Download Chapter 3 Section 1 Newton`s Second Law

Chapter 3 Section 1
Newton’s Second Law
Newton’s First Law
Law of Inertia (first stated by Galileo)
• An object at rest will remain at rest unless it
experiences a net external force. An object in
motion will continue in motion at the same
speed and in the same straight-line direction
unless it experiences a net external force.
• Corollary: If either the magnitude or the
direction of the velocity of an object changes,
someone or something has exerted a net force
on the object.
Newton’s 1st Law
Law of Inertia
• The motion of an object changes
if and only if an unbalanced force
acts on the object.
Newton’s Second Law of Motion
• The acceleration of an object is directly
proportional to the net resultant force acting
on the object and is inversely proportional to
the mass of the object. The direction of the
acceleration is in the direction of the net
resultant force
F
aα
m
F=ma
Acceleration and Force
• For a given object (mass) the greater the
force, the greater the acceleration.
Acceleration and Mass
• For a given force, the greater the mass, the
smaller the acceleration.
Units
• Base Units
– length
– mass
– time
meter
m
kilogram kg
second
s
• Derived Units
– velocity = distance/time m/s
– acceleration= ∆v/∆t = m/s/s
– Force= ma = kg m/s2
Newton in honor of Isaac Newton
Newton
• 1 Newton (1N) is the force
needed to accelerate a
1 kilogram (kg) mass at 1 m/s/s.
September 23, 2011
• Read Page 68 and 69
• Page 69 Practice Problems 1,2,3,4
Page 69 Practice Problems 1-4
• You push a wagon that has a mass of 8 kg. If
the net force on the wagon is 4 N, what is the
wagon’s acceleration
• Given
• Asked
• Formula
• Given:
• Asked:
• Formula:
• Substitute:
• Answer:
Problems
September 24
• Read the last paragraph on page 70 and first
paragraph on page 71
• Homework Assignment Page 74: 7-8
Schedule
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Monday 9/23 Page 68-69 Newton’s 2nd Law
– Page 69:1-4
Tuesday 9/24 Friction
– P74: AM 7,8
Wednesday 9/25 Air Resistance
Thursday 9/26 Friction Lab
Friday 9/27 Gravity
Monday 9/30 Projectile Motion
Tuesday 10/1 Circular Motion, Centripetal Force
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Wednesday 10/2 Thursday Section 3 Newton’s 3rd Law
Thursday 10/3 Momentum
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Page 86:1-4
Friday 10/4 Conservation of Momentum
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Page 82: 6,7,8
Practice Problems
Monday 10/1 NTWS Sections 1-2, DRCM 1
Tuesday 10/2 Chapter Review 1-14 and 24,25,28
Wednesday 10/3 Test Section 1-2 Chapter 2
Friction
Cause- Microwelds
(Figure 4 page 71)
Friction
Box Not Moving
Applied Force
Static Friction
Friction
Box Moving
Applied Force
Kinetic Friction
Friction
Rolling
Static Friction
Friction
Sliding
Kinetic Friction
• Static Friction- object not
moving
• Kinetic friction- object sliding
• Rotational
–rolling- static friction
–sliding- kinetic friction
• Static Friction > Kinetic Friction
Practical Application
• Driving a car
• Wheels rolling – Static Friction
• Wheels sliding – Kinetic Friction
• Static Friction > Kinetic Friction
• Anti Lock Brakes (ABS)- prevents slamming on
the brakes and locking the wheels
Assignment Page 74: 7-8
• Given:
• Asked:
• Formula:
• Substitute
• Answer with Units:
Air Resistance
• Without air resistance, all objects
would fall with the same
acceleration.
• The acceleration of gravity
• g =- 9.8 m/s2
• Feather and ball - see page 73
Air resistance
Air Resistance
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The amount of air resistance depends on the
Speed
Size
Shape of the object.
• The faster an object falls the greater the air
resistance.
Air Resistance (cont)
• An object falling from an air plane starts with
zero velocity but it is accelerating at -9.8 m/s2
• The object keeps falling faster and faster but
the air resistance increases the faster it falls.
Terminal Velocity
• Finally the force of air resistance
becomes equal to the force of gravity
and the object no longer accelerates.
• The velocity stays the same.
• The object does not stop and float.
• Terminal Velocity
Review
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Newton’s 1st Law – law of inertia
Newton’s 2nd Law - F= ma
Unit of Force: kg m/s2 called a newton
Friction
– Static, Kinetic (sliding), Rolling
– Static > kinetic
• Air Resistance
– Size, Shape, Speed
Chapter 3
Section 2 Gravity
Page 75-82
Gravity
• A fundamental characteristic of all matter.
