakers
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Draw the apparatus
What do you predict will
happen?
Record what does happen
Why do you think this
happened?
While most people know
what Newton's laws say,
many people do not know
what they mean (or simply do
not believe what they mean).
Newton’s Laws of Motion
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1st Law – An object at rest will stay at
rest, and an object in motion will stay in
motion at constant velocity, unless acted
upon by an unbalanced force.
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2nd Law – Force equals mass times
acceleration.
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3rd Law – For every action there is an
equal and opposite reaction.
1st Law of Motion
(Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.
Examples: tablecloth trick
table cloth trick mythbusters style
Compare
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Video 1
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tablecloth trick
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1.) What factors determine
the “success” of the
tablecloth trick?
2.) What physics vocab do
they use?
3.) Did increasing the
mass make the dishes
move more or less?
p29
Video 2
table cloth trick mythbusters
style
1.) What two forces did
Adam say must be
balanced?
2.) What did they change
to try and balance these
forces?
1st Law
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Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
motionless.
These pumpkins will not move unless acted on
by an unbalanced force.
Which has more inertia?
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A plate or a spoon?
An empty glass or a full glass?
An egg or a toilet paper tube?
Things with more mass have _________ inertia
Things with less mass have _______ inertia
Larger things have ________ inertia
Smaller things have _______ inertia
1st Law
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Once airborne,
unless acted on
by an
unbalanced force
(gravity and air
– fluid friction),
it would never
stop!
1st Law
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Unless acted
upon by an
unbalanced
force, this golf
ball would sit on
the tee forever.
Why then, do we observe every
day objects in motion slowing
down and becoming motionless
seemingly without an outside
force?
It’s a force we sometimes cannot see –
friction.
Objects on earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.
What is this unbalanced force that acts on an object in motion?
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There are four main types of friction:
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Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water resistance
Static friction: initial friction when moving an
object
Slide a book
across a table and
watch it slide to a rest
position. The book
comes to a rest
because of the
presence of a force that force being the
force of friction which brings the book
to a rest position.
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In the absence of a force of friction, the book
would continue in motion with the same speed
and direction - forever! (Or at least to the end
of the table top.)
Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.
Explain why the glassware stayed
on the table and the eggs fell into
the beakers (using the words
inertia and friction)
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Newton’s First Law: A Running
Start (AP p132)
p30
What do you think?
Investigate...record data and answer questions
1-5 in your notebook. The materials are at the
lab benches.
Put the lab materials back in the covered bin
and Get a stamp from your teacher when you
are finished at the lab station
Return to your seats and read p134-138.
Answer CU p138 (1-6)/ CDP 2-1 3-2
Last chance to get this week’s work stamped!
Model for Forces
DQ: How do unbalanced forces
change the motion of an object?
1.) An object at rest remains at rest
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2.) An object in motion remains in motion at a
constant speed unless acted upon by a force
3.) Rest is a special case of constant motion in a
straight line with a constant speed (v=0)
4.) Inertia is the property of an object to remain
at rest or in motion unless something causes it to
move. Inertia resists a change in an object’s state
of motion.
Model for Forces
DQ: How do unbalanced forces change
the motion of an object?
p32
5.) a ball should roll forever (once it is set in
motion). However, outside forces (friction) cause
it to slow down and stop.
6.) In the absence of friction, a ball released from
one side of a track should roll to the same height
on the other side of the track
7.) Two people, one on a train and the other
outside, will measure different speeds for the
same moving object. Both are correct for their
frame of reference
Physics in action
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Benchmark 1 9/3
Benchmark 2 9/18-9/19
Benchmark 3 9/22-9/26
Benchmark 4 9/26-10/3
Benchmark 5 10/3-10/17
Benchmark 6 10/17
Benchmark 7 (Presentation time!) 10/21
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Review
p34
CU (p138) 1-6
PtoGo (p143) 1-4 (p144) 10
CDO 2-1 3-2
p33
Get your notebook stamped when you are
finished. Then work on your project.
2nd Law
p33
2nd Law
The net force of an object is
equal to the product of its mass
and acceleration, or F=ma.
Which falls faster,
a penny or a feather?
