Transcript Introduction to Modern Physics PHYX 2710

Physics of Technology
PHYS 1800
Lecture 7
Introduction
Newton’s Laws
Section 0
Lecture 1
Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 1
PHYSICS OF TECHNOLOGY
Spring 2009 Assignment Sheet
Date
Day
Lecture
Chapter
Jan 5
M
Class Admin: Intro.Physics Phenomena
1
6
T
Problem solving and math
App. B, C
7
W
Units, Scalars, Vectors,
1
9
F*
Speed and Velocity
2
Jan 12
M
Acceleration
2
14
W
Free Falling Objects
3
16
F*
Projectile Motion
3
Jan 19
M
Martin Luther King
No Class
21
W
Newton’s Laws
4
23
F*
Mass and Weight
4
Jan 26
M
Motion with Friction
4
28
W
Review
1-4
1-4
29
Th
Test 1
30
F
Circular Motion
5
Feb 2
M
Planetary Motion and Gravity
5
4
W
Energy
6
6
F*
Harmonic Motion
6
Feb 9
M
Momentum
7
11
W
Impulse and Collisions
7
13Introduction
F*
Rotational
8
Section
0 Motion
Lecture 1 Slide 2
Feb 16
M
Presidents Day
No Class
17
Tu
Angular Momentum (Virtual Monday)
8
18
W
Review
5-8
19
5-8
H
Test 2
INTRODUCTION TO Modern Physics PHYX 2710
20
F*
Static Fluids, Pressure
9
Fall 2004
Feb 23
M
Flotation
9
25
W
Fluids in Motion
9
27
F*
Temperature and Heat
10
Mar 2
M
First Law of Thermodynamics
10
Physics of Technology—PHYS 1800
4
W Spring 2009Heat flow and Greenhouse Effect
10
Newton’s Laws
*Homework
Handout
6
F*
Climate Change
-
Homework Due
-
1
2
3
4
5
-
6
Lecture 7 Slide 2
7
Physics of Technology
PHYS 1800
Lecture 6
Newton’s Laws
Introduction
Section 0
Lecture 1
Slide 3
Introduction and Significance
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 3
Describing Motion
Position—where you are in space (L-meter)
Speed—how fast position is changing with time (LT-1
or m/s)
Acceleration—how fast speed is changing with time
(LT-2 or m/s2)
Introduction
Section 0
Lecture 1
Slide 4
Question: How do we get things to accelerate?
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 4
Acceleration
Acceleration is the rate at which velocity changes.
– Our bodies don’t feel velocity, if the velocity is constant.
– Our bodies feel acceleration.
• A car changing speed or direction.
• An elevator speeding up or slowing down.
Acceleration can be either a change in the object’s
speed or direction of motion.
Change in velocity V2 - V1
=
Average acceleration =
t
Time
interval
Introduction Section 0 Lecture 1 Slide
5
r
r
a = DV m s 2
t
In this Chapter acceleration is a variable, caused by FORCE.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 5
Acceleration Due to Gravity
• Earth exerts a gravitational force on objects that is
attractive (towards Earth’s surface).
• Near Earth’s surface, this force produces a
constant acceleration downward.
– To measure this acceleration, we need to slow down the
action.
– Galileo was the first to accurately measure this
acceleration due to gravity.
– By rolling objects down an inclined plane, he slowed the
motion enough to establish that the gravitational
acceleration
uniform,
Introduction Section 0is Lecture
1 Slide or
6 constant with time.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 6
How does this trajectory happen?
Key: - resolve motion into its HORIZONTAL and VERTICAL
components.
VH = constant
VTOTAL
VG (due to gravity)
But we know VG increases with time due to gravity acceleration!
VH (constant)
At any instant the total
velocity is vector sum of VH
and VG
Resultant TRAJECTORY
Introduction Section 0 Lecture 1 Slide 7
STEEPENS with increasing
time.
Uniform increase
in VG
with
time
INTRODUCTION TO Modern Physics PHYX 2710
As NO horizontal acceleration the ball moves
equal distances horizontally in equal time (assuming NO air resistance).
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 7
One Heck of a Ball Team!!!
Hart’s list of most influential people in the
history of the world:
Newton (2)*
Einstein (10)
Galileo Galilei (12)*
Aristole (13)***
Copernicus (19) *
Kepler (75) *
*(even though they got the wrong answer on the test)
Introduction
Section 0
Lecture
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Simmon’s list of most influential scientists in
the history of the world
Newton (1)* (and 2 and 6 and 40)
Einstein (2)
1 Slide 8
Galileo Galilei (7)*
Copernicus (9)
Kepler (10)
Tyco Brahe (22)
Aristole (an honorable mentioned)***
Newton’s Laws
Lecture 7 Slide 8
Newton’s Contribution
• Newton built on Galileo’s
work, expanding it.
• He developed a
comprehensive theory of
motion that replaced
Aristotle’s ideas.
• Newton’s theory is still
widely used to explain
ordinary motions.
