Planetary Motion Test

Question 1

Aristotle (384-322 BCE)

Answer 1

• Earth in the center
• Sun and other planets orbit around in perfect circle

Question 2

Ptolemy (100-178 BCE)

Answer 2

• Added epicycles to Aristotle’s model to explain retrograde motion
• Made better predictions, but didn’t explain why epicycles would exist

Question 3

Nicolaus Copernicus (1473-1543 CE)

Answer 3

• Sun is in the center! (Heliocentric)
• Earth and other planets orbit the Sun in perfect circles
• He added MORE epicycles
• Popular theory, widely published which was uncommon

Question 4

Tycho Brahe (1546-1601 CE)

Answer 4

• He was bothered by the inaccuracy of eclipse and astronomical event predictions
• He dedicated his whole life to this work
• His non-telescopic predictions got to an accuracy of 1/20th of a degree
• Took on Kepler as his assistant

Question 5

Johannes Kepler (1571-1630 CE)

Answer 5

• Brahe’s assistant
• When Brahe died, Kepler took his decades of data and observations and used them to devise the laws of planetary motion

Question 6

Galileo (1564-1642 CE)

Answer 6

• A friend of Pope Urban VIII
• Catholics disliked the new planetary theories, so the Pope asked Galileo to look over and deem the accuracy of Kepler’s laws
• Using a telescope, he proved the laws, as well as discovered Venus’s moons
• Stood trial for going against the church

Question 7

Isaac Newton (1642-1727 CE)

Answer 7

• Confirmed Kepler’s laws through his law of gravity

Question 8

Henry Cavendish (1731-1810 CE)

Answer 8

• Spent whole life trying to find value of “G”, the universal constant of gravity
• Cavendish balls

Question 9

What is the first law of planetary motion?

Answer 9

• All planets orbit in an ellipse around the Sun as a focus point

Question 10

What is the second law of planetary motion?

Answer 10

• A line joining a planet and the Sun sweeps out equal areas during equal time periods

Question 11

What is the third law of planetary motion?

Answer 11

• (Ta/Tb)^2 = (ra/rb)^3
• T: Orbital period
• r: average distance from center

Question 12

What is Newton’s law of gravity?

Answer 12

• Any two objects with mass have gravitational attraction
• Fg=(Gmamb)/(d^2) OR
• Fg (G*m)/(d^2)

Question 13

What is G?

Answer 13

• 6.67 x 10^(-11)
• Unit is (Nm^2)/(kg)^2

Question 14

What was Newton’s thought experiment?

Answer 14

• He wanted to know how fast he would have to launch a ball around the earth for it to reach all the way around the Earth and hit him in the back of the head

Question 15

What is the equation for the speed of orbit?

Answer 15

• v= sqrt(Gm/r)

Question 16

What is the equation for orbital period?

Answer 16

• T=2pi* sqrt(r^3/Gm)

Question 17

What is the equation for the ratio between two planets and their orbital periods?

Answer 17

• (Ta/Tb)^2 = (ra/rb)^3