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
 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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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
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 =ma
 Acceleration is in direction of net force
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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
The cause of acceleration is… Force
 _________
Mass resists acceleration
 The greater the force, the ________
greater the
acceleration
less
 The greater the mass, the _________
the
acceleration.
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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 = 20/0.145kg = 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?
 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