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Newton’s Laws of Motion
Forces change the motion of an object in very
specific ways – Sir Isaac Newton (1642-1727)
was able to state the Laws that describe the
effects of forces.
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Newton’s 1st Law of Motion
Law of Inertia
INERTIA is…
the tendency for an object to resist its change in motion.
It wants to stay where it is.
No mom,
I don’t
want to
go!
First Law basically states that… An object in
motion tends to stay in motion, while an
object at rest tends to stay at rest until a
force acts upon the object.
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Newton’s 1st Law of Motion - Continued
The greater the mass of the object – the greater it’s inertia.
Think about it like this…
The book is sitting on the table is at rest.
The book will stay there (remain at rest)
until someone moves it (the outside
force).
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Newton’s 1st Law of Motion - Continued
Another example:
If you were standing the same distance from a softball
being hit or a baseball being hit which one would hurt
more if it hit you?
Which has more inertia?
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Newton’s 2nd Law of Motion
Law of Acceleration
Second Law basically states that…a force
acting on an object causes the object to
accelerate in the direction of that force.
Force causes acceleration.
Greater the mass – the greater the
resistance to acceleration.
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Newton’s 2nd Law of Motion - Continued
Let’s think about this one…if the soccer
player kicks the soccer ball – which direction
will the soccer ball go?
Force causes acceleration of the object
to accelerate in the direction of that
force (the kicking of the ball).
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Newton’s 2nd Law of Motion - Continued
Think about it like this…
If the blue truck has a mass of 25,000kg and the red
truck has a mass of 30,000kg which truck is going to
take longer to speed up?
The RED Truck – 30,000kg
It goes back to the greater the mass – the
greater the resistance to acceleration.
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Newton’s 2nd Law of Motion - Continued
Newton’s 2nd Law of Motion can be
expressed in an equation:
Force = Mass X Acceleration
F = ma
divide
F
m
a
multiply
Force units = kg.m/s2 OR Newton
Mass units = kilograms (kg)
Acceleration units =
m/s2
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Newton’s 2nd Law of Motion - Continued
Problem:
What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 1
Read the problem.
Draw a picture.
1000kg
15m/s2
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What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 2
Write down what you know.
What are you trying to find?
1000kg
15m/s2
F=
M=
1000kg
A=
15m/s2
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What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 3
Set up the formula.
1000kg
15m/s2
Formula Substitute
F=
M=
1000kg
A=
15m/s2
Answer
F = ma
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What force is exerted on a 1000kg car
accelerating at a speed of 15m/s2?
Step 4
Substitute the numbers.
Solve. Box Answer with Units.
1000kg
15m/s2
Formula
F = 15,000N
M = 1000kg
A = 15m/s2
F = ma
Substitute
F = 1000kg x
15m/s2
Answer
F = 15,000
kg.m/s2 OR
F = 15,000 N
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Do problems 1-3
in your notes.
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Newton’s 2nd Law of Motion - Continued
The acceleration for any object moving under the
sole influence of gravity.
Any moving object being acted upon only by the force
of gravity is said to be "in a state of free fall."
Characteristics of Free-Falling Objects:
•do not encounter air resistance.
• (on Earth) accelerate downwards at a rate of
9.8 m/s2.
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Let’s Look at a Diagram
Assuming that the position of a
free-falling ball dropped from a
position of rest is shown every 1
second, the velocity of the ball
shown to increase as the time
increases.
What relationship do you notice
about the distance per every 1
second?
So…what type of acceleration
does the ball have? Positive or
Negative.
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Remember the formula to find
acceleration?
a=
Vf - Vi
t
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Let’s Look at a Problem
What is the velocity of a rubber ball dropped
from a building roof after 5 seconds?
Step 1
Step 2
What
trying to
Read the problem.
Draw a picture.
Write down what
you know.
are you
find?
Vf =
Vi = 0 m/s
t= 5s
a = 9.8 m/s2
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What is the velocity of a rubber ball dripped from a building
roof after 5 seconds?
Step 3
Set up the formula.
a=
Step 4
Vf - Vi
t
Substitute the numbers.
Solve. Box Answer with Units.
Formula
a= Vf - Vi
t
Substitute
Answer
49m/s = Vf – 0m/s
9.8m/s2 = Vf – 0m/s + 0m/s = + 0m/s
5s
49m/s = Vf
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Do problems 4 & 5
in your notes.
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Newton’s 3rd Law of Motion
Law of Action / Reaction
Third Law basically states that…to every ACTION
there is an equal and opposite REACTION.
Action/Reaction forces act in PAIRS.
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Newton’s 3rd Law of Motion - Continued
In this example, when the lady is
walking, her feet push against the
ground while the ground pushes
against her feet.
Thus action/reaction – pair is
the feet and ground pushing off
of one another.
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Newton’s 3rd Law of Motion - Continued
Another example:
Let’s think about this one…what are the pairs of
forces that are acting against each other in
order for the shuttle to launch?
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MOMENTUM
Property of a moving object because of its
mass and velocity.
An increase in mass or velocity causes an
increase in momentum.
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Momentum can be calculated
with the following formula:
Momentum = Mass X Velocity
P = mv
P
m
v
Units for Momentum are kg.m/s
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Problem: Find the momentum of an 7.2kg rock that is
rolling down a hill with a velocity of 3.0m/sec.
Step 1
Step 2
Read the problem.
Draw a picture.
Write down what you know.
What are you trying to find?
P = 21.6kg.m/s
m = 7.2kg
v = 3.0 m/s
Formula
P = mv
Step 3
Set up the formula.
Step 4
Substitute the numbers.
Solve. Box Answer
with units.
Substitute
Answer
P = 7.2kg x 3.0m/s P = 21.6 kg.m/s
Units, units, units!
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Do problems 6 & 7
in your notes.
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Law of Conservation of Momentum
• In the absence of external forces applied to
a system, the total momentum cannot
change.
• m1 v1 = m2v2
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Do problems 8 & 9 in
your notes.
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