• Einstein would say it is a fundamental characteristic of
space and time.
• Anything that has mass is attracted by the force of gravity.
• Anything that has mass attracts other things that have
mass.
• Newton found the formula for finding the force of gravity.
FG = G m1m2
d2
G =6.67 X 10 -11N m2/kg2
Gravity
Fundamental Forces
• Electromagnetic force
– Attracts or repels
• Strong Nuclear force
• Weak Nuclear force
• Nuclear forces only over very short distances.
• Gravitational Force
– Always attraction
– Long distance
Earth’s Gravity
The earth exerts a force of attraction on every
object. At the same time that object exerts a
force of attraction on the earth.
Fg = m g
g = -9.8 m/s2
W = m g = m *-9.8 m/s2
W = m g = kg*m/s2 = newtons
Weight Varies
Weight vs Mass
• Weight and mass are not the same
• Mass is a measure of the inertia of an object,
how much the object resists a change in
velocity.
• Mass is a measure of the amount of substance
• Weight is a force
• Weight is the force of attraction between an
object and the earth
Weightlessness
• If you are in an elevator that is in free fall
(being accelerated toward the ground with the
acceleration of g) then you would be
weightless, but not massless.
• See Figure 15, Page 79
• Elevators don’t free-fall !
Projectile motion
• Projectile- anything that is thrown or shot
through the air.
• Projectile- an object moving in the horizontal
direction at a constant velocity and under the
influence of gravity in the vertical direction.
• The object (projectile) follows a curved path,
in fact the curve has a name- parabolic
motion.
Projectile Motion
Page 80
Projectile Motion
Parabolic Motion
Time
Constant horizontal
velocity
Accelerating vertical
velocity
Both balls hit ground at same time!
Tuesday October 1
• Read Page 81 and 82
Motion in a Circle (p81)
Velocity is constantly changing
Changing Velocity
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Velocity is constantly changing
Changing velocity = acceleration
Acceleration toward the center of the circle
Acceleration means Force
Force toward the center of the circle
Motion in a Circle
Centripetal Force/Acceleration
v1
Fcenter
v2
2
mV
Fc =
r
Motion in a Circle
Centrifugal Force/Acceleration
v1
Fcenter
Foutward
v2
Centrifugal Force is a “made up” force that
is really the inertia of the object.
Centrifugal Force
• Fugitive- some one fleeing the law
• Centrifugal Force – the force fleeing the center
• Experience going around a curve in a car“thrown” to the outside.
• Centrifugal force is not a real force but is
inertia of the body trying to stay moving in a
straight line while the car is turning.
Source of Centripetal Force
• What provides the force that causes the moon
to go around a curve rather than following
inertia to keep going in a straight line?
Source of Centripetal Force
Gravity
Source of Centripetal Force
• What provides the force that causes a car to
go around a curve rather than following
inertia to keep going in a straight line?
Chapter 3 Assignments
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Page 69: 1-4
Page 74: 7,8
Page 82: 6-8
Lab/Demonstration - Friday
Note Taking Worksheet - Turn in Monday
Chapter Review (Page94-95) 1-14, 24-28
Tuesday
• Test- Wednesday
Chapter 3 Section 3
Newton’s 3rd Law of Motion
Page 83-88
Newton’s 3rd Law
• When one object exerts a force on a second
object, the second object exerts a force on the
first object that is equal in strength and
opposite in direction.
• For every action force there is an equal and
opposite reaction force.
• The ladder law- if you jump off of a ladder the
ladder exerts a force that moves you but you
also exert a force on the ladder that may move
the ladder.
Examples of 3rd Law
• When you push on a wall, the wall pushes back on
you.
• A skater exerts a force on a second skater and the
second skater exerts an equal force on the first
skater.
• When you swim, you push the water back but the
water pushes the swimmer forward.
• A rocket engine exerts a force that pushes hot gases
out the back of the rocket, but the hot gases exert a
force that propels the rocket forward.
Why don’t these opposite forces
cancel each other?
• Because the action force acts on one object
and the reaction force acts on another object.
– Force on wall
- force on you
– Force on you
- force on ladder
– Force on water
- force on swimmer
– Force on #1 skater - force on #2 skater
– Force on gases
- force on rocket
Momentum (p86)
• The Momentum is a property of a moving
object that is related to the force that is
needed to change the motion of the object.
