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LPH105 W15 5:intro

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Richard Datwyler

on 10 May 2016

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Transcript of LPH105 W15 5:intro

Main Ideas for the chapter
Uniform Circular Motion
Kinematics
Dynamics
Wait until chapter 8
Newton's Law of Gravity:
Orbits
Kepler's Laws
little g, big G
Forces and velocities
Review concepts
Position - velocity - acceleration
Changes in time
Changes in time
Scalar vs vector
If my velocity is constant can I have an acceleration?
If my speed is constant can I have an acceleration?
Uniform circular motion : the speed is uniform, and the acceleration is too. But only as we change our coordinate system
Kinematics of UCM

Using some trig relationships (similar triangles) and the definition of acceleration gives
An object moving in a circle of radius r at constant speed v has an acceleration pointing to the center with a value of:
Useful relationship
More Kinematics
Time to complete one revolution
Measured in seconds
(s)
Revolutions in one second
Measured in Hertz
(Hz)
Period:
Frequency:
Dynamics of UCM
Using Newton’s second law we see that there is a force required to move in a circular path.
This is not an additional force!!! Rather the net force. Meaning the sum of all forces in this radial direction.
It comes from other objects, and their associated forces.
Examples
Un-banked curves
Banked curves
Roller coaster
Tether ball
‘corners’
Aside: Centripetal vs. Centrifugal forces
Others?
F
F
F
fr
G
N
If a 1000 kg car goes around a 50 m radius turn, having tires with a static coefficient of friction of 0.6, how fast could it go before skidding out?
F
N
F
G
If it moves in a circle. a
If it goes faster or slower, than a
t
r
Non UCM
Newton's
Law of Universal Gravity.
It states that every particle in the universe
attracts every other particle.
and the force is proportional to the product
of the masses and inversely to the distance
between them squared

Further, that force is on a line between the
two particles
This is described in this equation:
Where G is a constant:
Acceleration due to gravity.
Yet we have said the force of gravity is
Force of
Gravity
mg
Weightlessness
When does someone 'feel' weightless?
Would both of these people 'feel' weightless?
You recall that if an object is to move
in a circular path, there must be a net
force acting on it pointing towards the
center.
This gravitational force is that force in
space physics.
Orbital Velocity
Speeds in orbit
slow down
Speed up
Closer =
faster

1. Path of each planet is ellipse with sun at one focus
2. Planets paths sweep equal areas in equal time
3. Ratio of period to radius between any 2 objects orbiting another is same
Kepler's Laws
For 3rd law, we will solve for this relationship in lab.
Which of these pairs can be compared using Kepler's 3rd Law?

A. Earth, and Moon
B. Earth and Satellite
C. Mars and Moon
D. Mars and Earth
E. Satellite and Mars
Try one
how long does it take Saturn to complete
one orbit?

Re = 149.6 x 10 km
Rs = 1427 x 10 km
6
6
What happens to a satellite in orbit
when its tangential or forward velocity increases:

A. It moves closer to earth.
B. Orbit stays the same.
C. It moves farther from earth.
Now that it is farther from earth what must it do to maintain that orbit distance?

A. Maintain this increased velocity
C. Slow down to new velocity
D. Speed up to a new tangential velocity
"Also can you explain deterministic and weightlessness?"
"Should we go about the same set for for these problems like we did in chapter 4?"
"When talking about uniform circular motion, I couldn't help but wonder how a free body diagram is supposed to be drawn in order to show force in every circular direction. I'm guessing there's an equation that makes up for it?"
"Can we go over centrifuges?"
"How does the size not factor in to the force acting between them i mean in chemistry it does the bigger the more it pulls?"
7.36*10^22
5.98*10^24
1.90*10^27
1.99*10^30
2.80*10^30
3.50*10^30
DEMOS
Example: weight, when I lower an object
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