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Demo
Conclusion:
In air (and therefore with air resistance
called fluid friction) __________ will fall
faster
In space (no air resistance)
_______________________________________
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
F = ma
98 N = 10 kg x 9.8 m/s/s
9.8 N = 1 kg x 9.8 m/s/s
2nd Law
When mass is in kilograms (kg) and
acceleration is in m/s/s,
the unit of force is in newtons (N).
 One newton is equal to the force required
to accelerate one kilogram of mass at one
meter/second/second.
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2nd Law (F = m x a)
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How much force is needed to accelerate a 1400
kilogram car 2 meters per second/per second?
Write the formula
F=mxa
Fill in given numbers and units
F = 1400 kg x 2 meters per second/second
Solve for the unknown
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2800 kg-meters/second/second or 2800
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N
If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.
Types of Forces
Contact
Forces
Distance
Forces
•
•
•
•
•
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• Gravitational (weight)
• Electrical
• Magnetic
Friction
Tension
Normal
Air resistance
Applied
Spring
Check Your Understanding
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1. What acceleration will result when a 12 N net
force applied to a 3 kg object? A 6 kg object?
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2. A net force of 16 N causes a mass to accelerate
at a rate of 5 m/s2. Determine the mass.
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3. How much force is needed to accelerate a 66 kg
skier 1 m/sec/sec?
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4. What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/sec/sec?
Check Your Understanding
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1. What acceleration will result when a 12 N net force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s
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2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the
mass.
16 N = 3.2 kg x 5 m/s/s
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3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
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4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
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9800 kg-m/sec/sec or 9800 N
Gravitational Force (Weight)
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Weight (a force) is caused by gravity
(acceleration) pulling you (your mass)
towards the earth
Using Newton’s 2nd Law F=ma where
force is your weight in newtons, mass
must be in kg (not pounds), and gravity is
9.8 m/s2 (round off…g=10 m/s2)
Weight= mass x gravity
W=mg
Weight depends on gravity
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Gravity on earth is
9.81 m/s2
What is your weight
on earth if your mass
is 50 kg?
W=mg
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Gravity on the moon
is 1.622 m/s2
What is your weight
on the moon if your
mass is 50 kg?
W=mg
Your mass remains the same, but your
weight is different on the earth than on
the moon because there is more gravity
on earth
Hoop Dreams
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Play (10 min)
 Record
Draw a picture and describe your technique(s).
Show on the diagram where to grab the hoop and
the correct direction to move it so that the result
is the hex nut falling into the bottle.
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When you are finished with your picture, raise
your hand and show your teacher for a stamp,
then go on to the next page.
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Talk. Take turns. Each partner gets to say one
sentence at a time (if there is an adult
observing, wait for the words to be recorded
before the next person takes a turn).
1.) Why does the hex nut fall into the bottle
when the hoop is pulled/pushed?
2.) Why does that technique work while
others do not (trust me…there is essentially
only one way to make the trick work)
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Watch
p36
http://www.youtube.com/watch?v=uOSBC0S
XVR4
Record the important physics vocab that are
used in the video:
Write. Now use these words in your answer to
the driving question (again):
Why does the hex nut fall into the vessel when
the hoop is pushed/pulled?
You may add a labeled picture to help explain
what you mean, but your answer must be in
paragraph form…
Finish
p31
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CU (p138) 1-6
PtoGo (p143) 1-4 (p144) 10
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CDP 3-1
p34
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Get your work stamped before you go
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nd
2
Newton’s
Law: Push or Pull
(AP p157-159)
p37
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What is force? What do you think?
Investigate...record data and answer questions
1-9 in your notebook. The materials are at the
lab benches.
Put the lab materials back in the covered bin
and Get a stamp from your teacher when you
are finished at the lab station
Return to your seats and read p160-167.
p38
Answer CU p167 (1-4)
Model for Forces
DQ: How do unbalanced forces
change the motion of an object?
8.) Forces are pushes or pulls. Forces add
p32
together to give the net force.
9.) Forces add together as vectors (parallelagram
rule)
10.) The total net force on an object is related to its
mass and acceleration by Newton’s 2nd Law F=ma
11.) Weight is the force due to gravity (weight is
proportional to mass) W=mg (on earth g=9.81≈10)
Quiz tomorrow!
