phy191 exam
Jocelyn yang 
Ch.13_1.pptxGravity
Warping the very fabric of reality.
4/4/2108
1
Consider the following…
Gravity is an inverse square force
Gravitational forces act along the line connecting the centers of mass
Gravity is always attractive
Newton’s Theory of Gravity
2
Only works with spherical symmetry
Inverse square give elliptical orbits
2
Whiteboard Problem 13-1
What is the ratio of the Sun’s gravitational force on the Moon to the Earth’s gravitational force on the Moon? (LC)
3
Earth’s Pull
The acceleration caused by gravity has a value of 9.8m/s2
This is merely an approximation, the effect changes as you move around Earth
We now have the tools to calculate the effect of gravity some distance away from the Earth:
4
So, the acceleration of gravity is:
Earth
(from the
Earth’s center)
1 gal == 1cm/s2, or .01m/s2, so a miligal is a difference of .00001m/s2, or .00001% of our daily gravity
4
Whiteboard Problem 13-2
Planet Zed is 10,000km in diameter. The free-fall acceleration on the surface of Zed is 8.0m/s2.
What is the mass of Zed?
What is the free fall acceleration 10,000km above Zed’s North pole? (LC)
5
With the acceleration caused by gravity changing, Ug = mgh is not true for every situation
Your text describes a new form of potential energy which works on larger scales:
Some notes:
Zero potential energy is located at infinity
The negative sign is important, and expresses that gravity is attractive
The potential energy varies as 1/r, not 1/r2
Gravitational Potential Energy
6
Whiteboard Problem 13-3
The escape speed is the minimum speed needed to escape from the surface of a planet and leave its gravitational influence.
Use conservation of energy to find an expression for the minimum initial speed the rocket needs to escape to infinity.
Calculate the escape speed from the surface of the Earth. (LC) (ME = 5.98 X 1024 kg; RE = 6.37 X 106 m)
7
Exploring Orbits
To better prepare for calculating orbits, there is a PhET assigned where you can play with various parameters of a solar system.
Use a computer with Flash (Version 8 or newer, but really just update your flash player) to run the simulation. You can find a link to the simulation in the google doc under the assignment on Canvas.
Finish the assignment as a group, submitting 1 copy per group. If you finish early, you’re free to go. If you don’t be sure to submit the finished document by Friday at noon.
8