Introduction
Section 0
Lecture 1
Slide 9
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 9
Aristotle’s View
• A force is needed to
keep an object
moving.
• Air rushing around a
thrown object
continues to push the
object forward.
Introduction
Section 0
Lecture 1
Slide 10
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 10
Galileo’s Contribution
• Galileo challenged Aristotle’s ideas that
had been widely accepted for many
centuries.
• He argued that the natural tendency of
a moving object is to continue moving.
– No force is needed to keep an object
moving.
Introduction Section 0 Lecture 1 Slide 11
– This goes against what we seem to
experience.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 11
Newton’s First and Second Laws
 Put Galileo's notions of motion on a mathematical footing with
calculus
 Set up the framework to explain motion.
 How do forces affect the motion of an object?
 What exactly do we mean by force? Is there a difference between,
say, force, energy, momentum, impulse?
 What do Newton’s first and second laws of motion tell us, and how
are they related to one another?
 Developed the first hints of a concervation law— Newton’s 3rd Law
of Motion.
 Developed the first formulation of a force—the gravitational force.
Introduction
Section 0
Lecture 1
Slide 12
 Made seminal contributions in thermodynamics and optics.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 12
Inconsistencies in Physics cira 1900
Statistical Mechanics
• Boltzmann Distribution
• Entropy and counting states
Electricity & Magnetism
• Medium for propagation of light
• Obeys Lorentz transformation
•
•
•
•
Mechanics (Gravity)
• Discrete atomic spectra
• Radioactive decay
Obeys Galilean transformation
Introduction
Section 0
• Brownian motion
Lecture 1
Blackbody radiation
Wein’s Law
Photoelectric effect
Diffraction of x rays
Slide 13
•
Existence of Atoms!
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 13
Newton’s Law of Universal Gravitation
•
Newton recognized the similarity
between the motion of a projectile on
Earth and the orbit of the moon.
•
If a projectile is fired with enough
velocity, it could fall towards Earth
but never reach the surface.
•
The projectile would be in orbit.
•
Newton’s law of universal gravitation
says the gravitational force between
two objects is proportional to the
mass of each object, and inversely
proportional to the square of the
distance between the two objects.
Introduction
•
Section 0
Lecture 1
Slide 14
G is the Universal gravitational
constant G.
Fgravity =
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Gm1m2
r2
Lecture 7 Slide 14
Dennison’s Laws of Motion
1. Stuff happens (or not).
2. The bigger they are the harder
they fall.
3. You get what you give.
Introduction
Section 0
Lecture 1
Slide 15
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 15
Newton’s First Law of Motion
An object
remains at rest,
or in uniform
motion in a
straight line,
unless it is
compelled to
change by an
externally
imposedIntroduction
force. Section 0
Lecture 1
Slide 16
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 16
Newton’s Second Law of Motion
The acceleration of an
object is directly
proportional to the
magnitude of the imposed
force and inversely
proportional to the mass
of the object.
The acceleration
is the
Introduction Section
0 Lecture 1
same direction as that of
the imposed force.
Slide 17
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 17
Newton’s Second Law of Motion
 Note that a force is proportional to an object’s
acceleration, not its velocity.
 Precise definitions of some commonly used terms:
 The mass of an object is a quantity that tells us how much
resistance the object has to a change in its motion.
 This resistance to a change in motion is called inertia.
F = ma
Introduction
Section 0
Lecture 1
Slide 18
units: 1 newton = 1 N = 1 kg m s2
Force has dimensions of (MLT-2)
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 18
Fstring = 10 N (t o the right)
Net Forces
 It is the total force or net force
that determines an object’s
acceleration.
 If there is more than one vector
acting on an object, the forces are
added together as vectors, taking
into account their directions.
ftable = 2 N (t o the left )
Fnet = 10 N - 2 N
= 8 N (t o the right)
a=
Fnet 8 N
=
m 5 kg
= 1.6 m s2 (t o t he right
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Slide 19

Newton’s Laws
Lecture 7 Slide 19
Two equal-magnitude horizontal forces act on a box.
Is the object accelerated horizontally?
a)
b)
c)
Since the two forces
are equal in size, and
are in opposite
directions, they cancel
each other out and
there is no
acceleration.
Yes.
No.
You can’t tell from
this diagram.
Introduction
Section 0
Lecture 1
Slide 20
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 20
Is it possible that the box is moving, since the forces
are equal in size but opposite in direction?
a)
a)
Yes, it is possible for
the object to be moving.
No, it is impossible for
the object to be moving.
Introduction
Section 0
Lecture 1
Even though there is no
acceleration, it is possible the
object is moving at constant
speed.
Slide 21
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 21
Two equal forces act on an object in the directions
shown. If these are the only forces involved, will
the object be accelerated?
a)
b)
c)
Yes.
No.
It is impossible to determine
from this figure.
The vector
sumSection
of the
forces
in a
Introduction
0 two
Lecture
1 Slideresults
22
force directed toward the upper right corner.