• Momentum is the product of the mass and
the velocity of an object.
• Momentum is given the symbol “p”
• p (momentum) = m (mass) * v (velocity)
Momentum and Force
p= mv
F= ma
v f – vi
∆v
a=
=
∆t
∆t
F = m v f – vi
∆t
F = m ( vf – vi )/ ∆t
F ∆t = m ( vf – vi )
Impulse = Change in Momentum
Practice Problems Page 86
Assignment 1-4
• At the end of a race, a sprinter with a mass of
80.0 kg has a speed of 10.0 m/s. What is the
sprinters momentum.
• P = m*v
• #1 asks for momentum
• #2 asks for speed
• #3 asks for mass
Problems
• Given:
• Asked:
• Formula:
• Substitute:
• Answer (with units):
Units:
Conservation of Momentum
• If there is no outside force acting on an object
or a group of objects then F= O so the change
in momentum is zero. The momentum before
the event is the same as the momentum after
the even
F ∆t = m vf – mvi
F ∆t =0 and m vf – mvi = 0
m vf = mvi
Examples of conservation of
Momentum
• A pool ball moving at a velocity strikes a group of
pool balls at rest. Momentum is transferred from
the moving ball to the balls at rest but after the
collision, the total momentum must be the same.
• An ice hockey puck moves across the ice and
strikes a second puck. The first puck stops but
the second puck moves away with the same
momentum that the first puck had before the
collision.
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Review
Newton’s 1st Law – law of inertia
Newton’s 2nd Law - F= ma
Newton’s 3rd Law- Action/Reaction
Unit of Force: kg m/s2 called a newton
Friction
– Static, Kinetic (sliding), Rolling
– Static > kinetic
• Air Resistance
– Size, Shape, Speed
• Anything that has mass attracts other things
that have mass.
FG = G m1m2
d2
G =6.67 X 10 -11N m2/kg2
Fg = m g
W = m g = kg*m/s2 = newtons
Weight vs Mass
• Weight and mass are not the same
• Mass is a measure of the amount of substance
• Weight is the force of attraction between an
object and the earth
Dear TJCA Parents, grades 7-12:
This is an important reminder that any student who has a grade below 70 on
midquarter or end of quarter report cards in a core subject area MUST attend the
required after school tutoring from 3-4PM on the designated days for the respective
courses, listed below, until the end of the next grading period. PLEASE note that
students who do attend mandatory tutoring will be placed in In School Suspension
the following day. We cannot stress enough how important it is to attend these
tutoring sessions to help students to bring these grades up.
Mondays: English, History (if failing both- teachers will rotate the schedule)
Tuesdays: Science
Thursdays: Math/computer
Fridays: Latin, logic, foreign language
We are all making sacrifices to help assure your child's success, so please make
sure your child attends these mandatory sessions.
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Chapter 3 Assignments
Page 69: 1-4
Page 74: 7-8
Page 82: 6-8
Page 86: 1-4
Note Taking Worksheet/Reinforcement Sheet
Turn in Thursday
Chapter Review (Page94-95) 1-14, 24-28
Test –
Problem 24
• Find your mass if a scale on Earth reads 650N.
• Given:
• Asked:
• Formula:
• Substitute:
• Answer (with units)
Problem 25
• You weigh yourself at the top of a high mountain and
the scale reads 720N. If your mass is 75 kg, what is
the acceleration of gravity at your location?
• Given:
• Asked:
• Formula:
• Substitute:
• Answer (with units)
Problem 26
• The 2kg metal ball moving at a speed of 3m/s strikes
a 1kg wooden ball that is at rest. After the collision,
the speed of the metal ball is 1m/s. Assume
momentum is conserved, what is the speed of the
wooden ball after the collision?
• Given:
• Asked:
• Formula:
• Substitute:
• Answer (with units)
Problem 27
• Find the mass of a car that has a speed of
30m/s and a momentum of 45,000 kg m/s.
• Given:
• Asked:
• Formula:
• Substitute:
• Answer (with units)
Problem 28
• A box being pushed with a force of 85N slides
along the floor at a constant speed. What is t
• Given:
• Asked:
• Formula:
• Substitute:
• Answer (with units) he force of sliding friction
on the box?
Tuesday September 27, 2011
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Need your book, calculator, Reference Sheet
Turn in all homework with your test
Use the blue (gray) side of scantron
Question 1: a=true, b=false
Show all work on the problems
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Given
Asked
Formula
Substitution
Answer with unit
– After the test
– Turn in all homework for chapter 3
– Read pages 100-102