Download Basic Physics Topics For Today`s Class Newton`s Laws of Motion (1

PHYS 1411 Introduction to Astronomy
Basic Physics
Chapter 5
Topics For Today’s Class
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Recap of Newton’s Laws
Mass and Weight
Work, Energy and Conservation of Energy
Rotation, Angular velocity and acceleration
Centripetal Force
Angular Momentum
Universal Law of Gravity
Newton’s Laws of Motion (1)
1. A body continues at
rest or in uniform
motion in a straight
line unless acted
upon by some net
force.
An astronaut floating in space
will continue to float forever in
a straight line unless some
external force is accelerating
him/her.
Newton’s Laws of Motion (2)
2. The acceleration a
of a body is
inversely
proportional to its
mass m, directly
proportional to the
net force F, and in
the same direction
as the net force.
a = F/m  F = m a
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Newton’s Laws of Motion (3)
How to find the Weight on Earth?
Weight is a Force = mass x acceleration due to gravity
3. To every action,
there is an equal
and opposite
reaction.
Weight in Newtons = mass in kg x 9.8 m/s2 for Earth
Mass is a property of matter, it is not equal to weight
M = 70 kg
V=?
The same force that is
accelerating the boy
forward, is accelerating
the skateboard backward.
Apple (m)
Earth (M)
m = 1 kg
v = 7 m/s
Weight of apple = mg = m x 9.8 m/s2 = Fg
https://www.youtube.com/watch?v=PrJnWTcW55s
Work and Energy
Work
• Work = Force x distance
SI units: Newton's x meter = Joules
– Example: A 1 kg object at a height of 1 m from ground
experience a force of 9.8 N, when it falls to the
ground it does work that is
W = 9.8 N x 1 m = 9.8 N.m = 9.8 J
Work and Energy
Gravitation Potential Energy
• Work due to Gravity is called Gravitational
Potential Energy (GPE) measured in Joules
GPE = mass x g x h (h = height with respect to a
reference point)
– If a 1 kg object falls from a height of 1 m, its GPE is
GPE = 1kg x 9.8m/s2 x 1m = 9.8 J
Work and Energy
Energy
• Ability of a body to do work
• Work due to motion is called Kinetic Energy (KE)
KE = ½ x mass x velocity2 measured in Joules
– Example: If a 1 kg object fall from a height of 1 m
with a velocity of 4.43 m/s
KE = ½ x 1kg x (1 m/s)2 = 9.8 J
Fundamental Law of Nature
Conservation of Energy
• In all the three examples above we find that
Work = KE = GPE
• Energy can neither be created or destroyed: It
only transforms from one form to the other.
Example: Dropping a ball converts GPE to KE
and at impact KE to heat, sound and work
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Work and Energy?
Work and Energy
This is called conservation of energy.
Energy is transformed from gravitational to
kinetic and to heat, sound and mechanical
As you walk towards the front of this class room from
your seat, which type(s) of energy do you exhibit?
a) potential energy only
b) kinetic energy only
c) both potential and kinetic energy
d) neither, only thermal energy
http://hea-www.harvard.edu/~pgreen/
Power
Power
• Rate at which energy is expended
Power (P) = Work / time
= Joules /seconds = watts (w)
Conservation of Energy Demo
https://www.youtube.com/watch?v=Ehx1P4adv6I
Mindcraftwounderhowto.com
Conservation of Energy and Kepler's Second Law
of Planetary Motion
Things that Move in Circles: Units
Radian: An angle at the center of a circle
whose arc is equal in length to the radius
Units of Measure: Radian, Degrees
and Revolutions
1 radian = 57.3 degrees
Source: wikipedia
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Things that Move in Circles
Angular velocity (ω) = change in angle / change in time
rad/s or rev/s, or deg/s
Angular Acceleration (α) = change in ω / change in time
rad/s2 or rev/s2, or deg/s2
Newton’s Second Law for
Rotating Bodies
• Any object that moves
in a circle or an arc has
centripetal force
• Centripetal force (Fc)
r
measured in Newton’s
Fc = ma = m v2 / r
• Centripetal (radial)
Circumference = 2 π r
acceleration
ac = v 2 / r
v=2πr/t
v
Source: share.ehs.uen.org
Source: faculty.wcas.northwestern.edu
Visual Demo of Centripetal Force
Consequences of Centripetal Force
Why planets are not perfect spheres?
https://www.youtube.com/watch?v=Tctr8CIMOZA
Image source: NASA, STSI
Angular Momentum
Rotating and or orbiting object poses angular momentum
Solar System
Source: Wikipedia
Galaxy
Angular Momentum is
Conserved
I is called Moment of Inertia ~ MR2
M is mass and R is radius of disk
Source: wikipedia
https://www.youtube.com/watch?v=V3UsrfHa4MQ
https://www.youtube.com/watch?v=0RVyhd3E9hY
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Angular Momentum?
Attendance
Which of the following equation gives angular
momentum>
a) L = mv
b) L = Iω
c) L = mα
d) L = mr2
http://hea-www.harvard.edu/~pgreen/
Newton’s Law of Gravitation
Any two bodies are attracting each other
through gravitation, with a force (Fg)
proportional to the product of their masses
(M,m) and inversely proportional to the square
of their distance (r). G is called gravitational
constant.
Fg = GMm
r2
Campus.kellerisd.net
Gravity and Distance: The InverseSquare Law
Inverse-square law -• relates the intensity of an effect to the inversesquare of the distance from the cause
• in equation form: intensity = 1/distance2
• for increases in distance, there is decreases in
force
• even at great distances, force approaches but
never reaches zero
YouTube
Inverse-Square Law
Force of Gravity and Inverse-Square Law
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Gravitational Constant
How to find the value of G?
G=6.67x10-11 N.m2/kg2
Fg = Gm1m2
d2
Gravity?
Gravity is
a) sometimes a repulsive force and sometimes an
attractive force.
b) always a repulsive force.
c) always an attractive force.
d) none of the above.
Gravitational force is significant only for very large masses and small
separation distance
http://hea-www.harvard.edu/~pgreen/
Gravity?
Compared to your mass here on Earth, your mass out in
the space between the stars would be __________.
a) zero
b) negligibly small
c) much much greater
d) the same
e) the question cannot be answered from the
information given
http://hea-www.harvard.edu/~pgreen/
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
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How to find g of a Planet?
How to find the mass of Earth?
Weight = Gravitational Force
Weight = Gravitational Force
=
mg = GMm
Apple (m)
Apple (m)
r2
=
M=
Earth (M)
g
r2
Earth (M)
G
If we know G, M and r then we can find g
Since g, r and G are known we can calculate the mass of Earth = 6 x 1024 kg
Exam 1 Statistics
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Hi – 90
Low – 46
Average – 70
Includes 10 points curve
Good News: I drop the lowest exam grade
so there is still a chance to make an A.
Suggestions
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Read Chapter ahead of class
Study power points after class
Practice all questions in power points
Use MindTap study resource
Come to help session with questions
Try other questions from ClassAction at
UNL site (see links in the PowerPoint).
Acknowledgment
• The slides in this lecture is for Tarleton:
PHYS1411/PHYS1403 class use only
• Images and text material have been
borrowed from various sources with
appropriate citations in the slides,
including PowerPoint slides from
Seeds/Backman text that has been
adopted for class.
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