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Correct CU (p167) 1-4
PtoGo (p171-172) 1-8
p34
p38
Project time today and tomorrow. Finish
Benchmark 4 (second page of sportsdraft and
work on google slides)
Notebook check Pages 32-38
Quiz today!
No electronics during the test! You
may use your notebook and a
calculator
 Notebook check (p32-38)
 Work on project if any remaining
time…Benchmark 4 (second page
of sportsdraft and google slides)
PtoGo (p171-172) 1-8 Answers
F
1.)
= m
a
m/s2
350 N
70 kg
5
800N
80 kg
10 m/s2
70 N
7 kg
10 m/s2
400 N
80 kg
5 m/s2
-1500 N
100 kg
-15 m/s2
-3000 N
100 kg
-30 m/s2
p38
2.a) the long jump and shot put involve freefalling objects; therefore the acceleration is due
to gravity (g=10 m/s2)
b) the negative sign shows that the force and
acceleration are in opposite directions
3.) 42/0.30= 140 m/s2
4.) F=ma=(0.040)(20)= 0.8 N
5.a) A bowling ball has greater inertia (mass)
than a baseball therefore a bowling ball has a
greater tendency to stay at rest or in motion
b) More force is required to make a bowling
ball accelerate
6.) F=ma=(0.1)(10)= 1 N thus a “newtonburger”
7.a) if you weigh (150 lbs)(4.38)= 657 N
b) F=ma
657= (m)(10) m=65.7 kg
3rd Law
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For every action, there is an
equal and opposite reaction.
p40
3rd Law
According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.
3rd Law
There are two forces
resulting from this
interaction - a force on
the chair and a force on
your body. These two
forces are called action
and reaction forces.
Name an action-reaction pair
Free Body Diagrams
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Use arrows (vectors)
to show all the forces
acting on an object
The size of the arrow
represents the
strength of each
force
The direction of the
arrow is the direction
of the force
Draw the free-body diagram for
this person
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What forces are at
work?
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What is the sum of
these forces?
What is the sum of the forces?
1
3
3
3
4
3
Finish the net force problems
P40
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Get your work stamped before you leave
Finish the net force problems
Finish and get a stamp
P42
rd
3
Newton’s
Law: Run and
Jump (AP p198-200)
Part A (together)
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What do you see?
What do you think?
Investigate 1-4
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Part B (optional)
rd
3
Newton’s
Law: Run and
Jump (AP p198-200) P41
Part A (together)
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What do you see?
What do you think?
Investigate 1-4
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Part B (Steps 1-4)
Get a stamp when finished
Model for Forces
DQ: How do unbalanced forces
change the motion of an object?
p32
12.) Forces come from interactions
between objects. This means that
forces come in pairs.
13.) For each pair of forces, the forces
are equal, but in opposite directions
(Newton’s 3rd Law)
CU (p205) 1-3 and Show the Forces
P42
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Model notes (p32)
Finish CDP 7-2 and Show the forces (p42)
Quiz 3 corrections (p39)
Progress report reflection (p43)
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Highlight current grade
Highlight 3 lowest scores (by percentage)
Write 3 goals to improve your grade
Get parent signature
Newton’s 3rd Law in Nature
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Consider the propulsion of a
fish through the water. A
fish uses its fins to push
water backwards. In turn,
the water reacts by pushing
the fish forwards, propelling
the fish through the water.
The size of the force on the
water equals the size of the
force on the fish; the
direction of the force on the
water (backwards) is
opposite the direction of the
force on the fish (forwards).
3rd Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their wings,
the air pushes their
wings up and gives
them lift.
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Consider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the bird
upwards.
The size of the force on the air equals the size of the
force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on
the bird (upwards).
Action-reaction force pairs make it possible for birds
to fly.
Other examples of Newton’s
Third Law
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The baseball forces the
bat to the left (an
action); the bat forces
the ball to the right (the
reaction).
3rd Law
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Consider the motion of
a car on the way to
school. A car is
equipped with wheels
which spin backwards.
As the wheels spin
backwards, they grip the
road and push the road
backwards. Friction
from the road pushes
the car forwards.
3rd Law
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
direction.