Download newtons 1st and 2nd law

Sir Isaac Newton
1st and 2nd laws of motion
Sir Isaac Newton
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Newton made many contributions to science.
 He developed the scientific method.
 You can thank him for calculus.
His biggest contribution was developing laws of forces and
gravity.
Born in England in 1642 to a poor farming family.
He studied math, physics, astronomy
Knighted by Queen Anne in 1705
He never married
Died in 1727 at age 84
Buried in Westminster Abby
Sir Isaac Newton
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There is a famous myth about Newton that many of you many
have heard.
 He was sitting in his garden when an apple fell on his head.
Right then and there he discovered gravity along with the
other forces.
 It is doubtful this really happened but it’s a good story.
 It was his understanding of math that ultimately allowed
him to determine the three laws of motion for the universe.
Newton’s First Law
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An object at rest will stay at rest and an object
in motion will stay in motion unless it is acted
upon by an unbalanced force.
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An object tends to stay “doing what it is doing”
unless another force changes that.
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Water in a cup that is sitting in the cup holder of the car.
What happens to you in the car when it comes to an
abrupt stop?
Why do cars have head rests?
1st Law
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Inertia is the
tendency of an object
to resist changes in
its velocity: whether
in motion or
motionless.
mass = inertia
mass = inertia
These pumpkins will not move unless acted on
by an unbalanced force.
Force
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Force- - A force is a push or pull on an object
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This happens when an object interacts with
another object
Balanced and Unbalanced Forces
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Balanced Forces- Forces on an object are
equal in size and opposite in direction. The
object does not accelerate.
Unbalanced forces- Forces on an object that
are unequal in size and in the opposite
direction. The object accelerates in the
direction of the larger force.
Activity- With a partner, apply a set
of balanced forces on a book. What
happens to the book?
 Now apply a set of unbalanced forces
to make the book accelerate to the
right. How many ways can you
achieve this? Explain.
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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.)
Free Body Diagrams
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Shows all forces acting on an object
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Abbreviations to know: app = push or pull; norm =
normal (floor, ground, chair, table); frict= friction;
grav= gravity
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.
The End
For today….
Newton’s 2nd Law
Newton’s 2nd Law
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The net force acting on an object causes the
object to accelerate in the direction of that net
force.
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It explains how the velocity of an object changes
when an external force acts on it.
The net force of an object is equal to the
product of its mass and acceleration
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F=ma (Force = mass x acceleration)
SI Units
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F = Force in Newtons (N)
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The unit kg × m/s2 is called a Newton
m = Mass in kilograms (kg)
a = Acceleration in (m/s2)
Newton’s 2nd Law (F = ma)
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How much force is needed to accelerate a 1400
kilogram car 2 m/s2?
Write the formula
F = ma
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/second2 or 2800
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N
Practice problems
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If 12N net force is applied to a 3 kg object,
what is the acceleration? (write formula, show
substitution with units, write answer with
correct units)
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Rearrange the formula F=ma
a=F/m (force divided by mass)
a= 12N / 3kg
a= 4 m/s2
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A force of 15 N is exerted on a book causing it
to accelerate 5 m/s2, what is the mass. (show
work)
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Rearrange the formula F=ma
m=F / a (force divided by acceleration)
m=15N / 5 m/s2
m= 3 kg
Gravity
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Anything that has mass is attracted by the
force of gravity.
Law of Gravitation
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Any two masses exert an attractive force on each
other.
The attractive force depends on the mass of the
two objects and the distance between them.
Gravity
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Gravity increases as the mass of either object
increases.
Gravity decreases as the distance between the
two objects increases.
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You can’t feel the gravitation pull of the desk on
you because it doesn’t have enough mass.
You can feel the gravitational attraction of the
earth because it is close enough and has a large
mass.
Gravitational Acceleration of
Earth
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Near the Earth’s surface, the gravitational
acceleration is 9.8 m/s2
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You can round this to 10.0 m/s2
The force of Earth’s gravity is always down.
All objects fall at the same acceleration no
matter how large or small they are.
Weight
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Standing here, my acceleration is zero and the
net force on me is zero.
The Earth is still pulling me down but the floor is
pushing me up with the same force. I’ll try not to
fall. 
Weight is the gravitational force on an object.
Weight = mass x acceleration due to gravity (W=mg)
W = m x 9.8 m/s2 (or 10.0 m/s2)
metric units are in Newtons
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Weight vs Mass
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Remember: Mass is the amount of matter in an object
The difference?
 Mass can’t change, weight can change with gravity.
What is the weight of a 70 kg person?
(write formula, substitution with units, answer
with units)
W= m*10.0 m/s2
W= 70kg *10.0 m/s2
W= 700 N
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What is the force of gravity on a 3.1 kg
dog?
W=mg
W= 3.1 * 10.0 m/s2
W= 31.0 N
Velocity vs Acceleration
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When we talk about Newton’s laws, we will talk a lot
about velocity and acceleration.
Velocity is how an object changes position from one
point to another.
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It is how fast or slow the object changes position and in which
direction it travels.
Acceleration measures how fast or how slow an object
speeds up or slows down.
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How fast or how slow it takes an object to reach its traveling
speed.
Citations
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Images from Google
Physical Science text
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