Transcript Introduction to Modern Physics PHYX 2710

Physics of Technology
PHYS 1800
Lecture 13
Introduction
Section 0
Energy
Lecture 1
Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 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
Energy
*Homework
Handout
6
F*
Climate Change
-
Homework Due
-
1
2
3
4
5
-
6
Lecture 13 Slide 2
7
Physics of Technology
PHYS 1800
Lecture 13
Energy
Introduction
Section 0
Lecture 1
Slide 3
Introduction
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 3
Dennison’s Law of minimum effort:
MAKE IT SIMPLE STUPID!!!
Introduction
Section 0
Lecture 1
Slide 4
INTRODUCTION TO Modern Physics PHYX 2710
Einstein on the Theory of General Relativity:
“Everything should be made as simple as possible, but not simpler. ... “
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 4
Can we build on
what we’ve learned
to make our life
easier…
... Is there some
sort ofIntroduction
conservation
Section 0 Lecture 1
law that is related
to Newton’s Laws of
motion…
Slide 5
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 5
Describing Motion and Interactions
Position—where you are in space (L or meter)
Velocity—how fast position is changing with time (LT-1 or m/s)
Acceleration—how fast velocity is changing with time (LT-2 or
m/s2)
Force— what is required to change to motion of a body
(MLT-2 or kg-m/s2)
Introduction
Section 0
Lecture 1
Slide 6
In this chapter we will develop on of the most useful concepts
in science…ENERGY…and learn what it means to
conserve energy.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 6
Newton’s Laws in Review
•
1st Law —a special case of the 2nd Law for statics,
with a=0 or Fnet=0
• An objects velocity remains unchanged, unless
a force acts on the object.
•
2nd Law (and 1st Law)—How motion of a object is
effected by a force.
– 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 same
direction as that of the imposed force.
F  ma
units: 1 newton = 1 N = 1 kg m s2
•
Introduction
Section 0
Lecture 1
Slide 7
3rd Law —Forces come from interactions with
other objects.
• 
For every action (force), there is an equal but
opposite reaction (force).
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 7
What can apply to all these
sorts of problems…
Forces are acting in each
case, but force is not
conserved really.
Introduction
Section 0
Lecture 1
Slide 8
F||
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 8
Physics of Technology
PHYS 1800
Lecture 13
Energy
Introduction
Section 0
Lecture 1
Slide 9
Work and Energy
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 9
Defining Work
• Work is equal to the force applied times the
distance moved.
– Work = Force x Distance:
– Work output = Work input
W=Fd
• units: 1 joule (J) = 1 Nm = 1 kg m2 / s2 [ML2T-2]
Introduction
Section 0
Lecture 1
Slide 10
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 10
Work and Power
• Only forces parallel to the motion do work.
• Power is the rate of doing work
– Power = Work divided by Time:
P=W/t
units: 1 watt (W) = 1 J / s = 1 kg m2 / s3 [ML2T-3]
Introduction
Section 0
Lecture 1
Slide 11
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 11
A string is used to pull a wooden block across the floor
without accelerating the block. The string makes an
angle to the horizontal. Does the force applied via the
string do work on the block?
a)
b)
c)
d)
Yes, the force F
does work.
No, the force F
does no work.
Only part of the
force F does work.
YouIntroduction
can’t tell
from
Section
0 Lecture
this diagram.
1
Slide 12
Only the part of the force that is parallel to the distance moved does work
on the block. This is the horizontal part of the force F.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 12
If there is a frictional force opposing the motion of
the block, does this frictional force do work on the
block?
a)
b)
c)
d)
Yes, the frictional
force does work.
No, the frictional
force does no
work.
Only part of the
frictional force
does work.
YouIntroduction
can’t tell
from
Section
0 Lecture
this diagram.
1
Slide 13
Since the frictional force is antiparallel to the distance moved, it does
negative work on the block.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 13
Does the normal force of the floor pushing upward on
the block do any work?
a)
b)
c)
d)
Yes, the normal
force does work.
No, the normal
force does no
work.
Only part of the
normal force does
work.
You can’t tell from
this diagram.
Introduction
Section 0
Lecture 1
Slide 14
Since the normal force is perpendicular to the distance moved, it does no
work on the block.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 14
A force of 50 N is used to drag a crate 4 m across a floor. The
force is directed at an angle upward from the crate as shown. What
is the work done by the horizontal component of the force?
a)
b)
c)
d)
e)
120 J
160 J
200 J
280 J
0J
The horizontal component
of force is 40 N and is in
the direction
of motion:
Introduction
Section 0 Lecture
1
Slide 15
W=F·d
= (40 N) · (4 m)
= 160 J.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 15
What is the work done by the vertical component of the force?
a)
120 J
b) 160 J
c)
200 J
d) 280 J
e) 0 J
The vertical component of
force is 30 N but isn’t in the
direction of motion:
W = Introduction
F · d Section 0 Lecture 1
= (30 N) · (0 m)
= 0 J.
Slide 16
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 16
What is the total work done by the 50-N force?
a)
120 J
b) 160 J
c)
200 J
d) 280 J
e) 0 J
Only the component of force
in the direction of motion
does work:
W = Introduction
F · d Section 0 Lecture 1
= (40 N) · (4 m)
= 160 J.
Slide 17
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 17
Physics of Technology
PHYS 1800
Lecture 13
Energy
Introduction
Lecture 1 Slide 18
Energy
and Simple Machines
Section 0
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 18
Simple Machines, Work, and Power
• A simple machine multiplies the effect of an
applied force.
– For example, a lever :
• A small force applied to one
end delivers a large force to
the rock.
• The small force acting
through aIntroduction
large distance
Section 0 Lecture 1
moves the rock a small
distance.
Slide 19
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
F2 d2 = F1 d1
Energy
Lecture 13 Slide 19
Simple Machines, Work, and Power
• A simple machine multiplies the effect of an
applied force.
– For example, a pulley :
• A small tension applied
to one end delivers twice
as much tension to lift the
box.
• The small tension acting
through Introduction
a large distance
Section 0 Lecture
moves the box a small
distance.
1
Slide 20
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 20
Mechanical Advantage and Simple Machines
• The mechanical advantage of a simple machine is the ratio
of the output force to the input force.
– For the pulley example, the mechanical advantage is 2.
• Work is equal to the force applied times the distance moved.
– Work = Force x Distance:
W=Fd
– Work output = Work input
• units: 1 joule (J) = 1 Nm= 1 kg m2 / s2 [ML2T-2]
Introduction
Section 0
Lecture 1
Slide 21
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 21
Physics of Technology
Next Lab/Demo:
Energy & Oscillations
Collisions and Momentum
Thursday 1:30-2:45
ESLC 53
Ch 6
Next Class:
Friday 10:30-11:20
Slide 22
BUS
318 room
Review Ch 6
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Energy
Lecture 13 Slide 22