chapter 2 newton’s first law of motion aristotle on motion (350 bc) aristotle attempted to...
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Chapter 2Chapter 2
Newton’s First Newton’s First Law of MotionLaw of Motion
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Aristotle on Motion Aristotle on Motion (350 BC)(350 BC)
Aristotle attempted to understand motion by classifying motion as either
• (a) natural motion• forces acting at a distance
• (b) or violent motion• contact forces
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Large object tend to 'strive harder'.
“The Earth remains at rest.”
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Geocentric Model - Earth Centered Universe
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Copernicus (1500's)Copernicus (1500's)
"The Earth and planets orbit the Sun.”
He reasoned this from his astronomical observations.
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Galileo (1600's)Galileo (1600's)Scientist who supported
Copernicus
Dropped objects with different weights from the Leaning Tower of Pisa
Found that all objects fall at the same rate if you can account for air resistance
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Friction - a force that resists motion
• e.g. air resistance and sliding on rough surfaces
Inertia - the resistance of an object to change in its state of motion
Demo: Ball and incline plane
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Galileo’s Incline PlanesGalileo’s Incline Planes
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Isaac Newton (1642-1727)Isaac Newton (1642-1727)
His three laws of motion first appeared in his book called Principia.
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Newton’s First LawNewton’s First Lawa.k.a “Law of Inertia”
A body remains at rest or moves in a straight line at a constant speed unless acted upon by a force.
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Newton’s First Law ExamplesWeight and string
Card, cup, and coin
Fixing a Hammer
Demo - Coins on elbowDemo - Lead Brick and HammerDemo - Table setting
Figure 2.4
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Mass Mass the quantity of matter in an object
the measurement of the inertia
measured in kilograms (kg)
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WeightWeightthe force upon an object due to
gravity
Weight = Mass Acceleration of gravity
W = mg
measured in Newtons (N) in the metric system or pounds (lb) in the British system
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The weight of a 10 kg brick is...• A) 98 N • B) 10 kg • C) 9.8 kg• D) 10 N • E) 98 kg
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Mass and Weight should not be confused with...
Volume• the quantity of space an object occupies
Density • the quantity mass per unit volume
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Mass and Weight
On the Moon the gravitational force is only 1/6 as strong as on the Earth.
In space you are “weightless” but not “massless”.
Your mass does not depend on where your are.
• (e.g. Earth, Moon, or space).
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WeightLocation Mass
Earth
Moon
Space
18.4 kg
18.4 kg
18.4 kg
180 N
30 N
0 N
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Chapter 3Chapter 3
Linear MotionLinear MotionSee Homework 5See Homework 5
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Chapter 4Chapter 4
Newton’s Newton’s Second Law of Second Law of
MotionMotion
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NEWTON'S 2nd LAW OF MOTION
Fa
or amF
F am
F am
m
F a
m
m
m
F a
F a
F aM
m
a1
M
M
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Newtons’ Second LawNewtons’ Second LawF = m a
The acceleration of an object is directly proportional to the net force acting on the object…
…and inversely proportional to the mass of the object.
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Example QuestionsExample Questions
How much acceleration does a 747 jumbo jet of mass 30,000kg experience in takeoff when the thrust of all of the engines is 120,000N?
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Example QuestionsExample Questions
The same net force is applied to two blocks.
If the blue one has a smaller mass than the yellow one, which one will have the larger acceleration?
F F
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If the net force is parallel to the velocity, then the speed of the object increases.
If the net force is anti-parallel to the velocity, then the speed of the object decreases.
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If the net force is perpendicular to the velocity, the direction of the velocity changes.
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Force and acceleration are vector quantities.
If v is parallel to F, speed increases.If v is antiparallel to F, speed
decreases.If v perpendicular to F, direction of
v changes.
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When Acceleration Is Zero - Equilibrium
Static EquilibriumVelocity is zero
Examples:
Hanging from a tree
Weighing yourself on a set of scales
Computer setting on a table
Car parked on an incline
Normal up
Weight down
Weight down
Weight downWeight down
Scales pushing up
Tree pulling up
NormalFriction
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When Acceleration Is Zero...…we say the object is in Mechanical
Equilibrium.
…the net force is zero.
For Static Equilibrium the velocity is zero.
For Dynamic Equilibrium the velocity is constant.
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Dynamic EquilibriumVelocity is nonzero and constantExamples:Driving at constant velocity
Force from road
Weight down
Friction
Normal up
Weight downAir resistance
Terminal velocity in parachuting
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When the Acceleration is g...…the object is in Free Fall.Consider a 1kg rock and a 1gram
feather.– Which object weighs more?
• Answer: The rock
– On which is the gravitation force stronger?• Answer: The rock
– Which has a greater acceleration when dropped from rest?
• Answer: Both have the same acceleration, g.
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When the Acceleration Is Less Than g...…the object is not in Free Fall.
In this case there is a force other than gravity.
That force is air resistance.
Air resistance depends on size and speed.
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Example: A heavy parachutists will fall faster than a light one.
When the force of air resistance is equal to weight of the falling object, the object will reach a Terminal Velocity.
See Questions on page 66,67 and 69.
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After jumping from an airplane a skydiver will fall until the air resistance equals her weight. At that point...– A) she will fall with constant speed– B) she will fall no farther– C) she will fall faster– D) she opens her parachute – E) she will hit the ground
*