Chapter 3: The laws of motion

Question 1

Explain the geocentric model by Ptolemy. What were some problems with it?

Answer 1

• Earth-centered model
• the sun, the moon, and the planets visible to naked eye (Mercury, Venus, mars, Jupiter, saturn) all moved in circles around a stationary Earth
• problems: Moons of Jupiter, Imperfections of Moon and Sun, Each planet treated differently, Phases of Venus

Question 2

How did Copernicus revolutionize astronomy? What is this model called? What happens in this model?

Answer 2

• by placing Sun at center of model
• heliocentric model
• the outer planets (Mars, Jupiter, Saturn) undergo apparent retrograde motion when Earth overtakes them in their orbits
• inner planets (Mercury and Venus) move in apparent retrograde motion when overtaking Earth

Question 3

What is apparent retrograde motion?

Answer 3

planets would seem to turn around, move westward for a while, and then return to their normal eastward travel

Question 4

What is a frame of reference?

Answer 4

a system within which an observer measures positions and motions using coordinates such as distance and time

Question 5

All solar system objects exhibit apparent retrograde motion except for ?

Answer 5

sun

Question 6

What is Kepler’s 1st Law of planetary motion?

Answer 6

• Planetary orbits are ellipses, each ellipse has two foci.
  -The Sun is at one focus of a planet’s elliptical orbit.
  -Ellipse described by:
  Semi-major axis: typically measured in AU; half of the longest length
  Eccentricity: number ranging from 0 to 1; describes how elongated the ellipse is

Question 7

What is Kepler’s 2nd law of planetary motion?

Answer 7

• the Law of Equal Areas
• The line between the Sun and the planet “sweeps” out equal areas in equal times.
• Planet travels fastest when it is closest to the Sun and slowest when it is farthest from the sun

Question 8

What is Kepler’s 3rd law of planetary motion?

Answer 8

• distant objects travel at lower speeds
• period (years it takes to get around a planet’s orbit) squared is equal to the semimajor axis of the planets orbit (average distance from the sun) (AU) cubed (p^2 = a^3)

Question 9

What do Newton’s laws generally explain?

Answer 9

• why objects in the universe move the way they do.
• how planets (and comets, asteroids, dwarf planet, etc.) orbit the Sun according to Kepler’s observationally-derived Laws.

Question 10

What is Newton’s 1st law of motion?

Answer 10

-describes inertia, An object at rest will remain at rest and an object in motion will continue moving along a straight line with a constant speed until an unbalanced force acts on it.

Question 11

What is Newton’s 2nd law of motion?

Answer 11

If a net force (an unbalanced force) acts on an object, then the object’s motion changes (causes acceleration).
-acceleration = force/mass

Question 12

What is Newton’s 3rd law of motion?

Answer 12

forces always occur in action-reaction pairs.

Question 13

Acceleration (a) =

Answer 13

• force (f) / mass (m)

- the rate at which the velocity of an object changes

Question 14

Velocity =

Answer 14

speed + direction

Question 15

What is the gravitational force?

Answer 15

the mutually attractive force between two objects with mass

Question 16

All objects on Earth have been experimentally shown to fall with the same gravitational acceleration (g), what is it?

Answer 16

g = 9.8 m/s^2.

Question 17

Weight is the product of your mass and the acceleration due to gravity. What is the equation?

Answer 17

𝐹(𝑤𝑒𝑖𝑔ℎ𝑡)=𝑚𝑔

Question 18

Define Newton’s universal law of gravitation. What does it depend on? Equation:

Answer 18

-Gravity is an attractive, mutual force between any two objects with mass.
-It depends on: Mass of first object, Mass of second object, Distance between objects
F (force) = G (constant) m1m2 (masses) / r2 (distance)

Question 19

What is the inverse square law?

Answer 19

At a particular Luminosity, the more distant an object is, the fainter its apparent brightness becomes as the square of the distance

Question 20

What is the universal gravitational constant?

Answer 20

Determines the strength of gravity between objects, and it is the same for all pairs of objects
G = 6.673 x 10^-11 m^3/(kg s^2)

Question 21

What is centripetal force?

Answer 21

a force toward the centre of a circle (Gravity provides the centripetal force that holds a satellite in its orbit)