1. Purdue University, Physics 220
Examination 1
Date: February 16, 2011
Time:
Place: Stewart 183
You may bring a crib sheet, handwritten, on 8.5 x 11 inch paper (you may write on both sides)
Material covered: Chapters 1-5 of textbook
Bring your student ID
Adaptive learners must notify your instructor by that you want to use an alternative testing location
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Purdue University, Physics 220

2. Purdue University, Physics 220
Lecture 08
Circular Motion
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Purdue University, Physics 220

3. Purdue University, Physics 220
Artificial Gravity
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Purdue University, Physics 220

4. Purdue University, Physics 220
Centrifuge
22,000 rpm
Rotor ,336 x g
Rotor ,228 x g
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Purdue University, Physics 220

5. Purdue University, Physics 220
The centrifuge works using the sedimentation principle, where the centripetal acceleration causes more dense substances to separate out along the radial direction (the bottom of the tube).
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Purdue University, Physics 220

6. Circular Motion – Centrifuge
A centrifuge is a device used in many laboratories
It can be used to separate particles or molecules
Or remove them
The effective force causes the particle to move to the bottom of the test tube
Similar to artificial gravity

7. Purdue University, Physics 220
A cell in a centrifuge
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Purdue University, Physics 220

8. Purdue University, Physics 220
iClicker
What is the direction of the acceleration?
A) Along the tension in the cable
B) In a horizontal direction away from the center
C) In a horizontal direction toward the center
First run at 4 second period, then do the PIQ. Then run at 2 second period. Stop it and show the forces. Acceleration is in the direction of the net force, which must be toward the center of the circle of motion.
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Purdue University, Physics 220

9. Circular Orbits of Planets
Ancient Greeks:
The Geocentric Model implies Earth-Centered Universe
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Purdue University, Physics 220

10. Circular Orbits of Planets
Copernicus (1500's)
Developed a mathematical model for a Sun-centered solar system
Published in De Revolutionibus
Dedicated the book to Paul III
The De Revolutionibus was placed on the Index of Prohibited Books in 1616 as a result of the Galileo affair
It was taken off the Index in 1758
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Purdue University, Physics 220

11. Circular Orbits of Planets
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Purdue University, Physics 220

12. Purdue University, Physics 220
Orbits of Planets
Tycho Brahe ( )
Johannes Kepler ( )
Made precise measurements of the positions of the planets
Described the shape of planetary orbits as well as their orbital speeds
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Purdue University, Physics 220

13. Purdue University, Physics 220
Kepler’s Laws
First law: The orbit of a planet about the Sun is an ellipse with the Sun at one focus.
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Purdue University, Physics 220

14. Purdue University, Physics 220
Kepler’s Laws
Second law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.
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Purdue University, Physics 220

15. Purdue University, Physics 220
Kepler’s Third Law
The square of a planet's orbital period is proportional to the cube of the length of its orbit's semimajor axis.
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Purdue University, Physics 220

16. Purdue University, Physics 220
Kepler’s Laws
Elliptical orbits…
Equal areas in equal time
T2 ~ R3
These were empirical laws
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Purdue University, Physics 220

17. Newton’s Law of Universal Gravitation
From Kepler's 3rd Law, Newton deduced inverse square law of attraction.
G=6.67 ´ N m2/kg2
Your book derives Kepler’s third law from Newton’s law for a circular orbit
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Purdue University, Physics 220

18. Purdue University, Physics 220
Satellites
Speed is independent of mass of satellite
Satellites in lower orbits have greater speeds
Geostationary orbits:
A circular orbit in Earth’s equatorial plane whose period is equal to Earth’s rotational period.
~35,786 km above ground
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Purdue University, Physics 220

19. Purdue University, Physics 220
iClicker
Two identical (except for color) satellites are in circular orbits around the Earth. The red satellite is farther from the earth than the green one. Which satellite has the greater velocity?
A) Red
B) Green
C) They are the same
a = v2/r and a goes as 1/r so v goes as 1/sqrt(r).
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Purdue University, Physics 220

20. Purdue University, Physics 220
Exercise
Two identical (except for color) satellites are in circular orbits around the Earth. The red satellite is farther from the earth than the green one. Which satellite has the greater centripetal acceleration?
A) Red
B) Green
C) They are the same
Green has the greater centripetal force so from second law it has the greater acceleration. If they have trouble with this, remind them to draw a FBD and give them F = GMm/r2
Lecture 8
Purdue University, Physics 220