The object will be accelerated toward the
upper right corner.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 22
Two forces act in opposite directions on a box. What is
the mass of the box if its acceleration is 4.0 m/s2?
a)
b)
c)
d)
e)
The net force is 50 N - 30 N = 20 N,
directed to the right.
5 kg
7.5 kg
12.5 kg
80 kg
120 kg
From F=ma, the mass is given by:
m = F/a
= (20 N) / (4 m/s2)
= 5 kg.
Introduction
Section 0
Lecture 1
Slide 23
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 23
A 4-kg block is acted on by three horizontal forces.
What is the net horizontal force acting on the block?
a)
b)
c)
d)
e)
10 N
20 N
25 N
30 N
40 N
The net horizontal force is:
5 N + 25 N - 10 N = 20 N
directed to the right.
Introduction
Section 0
Lecture 1
Slide 24
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 24
A 4-kg block is acted on by three horizontal forces.
What is the horizontal acceleration of the block?
From F=ma, the acceleration is given by:
a)
b)
c)
d)
e)
10 N
20 N
25 N
30 N
40 N
a = F/m
= (20 N) / (4 kg)
= 5 m/s2
directed to the right.
Introduction
Section 0
Lecture 1
Slide 25
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 25
A ball hangs from a string attached to the ceiling.
What is the net force acting on the ball?
a)
b)
c)
The net force is downward.
The net force is upward.
The net force is zero.
Since the ball is hanging from the
ceiling at rest, it is not
accelerating so the net force is
Introduction Section 0 Lecture 1 Slide 26
zero. There
are two forces acting
on the ball: tension from the string
and force due to gravitation.
They cancel each other.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 26
Two masses connected by a string are placed on a
fixed frictionless pulley. If m2 is larger than m1, will
the two masses accelerate?
a)
b)
c)
Yes.
No.
You can’t tell
from this diagram.
Introduction Section
Lecture
1 Slide 27
The acceleration
of0the
two
masses will be equal and will
cause m2 to fall and m1 to rise.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 27
Newton’s Third Law
• Where do forces come from?
• If we push on an object like a chair, does the
chair also push back on us?
• If objects do push back, who experiences the
greater push, us or the chair?
• Does our answer change if we are pushing
against a wall?
Introduction
Section 0
Lecture 1
Slide 28
• How does Newton’s third law of motion help us to
define force, and how is it applied?
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 28
Newton’s Third Law (“action/reaction”)
For every action (force),
there is an equal but
opposite reaction
(force).
Introduction
Section 0
Lecture 1
Slide 29
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 29
Identifying the forces acting on an object.
• The forces acting
on the book are W
(gravitational force
from Earth) and N
(normal force from
table).
• Normal force refers
to the perpendicular
force a surface
Introduction Section 0
exerts on an object.
Lecture 1
Slide 30
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 30
Third-Law Action/Reaction Pair
An uncompressed spring and the same spring supporting a
book.
The compressed spring exerts an upward force on the book.
Introduction
Section 0
Lecture 1
Slide 31
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 31
Third-Law Action/Reaction Pair
If the cart pulls back on the mule equal and
opposite to the mule’s pull on the cart, how
does the cart over move?
Introduction
Section 0
Lecture 1
Slide 32
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 32
Third-Law Action/Reaction Pair
The car pushes against the road, and the road, in
turn, pushes against the car.
Introduction
Section 0
Lecture 1
Slide 33
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 33
Newton’s Laws in Review
• 2nd Law (and 1st Law)—How motion of a
object is aeffected by a force.
• 3rd Law—Forces come from interactions
with other objects.
•
Two branches of Mechanics:
•
•
Statics using the 1st Law with a=0 or Fnet=0
Dynamics using the 2nd Law with a≠0 or Fnet ≠0
•
Note: At the most fundamental level, there are only 4
forces in nature, gravity, electricity and magnetism,
Introduction
Section
0 Lecture
1 and
Slide strong
34
tweak
nuclear
force
nuclear force.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 34
Dennison’s Laws of Motion
1. Stuff happens (or not).
2. The bigger they are the harder
they fall.
3. You get what you give.
Introduction
Section 0
Lecture 1
Slide 35
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 35
Free Body Diagrams
• Fancy Science: Vector analysis of complex force
problems is facilitated by use of a free body
diagram.
• Common Sense: A picture is worth a 100 words.
(A scale picture is worth an A!)
• Key is to:
• Isolate a single body and draw all the forces acting on it.
• Add up all the arrows (vectors).
Introduction Section 0 Lecture 1 Slide 36
• What’s left is the net force.
• Net force (and masses)  a.
• A plus initial conditions motion!
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 36
Physics of Technology
Next Lab/Demo:
Forces
Thursday 1:30-2:45
ESLC 53
Ch 3
Next Class:
Wednesday 10:30-11:20
Slide 37
BUS
318 room
Read Ch 4
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Newton’s Laws
Lecture 7 Slide 37