Transcript Newton`s Laws

Newton’s Laws
Three Laws of Motion
Aristotle’s Motion
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Natural Motion is up or down
 Down for falling objects
 Up for smoke
 Circular for heavenly bodies since without
end
Violent Motion
 Due to imposed forces such as wind
pushing a ship or someone pulling a cart
Natural state of motion is rest
 A force is needed to keep something
moving
Aristotle’s Basic Error
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Friction not understood as a force
Galileo’s Motion
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Force is a push or a pull
Friction is a force that
occurs when objects
move past each other
Friction due to tiny
irregularities
Only when friction is
present is a force
required to keep
something moving
Galileo’s Inclined Planes
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Ball rolling downhill
speeds up
Ball rolling uphill
slows down
He asked about ball
on smooth level
surface
Concluded it would
roll forever in
absence of friction
Inertia
Resistance to change in state of motion
 Resistance to Acceleration
 Galileo concluded all objects have
inertia
 Contradicted Aristotle’s theory of motion
 No force required to keep Earth in
motion around sun because no friction
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Newton
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Born 1665
Built on Galileo’s
ideas
Proposed three laws
of motion at age of
23
Newton’s First Law
Ourtesy www.lakeheadu.ca/~alumni/ hockey.gif
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Every object continues in its state of rest, or
of motion in a straight line at constant speed,
unless compelled to change that state by
forces exerted on it.
Also called Law of Inertia: things move
according to their own inertia
Things keep on doing what they are doing
Examples: Hockey puck on ice, rolling ball,
ball in space
Mass
Amount of inertia depends on amount of
mass…or amount of material (number
and kind of atoms)
 Measured in kilograms
 Question: Which has more mass, a
kilogram of lead or a kilogram of
feathers?
 Mass vs. Volume: volume is how much
space something occupies
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Experiencing Inertia
Inertia is resistance to shaking
 Which is easier to shake, a pen or a
person?
 Why is it so hard to stop a heavy boat?
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Mass vs. Weight
Mass is intrinsic property of any object
 Weight measures gravitational force on
an object, usually due to a planet
 Weight depends on location of object
 Question 1: How does mass of a rock
compare when on Earth and on moon?
 Question 2: How does its weight
compare?
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Review Mass vs. Weight
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What is mass?
Answer: quantity of
matter in something
or a measure of its
inertia
What is weight?
Answer: Force on a
body due to gravity
Weight of 1 Kilogram
9.8 Newtons
 About 2.2 pounds
 Compare the weight of 1 kg nails with 1
kg styrofoam
 Answer: Same
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Weight Examples
What does a 70 kg person weigh?
W = mg = 70 kg x 9.80 N/m2 = 686 N
 An object weighs 9800 n on Earth. What
is its mass?
 m = W/g = 9800 / 9.8 m/s2 = 1000 kg
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W
= mg
Inertia in a Car
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Discuss three examples of inertia in
a car
•Car hitting a wall
•Car hit from behind by a truck
•Car going around a corner
Newton’s Second Law
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Law of Acceleration
The acceleration produced by a net force on
an object is directly proportional to the
magnitude of the net force, and is inversely
proportional to the mass of the body.
Acceleration = net force ÷mass
F
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=ma
Acceleration is in direction of net force
Net Force
Net Force means sum of all forces
acting
 Sum is Vector sum
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F2
F1
Resultant force
Understanding the Second
Law
Force
The cause of acceleration is…
 _________
resists acceleration
Mass
 The greater the force, the greater
________ the
acceleration
__________
less
 The greater the mass, the _________
the acceleration.
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What Resists Acceleration of
Student in Chair?
Inertia
 Friction force
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These are two completely different
things
Units
F = ma
 Unit of force is the Newton (N)
 1 N = 1 kg m/s2
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F = ma is Three Equations
F and a are vectors
 So F = ma equation is really three
SFx = max SFy = may SFz = maz
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Examples
What force is required to accelerate a
1000 kg car at 2.0 m/s2 ?
Answer: F = ma = 1000 kg x 2.0 m/s2 =
2000 N.
 What is the acceleration of a 145 g
baseball thrown with a force of 20.0 N?
a = F/m = 20N/0.145kg = 138 m/s2
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Newton’s Third Law
Forces always come in pairs
 Two forces on different objects
 Whenever one object exerts a force on
a second object, the second exerts an
equal and opposite force on the first
 Example: hammer hits nail
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Example: pushing on wall
What are the forces when you push on
a wall?
 You exert force on wall
 You accelerate in the opposite direction
 Wall must have exerted a force on you
in the direction you accelerated (by 2nd
Law)
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Example: person walking
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Foot exerts force
backward on ground
Ground exerts force
forward on foot
Example: Throwing ball
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Pitcher exerts force
on ball
Ball exerts equal
and opposite force
on pitcher
Why doesn’t pitcher
move?
Example: Rocket
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Rocket engine
exerts rearward
force on gas
molecules
Molecules exert
forward force on
rocket.
Horse and Cart
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Horse exerts force
on cart
Cart exerts equal
and opposite force
on horse
Net force = zero
Cart can’t move
Huh?
The net force is NOT zero. Forces on different objects cannot
be added to make zero
Book on Table
The mass of the book is one kg. What
is the force (magnitude and direction)
on the book exerted by table?
 9.8 N upward
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More Examples
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Can you think of some more examples
of Newton’s Third Law in Action?
Review: Newton’s Laws of
Motion
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Newton’s First Law:
Every object continues in its state of rest, or of
motion in a straight line at constant speed, unless
compelled to change that state by forces exerted on
it.
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Newton’s Second Law:
The acceleration produced by a net force on an
object is directly proportional to the magnitude of the
net force, and is inversely proportional to the mass of
the body.
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Newton’s Third Law:
Whenever one object exerts a force on a second
object, the second exerts an equal and opposite force
on the first
Concept Check 1
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Which of these statements about an
object in free fall is false?
 (a) It accelerates
 (b) It is in equilibrium
 (c) It exerts an upward force on the
Earth
 (d) It’s mass equals the net force on it
divided by its acceleration
 (e) Only one force acts on it
Derivation of acceleration
of block sliding down a
plane with friction